19-2134; Rev 0; 8/01
SPI-Compatible Real-Time Clock in SOT23
Features
The MAX6902 SPI™-compatible real-time clock contains a real-time clock/calendar and 31 x 8 bits of static
random-access memory (SRAM). The real-time
clock/calendar provides seconds, minutes, hours, day,
date, month, year, and century information. A time/date
programmable polled ALARM is included in the
MAX6902. The end-of-the-month date is automatically
adjusted for months with fewer than 31 days, including
corrections for leap year up to the year 2100. The clock
operates in either the 24hr or 12hr format with an
AM/PM indicator. The MAX6902 operates with a supply
voltage of +2V to +5.5V, is available in the ultra-small 8pin SOT23 package, and works over the -40°C to
+85°C industrial temperature range.
♦ Real-Time Clock Counts Seconds, Minutes, Hours,
Day of Week, Date of Month, Month, Year, and Century
Applications
♦ Leap-Year Compensation Valid up to Year 2100
♦ +2V to +5.5V Wide Operating Voltage Range
♦ SPI Interface: 4MHz at 5V; 1MHz at 2V
♦ 31 x 8-Bit SRAM for Scratchpad Data Storage
♦ Uses Standard 32.768kHz, 12.5pF Watch Crystal
♦ Low Timekeeping Current (400nA at 2V)
♦ Single-Byte or Multiple-Byte (Burst Mode) Data
Transfer for Read or Write of Clock Registers or
SRAM
♦ Ultra-Small 8-Pin SOT23 Package
Point-of-Sale Equipment
♦ Programmable Time/Date Polled ALARM Function
Intelligent Instruments
♦ No External Crystal Bias Resistors or Capacitors
Required
Fax Machines
Ordering Information
Battery-Powered Products
Portable Instruments
PART
TEMP.
RANGE
PINPACKAGE
TOP
MARK
MAX6902EKA-T
-40°C to +85°C
8 SOT23-8
AAIT
Related Real-Time Clock Products
ALARM
FUNCTION
OUTPUT
FREQUENCY
PINPACKAGE
PART
SERIAL
INTERFACE
SRAM
MAX6900
I2C™ compatible
31 ✕ 8
—
—
6 SOT23
MAX6901
3 Wire
31 ✕ 8
Polled
32kHz
8 SOT23
MAX6902
SPI compatible
31 ✕ 8
Polled
—
8 SOT23
Pin Configuration
Typical Operating Circuit
+3.3V 0.1µF
TOP VIEW
6
VCC
+3.3V
MAX6902
1
µc
5
2
3
SCLK
X2
CS
DOUT
DIN
X1
SCLK 1
8
7
32.768kHz
CRYSTAL
8
X1
7
X2
3
6
VCC
GND 4
5
CS
DOUT 2
MAX6902
DIN
GND
4
SOT23
SPI is a trademark of Motorola, Inc.
I2C is a trademark of Philips Corp.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX6902
General Description
MAX6902
SPI-Compatible Real-Time Clock in SOT23
ABSOLUTE MAXIMUM RATINGS
VCC to GND ..............................................................-0.3V to +6V
All Other Pins to GND ................................-0.3V to (VCC + 0.3V)
Current into Any Pin..........................................................±20mA
Rate of Rise, VCC ............................................................100V/µs
Continuous Power Dissipation (TA = +70°C)
8-Pin SOT23 (derate 8.9mW/°C above +70°C)............714mW
Junction Temperature .....................................................+150°C
Storage Temperature Range…………………… -65°C to +150°C
ESD Protection (all pins, Human Body Model) ..................2000V
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
DC ELECTRICAL CHARACTERISTICS
(VCC = +2.0V to +5.5V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
Operating Voltage
Range
VCC
Active Supply
Current (Note 2)
ICC
Timekeeping Supply
Current (Note 3)
ITK
CONDITIONS
MIN
TYP
2
MAX
UNITS
5.5
V
VCC = +2V
0.3
VCC = +5V
1.1
VCC = +2V
0.4
0.8
VCC = +5V
1.3
2.2
mA
µA
SPI DIGITAL INPUTS (SCLK, DIN, CS)
2
Input High Voltage
VIH
Input Low Voltage
VIL
Input Leakage
IIL
VCC = +2V
1.4
VCC = +5V
2.2
V
VCC = +2V
0.6
VCC = +5V
0.8
VIN = 0 to VCC
-10
10
V
nA
Input Capacitance
SPI DIGITAL OUTPUTS (DOUT)
10
pF
Output Capacitance
15
pF
Output Low Voltage
VOL
Output High Voltage
VOH
VCC = +2.0V, ISINK = 1.5mA
0.4
VCC = +5.0V, ISINK = 4mA
0.4
VCC = +2.0V, ISOURCE = -0.4mA
1.8
VCC = +5.0V, ISOURCE = -1mA
4.5
_______________________________________________________________________________________
V
V
SPI-Compatible Real-Time Clock in SOT23
(VCC = +2.0V to +5.5V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TA = +25°C, unless otherwise noted.) (Figure 5,
Notes 1, 4)
PARAMETER
OSCILLATOR
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
X1 to Ground
Capacitance
25
pF
X2 to Ground
Capacitance
25
pF
SPI SERIAL TIMING
Maximum Input Rise
Time
trIN
DIN, SCLK, CS
2
µs
Maximum Input Fall
Time
tfIN
DIN, SCLK, CS
2
µs
Output Rise Time
trOUT
DOUT, CLOAD = 100pF
10
ns
Output Fall Time
tfOUT
DOUT, CLOAD = 100pF
10
ns
VCC = +2V
1000
VCC = +5V
238
SCLK Period
tCP
SCLK High Time
tCH
100
ns
SCLK Low Time
tCL
100
ns
SCLK Fall to DOUT
Valid
tDO
DIN to SCLK Setup
Time
tDS
100
ns
DIN to SCLK Hold
Time
tDH
2
ns
SCLK Rise to CS
Rise Hold Time
tCSH
2
ns
CS High Pulse Width
tCSW
200
ns
CS High to DOUT
High Impedance
tCSZ
CS to SCLK Setup
Time
tCSS
CLOAD = 100pF
ns
100
100
100
ns
ns
ns
Note 1: All parameters are 100% tested at TA = +25°C. Limits over temperature are guaranteed by design and characterization and
not production tested.
