AN154: Xicor Real Time Clock Family

Intersil Real Time Clock Family User’s Guide
®
Application Note
New Devices Integrate Crystal
Compensation Circuitry
Overall Functionality
Intersil Real Time Clock (RTC) products integrate the real
time clock function with microprocessor supervisory
functions and nonvolatile memory to form a vital system
element. Applications such as utility meters, security and
surveillance systems, entertainment systems and handheld
data loggers are just a few systems that require these
functions to operate reliably and accurately. This
functionality is provided in 8 or 14-pin surface mount
products, which along with a 32.768kHz crystal and a small
backup battery provide the entire RTC function.
Intersil has now integrated the oscillator compensation
circuitry on-chip, to adjust for crystal drift over temperature
and enable very high accuracy (<5ppm drift) and eliminating
the need for external components. Applications will be
discussed here for implementing this compensation, and an
evaluation board with software is available from Intersil
which demonstrates the functionality. The entire family of
devices is summarized in Table 1 below, indicating the
features available in each device.
Summary of Device Functions
Real Time Clock Function
The RTC function includes a clock/calendar and two alarms,
which use a set of registers for control, status and
programming. These registers provide seconds, minutes,
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hours, day of the week, month, and year, with automatic
correction for leap years. The X1286 and X1288 have
1/100th second resolution for precision applications. The
clock format is selectable for either AM/PM or 24 hour
(military) format. On power up, the clock will not function until
at least one byte is written to the clock register.
Alarm Registers
The Alarm function enables the system to generate an alarm
once every minute, hour, day, week, month or year. There
are two alarm registers and they are set up essentially the
same as the clock/calendar registers. Once an alarm
register matches the clock/calendar setting, an alarm flag is
set in the main status register for software interrupts. Also,
some RTC devices in the family have an IRQ- status pin for
a hardware flag. Note that an alarm flag is reset once the
status register contents are read.
CPU Supervisory Functions
Some devices have a RESET- pin which is intended to
provide a hardware reset to a microcontroller. The RESETpin is asserted low either when the Vcc supply voltage has
dropped below a certain threshold, or when the watchdog
timer period has expired. The devices are provided with a
choice of Vcc trip voltage threshold for 3.3V or 5V systems,
or can be user programmable. The watchdog timeout period
is programmable via the control registers and can be set to
0.25, 0.75, 1.75 seconds, or disabled. This pin is always an
open drain output and requires a pullup resistor of from 5K to
50kΩ.
TABLE 1. INTERSIL RTC PRODUCT FAMILY
WATCHDOG
TIMER
(.25, .75, 1.75s)
CLOCK
FREQUENCY
OUTPUT
ON-CHIP
OSCILLATOR
COMPENSATION
EEPROM
PACKAGES
PRODUCT
2 ALARMS
POWER-ON
RESET
(250ms)
X1205
Yes
-
-
Yes
Yes
0
8-TSSOP, SO
X1226
Yes
-
-
Yes
Yes
4k
8-TSSOP, SO
X1227
Yes
Yes
Yes
-
Yes
4k
8-TSSOP, SO
X1228
Yes
Yes
Yes
Yes
Yes
4k
14-TSSOP, SO
X1286
Yes
-
-
Yes
Yes
256k
14-TSSOP
X1288
Yes
Yes
Yes
Yes
Yes
256k
14-TSSOP
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 154
Frequency Output
Battery Backup Switchover Circuit
This is the PHZ output noted on the data sheets and shares
pin functionality with the IRQ- function. When the PHZ
function is enabled, the IRQ- function is disabled. Two bits in
the control registers (FO0 and FO1) select the functionality
for this pin as shown in Table 2 below:
There are two power supply pins for each RTC device, Vcc
and Vback. The Vcc pin is for the main board power supply
of 5V or 3.3V. The Vback pin is for a dedicated backup
supply only for the RTC chip. The pin can be tied to a
battery, a supercap, or tied to ground if not used. The RTC
devices contain internal circuitry to automatically switch over
to the backup battery when the main Vcc supply fails, and
switch back from battery to Vcc when the main supply
recovers (See Figure 1). This circuit contains assymetrical
hysteresis to address noise and glitch issues in Vcc lines.
There is approximately 150mV of hysteresis in the voltage
comparator when switching from Vcc to Vback, and 50mV of
when switching from Vback to Vcc.
TABLE 2. PHZ OUTPUT CONTROL
OUTPUT FREQUENCY
FO1
FO0
X1226, X1228
X1286, X1288
0
0
IRQ- Output
IRQ- Output
0
1
32.768kHz
32.768kHz
1
0
4096Hz
100Hz
1
1
1Hz
1Hz
The PHZ function can be used for clocking other devices in
the system, or as an accurate counter for miscellaneous
timing functions. It is also very useful for calibrating the
oscillator frequency as described in the oscillator section. In
some devices in the Intersil RTC family, the PHZ/IRQ- output
has a CMOS output. This output is driven even in battery
backup operation, so it is necessary to note the external
hardware connections to this pin to prevent excessive
current drain to the battery.
Since Lithium batteries are often used for battery backup,
knowledge of the backup circuitry is required for UL
approval. Figure 3 shows the internal switchover circuitry
illustrating the complementary control which disables one
input while enabling the other. Leakage from Vcc to Vback is
negligible (<100nA).
Transistor 1
Internal
VCC
VCC
Serial Data Interface
This interface consists of clock and data (SCL and SDA)
pins and have functionality similar to those in an I2C
interface. Start and stop conditions are used along with
acknowledge on address and data transfers. The devices
can be used with clock frequencies up to 400kHz, although
they go into a low current standby state if the SDA and SCL
are disabled (high). The SDA output is open drain and each
of the serial bus lines needs a pullup resistor somewhere on
the board for proper operation. It is highly recommended that
the serial interface pullup resistors are tied to Vcc and that
Vcc needs to go to 0V when powered down to avoid
excessive battery current drain.
Non-Volatile Memory
Either 4Kor 256K of EEPROM memory is available for
system use on these devices. The memory is useful in utility
meter applications for rate-schedule tracking and recording
readings as well as for general microcontroller memory. The
memory is addressed separately from the RTC control and
status registers, so there are separate slave address bytes
for each, as listed below.
TABLE 3. SLAVE ADDRESSES
SERIAL BUS SLAVE ADDRESS BYTE
NV Memory =
1
0
1
0
1
1
1
R/W-
Clock Control =
1
1
0
1
1
1
1
R/W-
2
VBACK
Transistor 2
FIGURE 1. BATTERY-BACKUP SWITCHOVER CIRCUIT
If this circuitry is not sufficient to meet the safety
requirements for battery leakage in an application, it is
suggested that a small schottkey barrier diode (like 1N5811
or ZC2811) be placed in series with Vback which minimizes
reverse current into the backup battery.
If the battery input (Vback) is not used, it should be tied to
ground, not to Vcc.
Operational Features
Crystal Oscillator
The Intersil RTC family uses an oscillator circuit with on-chip
crystal compensation network, including adjustable loadcapacitance. The only external