Note 2: ICC is specified with DOUT open, CS = DIN = GND, SCLK = 4MHz at VCC = +5V; SCLK = 1MHz at VCC = +2.0V.
Note 3: Timekeeping current is specified with CS = VCC, SCLK = DIN = GND, DOUT = 100kΩ to GND.
Note 4: All values referred to VIH min and VIL max levels.
_______________________________________________________________________________________
3
MAX6902
AC ELECTRICAL CHARACTERISTICS
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
TIMEKEEPING CURRENT
vs. SUPPLY VOLTAGE
MAX6902 toc01
10.0
SUPPLY CURRENT (µA)
MAX6902
SPI-Compatible Real-Time Clock in SOT23
1.0
0.1
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
Pin Description
PIN
NAME
1
SCLK
FUNCTION
Serial Clock Input. SPI clock for DIN and
DOUT data transfers.
2
DOUT
3
DIN
SPI Data Input
4
GND
Ground
5
CS
Chip Select Input. Active low for valid data
transfers.
6
VCC
Power-Supply Pin. Bypass VCC to GND
with a 0.1µF capacitor.
7
X2
External 32.768kHz Crystal
8
X1
External 32.768kHz Crystal
Table 1. Acceptable Quartz Crystal
Parameters
PARAMETER
SYMBOL
MIN
Frequency
f
Equivalent
Series
Resistance
(ESR)
RS
40
Parallel Load
Capacitance
CL
11.2
Q Factor
Q
40,000
TYP
MAX
32.768
kHz
SPI Data Output
12.5
60
kΩ
13.7
pF
60,000
Detailed Description
The MAX6902 is a real-time clock/calendar with an SPIcompatible interface and 31 x 8 bits of SRAM. It provides seconds, minutes, hours, day of the week, date of
the month, month, and year information, held in seven 8bit timekeeping registers (Functional Diagram). An onchip 32.768kHz oscillator circuit requires only a single
external crystal to operate. Table 1 specifies the parameters for the external crystal, and Figure 1 shows a
functional schematic of the oscillator circuit. The
MAX6902’s register addresses and definitions are
described in Figure 2 and in Table 2. Time and calendar
data are stored in the registers in binary-coded decimal
(BCD) format. A polled alarm function is included for
scheduled timing of user-defined times or intervals.
4
UNITS
MAX6902
25pF
25pF
X1
X2
EXTERNAL
CRYSTAL
Figure 1. Crystal Oscillator Circuit Schematic
_______________________________________________________________________________________
SPI-Compatible Real-Time Clock in SOT23
FUNCTION
MAX6902
REGISTER ADDRESS
REGISTER DEFINITION
A7 A6 A5 A4 A3 A2 A1 A0
VALUE
D7
D6 D5 D4
00-59
0
10 SEC
*POR STATE
0
D3
D2 D1
D0
CLOCK
SECONDS
RD 0
0
0
0
0
0
1
/W
MINUTES
RD 0
0
0
0
0
1
1
/W
HOURS
RD 0
00-59
*POR STATE
0
0
0
1
0
1
0
ALM
OUT
DATE
RD 0
0
0
0
1
1
1
/W
MONTH
RD 0
0
0
1
0
0
1
/W
DAY
RD 0
0
0
1
0
1
1
/W
YEAR
RD 0
0
0
1
1
0
1
/W
CONTROL
RD 0
0
0
00-23
12/24
01-12
1/0
0
0
1
1
1
CENTURY
RD 0
1
0
0
1
/W
0
1 MIN
0
0
10
HR 10
A/P HR
0/1
0
0
1 HR
0
0 10 DATE
*POR STATE
0
0
0
01-12
0
0
0 10M
*POR STATE
0
0
0
0
0
0
01-07
0
0
0
0
0
WEEK DAY
*POR STATE
0
0
0
0
0
0
0
0
0
1
0
0
0
1 DATE
0
0
0
1
1 MONTH
10 YEAR
0
0
1
1
1 YEAR
1
1
0
0
0
0
WP
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
00-99
*POR STATE
0
0
01-28/29
01-30
0-31
00-99
0
0
0
1
1
0
0
0
*POR STATE
0
0
*POR STATE
*POR STATE
/W
0
10 MIN
0
/W
0
1 SEC
1000 YEAR
0
0
0
100 YEAR
1
1
0
0
1
Note: *POR STATE defines power-on reset state of register contents.
Figure 2. Register Address Definition (Sheet 1 of 3)
_______________________________________________________________________________________
5
0
1
0
/W
RESERVED
Do not write
to this location.
RD 0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
*POR STATE
0
0
0
0
0
1
1
1
00-59
0
*POR STATE
0
00-59
0
*POR STATE
0
1
*POR STATE
0
1
0
1
1
1
/W
D5 D4
D3 D2 D1
D0
SECOND
1
VALUE
MINUTE
0
D6
DATE
RD 0
D7
MONTH
ALARM
CONFIG
A7 A6 A5 A4 A3 A2 A1 A0
DAY
FUNCTION
REGISTER DEFINITION
YEAR
REGISTER ADDRESS
HOUR
MAX6902
SPI-Compatible Real-Time Clock in SOT23
0
0
0
ALARM
THRESHOLDS
SECONDS
RD 0
0
1
1
0
0
1
/W
MINUTES
RD 0
0
1
1
0
1
1
/W
HOURS
RD 0
0
1
1
1
0
1
00-23
12/24
01-12
1/0
10 SEC
1
DATE
RD 0
0
1
1
1
1
1
/W
MONTH
RD 0
1
0
0
0
0
1
/W
DAY
RD 0
1
0
0
0
1
1
/W
YEAR
RD 0
1
0
0
1
0
1
/W
CLOCK
BURST
RD 0
1
1
1
1
1
1
1
1
0
1
01-28/29
01-30
01-31
0
0
*POR STATE
0
0
1
1
1
1
1
1
1
1
1 HR
1
10 DATE
1
1
1
1
1
1 DATE
1
1
1
1
01-12
0
0
0 10M
*POR STATE
0
0
0
1
1
01-07
0
0
0
0
0
WEEK DAY
*POR STATE
0
0
0
0
0
1
00-99
1 MONTH
10 YEAR
1
1
1
1
1
1
1
1
1 YEAR
1
1
1
1
/W
Figure 2. Register Address Definition (Sheet 2 of 3)
6
1
1 MIN
10 10
HR HR
A/P
0/1
*POR STATE
*POR STATE
1
1
10 MIN
0
/W
1
1 SEC
_______________________________________________________________________________________
1
1
SPI-Compatible Real-Time Clock in SOT23
FUNCTION
MAX6902
REGISTER ADDRESS
REGISTER DEFINITION
A7 A6 A5 A4 A3 A2 A1 A0
VALUE
D7
D6 D5 D4
D3
D2 D1
D0
RAM
RAM 0
RD 1
0
0
0
0
0
1
RAM DATA 0
x
x
x
x
x
x
x
x
x
x
x
x
/W
•
•
•
•
•
•
•
•
•
•
•
•
RAM 30
RD 1
1
1
1
1
0
1
1
1
1
1
1
1
RAM DATA 30
x
x
x
x
/W
RAM BURST
RD 1
/W
Note: *POR STATE defines power-on reset state of register contents.
Figure 2. Register Address Definition (Sheet 3 of 3)
Table 2. Register Address and Description
WRITE (HEX)
READ (HEX)
DESCRIPTION
POR CONTENTS (HEX)
01
81
Seconds
00
03
83
Minutes
00
05
85
Hours
00
07
87
Date
01
09
89
Month
01
0B
8B
Day
01
0D
8D
Year
70
0F
8F
Control
00
13
93
Century
19
15
95
Alarm Configuration
00
17
97
Reserved
07
19
99
Seconds Alarm Threshold
7F
1B
9B
Minutes Alarm Threshold
7F
1D
9D
Hours Alarm Threshold
BF
3F
1F
9F
Date Alarm Threshold
21
A1
Month Alarm Threshold
1F
23
A3
Day Alarm Threshold
07
25
A5
Year Alarm Threshold
FF
3F
BF
Clock Burst
Not applicable
_______________________________________________________________________________________
7
MAX6902
SPI-Compatible Real-Time Clock in SOT23
Table 2. Register Address and Description (continued)
8
WRITE (HEX)
READ (HEX)
DESCRIPTION
POR CONTENTS (HEX)
41
C1
RAM 0
Indeterminate
43
C3
RAM 1
Indeterminate
45
C5
RAM 2
Indeterminate
47
C7
RAM 3
Indeterminate
49
C9
RAM 4
Indeterminate
4B
CB
RAM 5
Indeterminate
4D
CD
RAM 6
Indeterminate
4F
CF
RAM 7
Indeterminate
51
D1
RAM 8
Indeterminate
53
D3
RAM 9
Indeterminate
55
D5
RAM 10
Indeterminate
57
D7
RAM 11
Indeterminate
59
D9
RAM 12
Indeterminate
5B
DB
RAM 13
Indeterminate
5D
DD
RAM 14
Indeterminate
5F
DF
RAM 15
Indeterminate
61
E1
RAM 16
Indeterminate
63
E3
RAM 17
Indeterminate
65
E5
RAM 18
Indeterminate
67
E7
RAM 19
Indeterminate
69
E9
RAM 20
Indeterminate
6B
EB
RAM 21
Indeterminate
6D
ED
RAM 22
Indeterminate
6F
EF
RAM 23
Indeterminate
71
F1
RAM 24
Indeterminate
73
F3
RAM 25
Indeterminate
75
F5
RAM 26
Indeterminate
77
F7
RAM 27
Indeterminate
79
F9
RAM 28
Indeterminate
7B
FB
RAM 29
Indeterminate
7D
FD
RAM 30
Indeterminate
7F
FF
RAM Burst
Not applicable
_______________________________________________________________________________________
SPI-Compatible Real-Time Clock in SOT23
Address/Command Byte
Each data transfer into or out of the MAX6902 is initiated
by an Address/Command byte. The Address/Command
byte specifies which registers are to be accessed, and
if the access is a read or a write. Figure 2 shows the
Address/Command bytes and their associated registers, and Table 2 lists the hex codes for all read and
write operations. The Address/Command bytes are
input MSB (bit 7) first. Bit 7 specifies a write (logic 0) or
read (logic 1). Bit 6 specifies register data (logic 0) or
RAM data (logic 1). Bits 5–1 specify the designated register to be written or read. The LSB (bit 0) must be logic
1. If the LSB is a zero, writes to the MAX6902 are disabled.
Clock Burst Mode
Sending the Clock Burst Address/Command (3Fh for
Write and BFh for Read), specifies burst-mode operation. In this mode, multiple bytes are read or written
after a single Address/Command. The first seven
clock/calendar registers (Seconds, Minutes, Hours,
Date, Month, Day, and Year) and the Control register
are consecutively read or written, starting with the MSB
of the Seconds register. When writing to the clock registers in burst mode, all seven clock/calendar registers
and the Control register must be written in order for the
data to be transferred. See Example: Setting the Clock
with a Burst Write.
RAM Burst Mode
Sending the RAM Burst Address/Command (F7h for
Write, FFh for Read) specifies burst-mode operation. In
this mode, the 31 RAM locations can be consecutively
read or written, starting at 41h for Writes, and C1h for
Reads. A Burst Read outputs all 31 bytes of RAM.
When writing to RAM in burst mode, it is not necessary
to write all 31 bytes for the data to transfer; each complete byte written is transferred to RAM. When reading
from RAM, data are output until all 31 bytes have been
read, or until CS is driven high.
Setting the Clock
Writing to the Timekeeping Registers
The time and date are set by writing to the timekeeping
registers (Seconds, Minutes, Hours, Date, Month, Day,
Year, and Century). During a write operation, an input
buffer accepts the new time data while the timekeeping
registers continue to increment normally, based on the
crystal counter. The buffer also keeps the timekeeping
registers from changing as the result of an incomplete
write operation, and collision-detection circuitry
ensures that a Time Write does not occur coincident
with a Seconds register increment. The updated time
data are loaded into the timekeeping registers after the
rising edge of CS, at the end of the SPI write operation.
An incomplete write operation aborts the update procedure, and the contents of the input buffer are discarded. The timekeeping registers reflect the new time
beginning with the first Seconds register increment
after the rising edge of CS.
Although both Single Writes and Burst Writes are possible, the best way to write to the timekeeping registers is
with a Burst Write. With a Burst Write, the main timekeeping registers (Seconds, Minutes, Hours, Date,
Month, Day, Year) and the Control register are written
sequentially following the Address/Command byte.
They must be written as a group of eight registers, with
8 bits each, for proper execution of the Burst Write
function. All seven timekeeping registers are simultaneously loaded into the clock counters by the rising edge
of CS, at the end of the SPI write operation. For a normal burst data transfer, the worst-case error that can
occur between the actual time and the written time
update is 1s.
If single write operations are used to enter data into the
timekeeping registers, error checking is required. If not
writing to the Seconds register, begin by reading the
Seconds register and save it as initial-seconds. Then
write to the required timekeeping registers, and finally
read the Seconds register again (final-seconds). Check
to see that final-seconds is equal to initial-seconds. If
not, repeat the write process. If writing to the Seconds
register, update the Seconds register first, and then
read it back and store its value (initial-seconds).
Update the remaining timekeeping registers and then
read the Seconds register again (final-seconds). Check
to see that final-seconds is equal to initial-seconds. If
not, repeat the write process.
AM/PM and 12Hr/24Hr Mode
Bit 7 of the Hours register selects 12hr or 24hr mode.
When high, 12hr mode is selected. In 12hr mode, bit 5
is the AM/PM bit, logic high for PM. In 24hr mode, bit 5
is the second 10hr bit, logic high for hours 20 through
23.
Write-Protect Bit
Bit 7 of the Control register is the Write-Protect bit.
When high, the Write-Protect bit prevents write operations to all registers except itself. After initial settings
are written to the timekeeping registers, set the WriteProtect bit to logic 1 to prevent erroneous data from
entering the registers during power glitches or interrupted serial transfers. The lower 7 bits (bits 0–6) are
unusable, and always read zero. Any data written to
_______________________________________________________________________________________
9
MAX6902
Command and Control
MAX6902
SPI-Compatible Real-Time Clock in SOT23
bits 0–6 are ignored. Bit 7 must be set to zero before a
single write to the clock, before a write to RAM, or during a Burst Write to the clock.
Example: Setting the Clock
with a Burst Write
To set the clock to 10:11:31PM, Thursday July 4th,
2002 with a burst write operation, write 3Fh as the
Address/Command byte, followed by 8 bytes, 31h, 11h,
B0h, 04h, 07h, 05h, 02h, and 00h (Figure 2). 3Fh is the
Clock Burst Write Address/Command. The first byte,
31h, sets the Seconds register to 31. The second byte,
11h, sets the Minutes register to 11. The third byte,
B0h, sets the Hours register to 12hr mode, and 10PM.
The fourth byte, 04h, sets the Date register (day of the
month) to the 4th. The fifth byte, 07h, sets the Month
register to July. The sixth byte, 05h, sets the Day register (day of the week) to Thursday. The seventh byte,
02h, sets the Year register to 02. The eighth byte, 00h,
clears the Write-Protect bit of the Control register to
allow writing to the MAX6902. The Century register is
not accessed with a Burst Write and therefore must be
written to separately to set the century to 20. Note the
Century register corresponds to the thousand and hundred digits of the current year and defaults to 19.
Example: Reading the Clock
with a Burst Read
Reading the Clock
To read the time with a Burst Read, send BFh as the
Address/Command byte. Then clock out 8 bytes,
Seconds, Minutes, Hours, Date of the month, Month,
Day of the week, Year, and finally the Control byte. All
data are output MSB first. Decode the required information based on the register definitions listed in Figure 2.
Reading the Timekeeping Registers
Using the Alarm
The main timekeeping registers (Seconds, Minutes,
Hours, Date, Month, Day, Year) can be read with either
Single Reads or a Burst Read. In the MAX6902, a latch
buffers each clock counter’s data. Clock counter data
are latched by the SPI Read Command (on the falling
edge of SCLK, after the Address/Command byte has
been sent by the master to read a timekeeping register). Collision-detection circuitry ensures that this does
not happen coincident with a Seconds counter increment to ensure accurate time data are being read. The
clock counters continue to count and keep accurate
time during the read operation.
The simplest way to read the timekeeping registers is to
use a Burst Read. In a Burst Read, the main timekeeping registers (Seconds, Minutes, Hours, Date, Month,
Day, Year), and the Control register are read sequentially, in the order listed with the Seconds register first.
They are read out as a group of eight registers, with 8
bits each. All timekeeping registers (except Century)
are latched upon the receipt of the Burst Read command. The worst-case error between the “actual” time
and the “read” time is 1s for a normal data transfer.
The timekeeping registers may also be read using
Single Reads. If Single Reads are used, it is necessary
10
to do some error checking on the receiving end,
because it is possible that the clock counters could
change during the Read operations, and report inaccurate time data. The potential for error is when the
Seconds register increments before all the registers are
read. For example, suppose a carry of 13:59:59 to
14:00:00 occurs during single read operations. The net
data read could be 14:59:59, which is erroneous. To
prevent errors from occurring with single read operations, read the Seconds register first (initial-seconds)
and store this value for future comparison. After the
remaining timekeeping registers have been read,
reread the Seconds register (final-seconds). Check that
the final-seconds value equals the initial-seconds
value. If not, repeat the entire Single Read process.
Using Single Reads at a 100kHz serial speed, it takes
under 2.5ms to read all seven of the timekeeping registers, including two reads of the Seconds register.
A polled alarm function is available by reading the ALM
OUT bit. The ALM OUT bit is D7 of the Minutes timekeeping register. A logic 1 in ALM OUT indicates the Alarm
function is triggered. There are eight registers associated
with the alarm function—seven programmable Alarm
Threshold registers and one programmable Alarm
Configuration register. The Alarm Configuration register
determines which Alarm Threshold registers are compared to the timekeeping registers, and the ALM OUT bit
sets if the compared registers are equal. Figure 2 shows
the function of each bit of the Alarm Configuration register. Placing a logic 1 in any given bit of the Alarm
Configuration register enables the respective alarm function. For example, if the Alarm Configuration register is set
to 0000 0011, ALM OUT is set when both the minutes and
seconds indicated in the Alarm Threshold registers match
the respective timekeeping registers. Once set, ALM OUT
stays high until it is cleared by reading or writing to the
Alarm Configuration register, or by reading or writing to
any of the Alarm Threshold registers. The Alarm
Configuration register is written with address 15h, and
read with address 95h.
______________________________________________________________________________________
SPI-Compatible Real-Time Clock in SOT23
shifted out on the negative edge. There is one clock
cycle for each bit transferred. Address and data bits
are transferred in groups of eight.
The static RAM is 31 x 8 bits addressed consecutively
in the RAM Address/Command space. Table 2 details
the specific hex Address/Commands for Reads and
Writes to each of the 31 locations of RAM. The contents
of the RAM are static and remain valid for VCC down to
2V. All RAM data are lost if power is cycled. The WriteProtect Bit (bit 7 of the Control register), when high, disallows any writes to RAM.
The SPI protocol allows for one of four combinations of
serial clock phase and polarity from the microcontroller,
through a 2-bit selection in its SPI Control register. The
clock polarity is specified by the CPOL Control bit,
which selects active-high or active-low clock, and has
no significant effect on the transfer format. The Clock
Phase Control bit, CPHA, selects one of two different
transfer formats. The clock phase and polarity must be
identical for the master and the slave. For the
MAX6902, set the control bits to CPHA = 1 and CPOL =
1. This configures the system for data to be launched
on the negative edge of SCLK and sampled on the
positive edge. With CPHA equal to 1, CS can remain
low between successive data byte transfers, allowing
burst-mode data transfers to occur.
SPI-Compatible Serial
Interface
Interface the MAX6902 with a microcontroller using a
serial, 4-wire, SPI interface. SPI is a synchronous bus
for address and data transfer, and is used with
Motorola or other microcontrollers that have an SPI
port. Four connections are required for the interface:
DOUT (Serial Data Out); DIN (Serial Data In); SCLK
(Serial Clock); and CS (Chip Select). In an SPI application, the MAX6902 acts as a slave device and the
microcontroller acts as the master. CS is asserted low
by the microcontroller to initiate a transfer, and
deasserted high to terminate a transfer. DIN transfers
input data from the microcontroller to the MAX6902.
DOUT transfers output data from the MAX6902 to the
microcontroller. A shift clock, SCLK, is used to synchronize data movement between the microcontroller and
the MAX6902. SCLK, which is generated by the microcontroller, is active only during address and data transfer to any device on the SPI bus. The inactive clock
polarity is usually programmable on the microcontroller
side of the SPI interface. In the MAX6902, input data
are latched on the positive edge, and output data are
Address and data bytes are shifted MSB first into DIN
of the MAX6902, and out of DOUT. Data are shifted out
at the negative edge of SCLK, and shifted in or sampled at the positive edge of SCLK. Any transfer
requires an Address/Command byte followed by one or
more bytes of data. Data are transferred out of DOUT
for a read operation, and into DIN for a write operation.
DOUT transmits data only after an Address/Command
byte specifies a read operation; otherwise, it is high
impedance.
Data Transfer Write timing is shown in Figure 3. Data
Transfer Read timing is shown in Figure 4. Detailed
Read and Write Timing is shown in Figure 5.
CS
SCLK
DIN
0
R*
A5
A4
A3
A2
A1
1
D7
D6
D5
ADDRESS/COMMAND BYTE
DOUT
D4
D3
D2
D1
D0
DATA BYTE
HIGH IMPEDANCE; NO ACTIVITY ON DOUT LINE DURING WRITES.
* R = RAM/REGISTER SELECT BIT; RAM = 1, REGISTER = 0.
Figure 3a. Single Write
______________________________________________________________________________________
11
MAX6902
Using the On-Board RAM
MAX6902
SPI-Compatible Real-Time Clock in SOT23
CS
SCLK
DIN
0
R*
1
1
1
1
1
1
D7
D6
D5
ADDRESS/COMMAND BYTE**
D4
D3
D2
D1
D0
D7
D6
D5
DATA BYTE 1
D4
D3
D2
D1
D2
D1
D0
DATA BYTE N
HIGH IMPEDANCE; NO ACTIVITY ON DOUT LINE DURING WRITES.
DOUT
* R = RAM/REGISTER SELECT BIT; RAM = 1, REGISTER = 0.
** ONLY ONE ADDRESS/COMMAND BYTE IS REQUIRED PER BURST TRANSACTION.
Figure 3b. Burst Write
CS
SCLK
DIN
1
R*
A5
A4
A3
A2
A1
1
ADDRESS/COMMAND BYTE
HIGH IMPEDANCE
DOUT
D7
D6
D5
D4
D3
D2
D1
D0
DATA BYTE
* R = RAM/REGISTER SELECT BIT; RAM = 1, REGISTER = 0.
Figure 4a. Single Read
CS
SCLK
DIN
1
R*
1
1
1
1
1
1
ADDRESS/COMMAND BYTE**
DOUT
HIGH IMPEDANCE
* R = RAM/REGISTER SELECT BIT; RAM = 1, REGISTER = 0.
** ONLY ONE ADDRESS/COMMAND BYTE IS REQUIRED PER BURST TRANSACTION.
D7
D6
D5
D4
D3
DATA BYTE 1
D2
D1
D0
D7
D6
D5
D4
D3
DATA BYTE N
Figure 4b. Burst Read
12
______________________________________________________________________________________
D0
SPI-Compatible Real-Time Clock in SOT23
MAX6902
tCSH
CS
tCSS
tCH
tCL
tCP
tCSW
SCLK
tDH
tDS
D7
D6
D5
D0
DIN
tCSZ
D7
DOUT
D0
tDO
Figure 5. SPI Bus Timing Diagrams
Chip Select
CS serves two functions. First, CS turns on the control
logic that allows access to the Shift register for
Address/Command and data transfer. Second, CS provides a method of terminating either single-byte or multiple-byte data transfers. All data transfers are initiated
by driving CS low. If CS is high, then DOUT is high
impedance.
Serial Clock
A clock cycle on SCLK is a rising edge followed by a
falling edge. For data input, data must be valid at DIN
before the rising edge of the clock. For data outputs, bits
are valid on DOUT after the falling edge of the clock.
Data Output (Single-Byte Read
and Burst Read)
A read from the MAX6902 is initiated by an
Address/Command Write from the microcontroller (master) to the MAX6902 (slave). The Address/Command
Write portion of the data transfer is clocked into the
MAX6902 on rising clock edges. Following the eighth
falling clock edge of SCLK, after tDO (Figure 4) data
begins to be output on DOUT of the MAX6902. Data
bytes are output MSB first. Additional SCLK cycles
transmit additional data bits, as long as CS remains low.
This permits continuous burst-mode read capability.
Applications Information
Data Input (Single-Byte Write)
Crystal Selection
Following the eight SCLK cycles that input a Single-Byte
Write Address/Command, data bits are input on the rising edges of the next eight SCLK cycles. Additional
SCLK cycles are ignored. Input data MSB first.
The MAX6902 is designed to use a standard
32.768kHz watch crystal. Table 1 details the recommended crystal requirements. Some suggested crystals are listed in Table 3. In addition to the specified
SMT devices, some of the listed manufacturers also
offer other package options.
Data Input (Burst Write)
Following the eight SCLK cycles that input a Burst-Write
Address/Command, data bits are input on the rising
edges of the following SCLK cycles. The number of
clock cycles depends on whether the timekeeping registers or RAM are being written. A Clock Burst Write
requires 1 Address/Command byte, 7 timekeeping data
bytes, and 1 Control register byte. A Burst Write to RAM
may be terminated after any complete data byte by driving CS high. Input data MSB first (Figure 3).
Frequency Stability and Temperature
Timekeeping accuracy of the MAX6902 is dependent
on the frequency stability, of the external crystal. To
determine frequency stability, use the parabolic curve
in Figure 6 and the following equations:
∆f = fk(T0 - T)2
where:
∆f = change in frequency from +25°C (Hz)
f = nominal crystal frequency (Hz)
______________________________________________________________________________________
13
Table 3. 32.768kHz Surface-Mount Watch Crystals
MANUFACTURER
PART NO.
MANUFACTURER
TEMP.
RANGE
CL (pF)
+25°C
FREQUENCY
TOLERANCE
(ppm)
Abracon Corporation
ABS25-32.768-12.5-B-2-T
-40°C to +85°C
12.5
±20
Caliber Electronics
AWS2A-32.768KHz, AWS2B-32.768KHz
-20°C to +70°C
12.5
±20
ECS INC International
ECS-.327-12.5-17
-10°C to +60°C
12.5
±20
Fox Electronics
FSM327
-40°C to +85°C
12.5
±20
M-tron
SX2010/ SX2020
-20°C to +75°C
12.5
±20
Raltron
RSE-32.768-12.5-C-T
-10°C to +60°C
12.5
±20
SaRonix
32S12A
-40°C to +85°C
12.5
±20
k = parabolic curvature constant (-0.035 ±0.005ppm/°C2
for 32.768kHz watch crystals)
T0 = turnover temperature (+25°C ±5°C for 32.768kHz
watch crystals)
T = temperature of interest (°C)
For example: What is the worst-case change in oscillator frequency from +25°C ambient to +45°C ambient?
∆fdrift = 32.768Hz × (-0.04ppm/ oC2 × (1× 10-6 ))
× (20o C - 45o C)2 = -0.8192Hz
]] }
{[ [
= {[1/[(32,768Hz - 0.8192Hz) / 32,768]] - 1s} / 1s
∆t drift = 1/ (f + ∆fdrift ) / 32,768 - 1s / 1s
= 0.000025s / s
Error due to 25°C initial crystal tolerance of ±20ppm:
{[
[
]]
}
∆tinitial = 1/ (f + ∆finitial ) / 32,768 - 1s / 1s
(
)
Total timekeeping error per second:
[
]]
}
∆tinitial = 1/ (32,768 - 0.65536) / 32,768 - 1 / 1s
= 0.000020s / s
∆t total = ∆t drift + ∆tinitial
∆t total = 0.000025s / s + 0.000020s / s = 0.000045s / s
14
-20
TEMPERATURE (°C)
0
20
40
60
80
100
-50
-100
-150
-250
TYPICAL TEMPERATURE CHARACTERISTICS
(k = -0.035 ppm/°C2; TO = +25°C)
Figure 6. Typical Temperature Curve for 32.768kHz Watch
Crystal
After 1 month that translates to:
hr min
s
∆t = (31day) 24
60
(0.000045s/s)
60
min
hr
day
=120.528s
∆finitial = 32,768Hz × -20ppm × (1 × 10-6 ) = 0.65536Hz
{[
-40
-200
What is the worst-case timekeeping error per second?
Error due to temperature drift:
∆t drift
0
f (ppm)
MAX6902
SPI-Compatible Real-Time Clock in SOT23
Total worst-case timekeeping error at the end of 1
month at 45°C is about 120s or 2min (assumes negligible parasitic layout capacitance).
Oscillator Start Time
The MAX6902 oscillator typically takes 5s to 10s to
begin oscillating. To ensure the oscillator is operating
correctly, the software should validate proper timekeeping. This is accomplished by reading the Seconds
register. Any reading of 1s or more from the POR value
of zero seconds is a validation of proper startup.
______________________________________________________________________________________
SPI-Compatible Real-Time Clock in SOT23
RESERVED Register
Address/Command 17h is reserved for factory testing
ONLY. Do not write to this register. If inadvertent writes
are done to this register, cycle power to the MAX6902.
Timekeeping Current—Backup Battery
Systems
Often a real-time clock (RTC) is operated in a system
with a backup battery. A microprocessor supervisory
circuit with backup battery switchover, or other switching arrangement, is used to switch power from VCC to
VBATT when VCC falls below a set threshold. Most of
these systems leave only the RTC and some SRAM to
run from VBATT. The microcontroller that communicates
with the RTC is powered only from V CC . When the
microcontroller is put into reset, its ports typically
become high impedance. This essentially floats DIN,
CS, DOUT, and SCLK. There is a potential for
increased timekeeping current (up to x100) as VCC falls
through the linear region of the gates for DIN, CS,
DOUT, and SCLK. Duration of this effect depends on
the discharge rate of VCC. To minimize current draw
from V BATT in such systems, ensure that V CC falls
rapidly at power down. One option is a VCC discharge
resistor of 100kΩ or less from VCC to ground. This also
ensures sufficient impedance, back through the microcontroller’s ESD protection, on VCC when it is gone to
keep DIN, CS, DOUT, and SCLK from floating, which
can cause excessive timekeeping current. Alternately,
a 100kΩ pulldown (for microcontroller port pins with
≤1µA input leakage) on each pin (DIN, CS, DOUT, and
SCLK) ensures that timekeeping current specifications
are met during the power switchover.
Power-Supply Considerations
For most applications, a 0.1µF capacitor from VCC to
GND provides adequate bypassing for the MAX6902. A
series resistor can be added to the supply line for operation in extremely harsh or noisy environments.
PC Board Layout Considerations
The MAX6902 uses a very-low-current oscillator to minimize supply current. This causes the oscillator pins, X1
and X2, to be relatively high impedance. Exercise care
to prevent unwanted noise pickup.
Connect the 32.768kHz crystal directly across X1 and X2
of the MAX6902. To eliminate unwanted noise pickup,
design the PC board using these guidelines (Figure 7):
1) Place the crystal as close to X1 and X2 as possible
and keep the trace lengths short.
2) Place a guard ring around the crystal, X1 and X2
traces (where applicable), and connect the guard
ring to GND; keep all signal traces away from
beneath the crystal, X1, and X2.
3) Finally, an additional local ground plane can be
added under the crystal on an adjacent PC board
layer. The plane should be isolated from the regular
PC board ground plane, and tied to ground at the
MAX6902 ground pin.
4) Restrict the plane to be no larger than the perimeter of
the guard ring. Do not allow this ground plane to contribute significant capacitance between X1 and X2.
Chip Information
TRANSISTOR COUNT: 26,418
PROCESS: CMOS
Power-On Reset
The MAX6902 contains an integral POR circuit that
ensures all registers are reset to a known state on
power-up. Once VCC rises above 1.6V (typ), the POR
circuit releases the registers for normal operation. When
VCC drops to less than 1.6V (typ), the MAX6902 resets
all register contents to the POR defaults (Figure 2).
______________________________________________________________________________________
15
MAX6902
Timekeeping Current
When DOUT is high impedance (CS = high or during a
DIN transfer segment), there is a potential for increased
timekeeping current (up to 100x) if DOUT is allowed to
float. If minimum timekeeping current is desired, then
ensure DOUT is not allowed to float. The microcontroller port pin attached to DOUT could be configured
as an input with a weak pullup. An alternate solution is
to use a 100kΩ, or less, pulldown or pullup resistor (for
microcontroller port pins with ≤1µA input leakage).
;;;;
;;
;;;;;;
;;
;;
;;;;
MAX6902
SPI-Compatible Real-Time Clock in SOT23
GROUND PLANE
VIA CONNECTION
*
VCC PLANE
VIA CONNECTION
GUARD RING
*
0.1µF
SM CAP
GROUND PLANE
VIA CONNECTION
*
**
**
*
MAX6902
*
*
*
*
*
**
2 LOCAL GROUND PLANE
** LAYER
CONNECT ONLY TO PIN 4
SM WATCH CRYSTAL
GROUND PLANE VIA
*
**
*
GROUND PLANE
VIA CONNECTION
*LAYER 1 TRACE
Figure 7. MAX6902 Crystal PC Board Layout
16
______________________________________________________________________________________
SPI-Compatible Real-Time Clock in SOT23
X1
X2
OSCILLATOR
32.768kHz
1Hz
DIVIDER
SECONDS
MINUTES
HOURS
DATE
VCC
GND
CONTROL
LOGIC
MONTH
DAY
SCLK
DIN
DOUT
CS
YEAR
INPUT SHIFT
REGISTERS
CONTROL
ADDRESS
REGISTER
CENTURY
ALARM
CONFIG
31x 8
RAM
RESERVED
ALARM
THRESHOLDS
CLOCK
BURST
RAM
BURST
ALARM OUT
ALARM
CONTROL
LOGIC
______________________________________________________________________________________
17
MAX6902
Functional Diagram
SPI-Compatible Real-Time Clock in SOT23
SOT23, 8L.EPS
MAX6902
Package Information
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
18 ____________________Maxim
Maxim
Integrated Products, 120
Integrated
San Gabriel
Products,
Drive,120
Sunnyvale,
San Gabriel
CA Drive,
94086Sunnyvale,
408-737-7600
CA ____________________
94086 408-737-7600
18
© 2001 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.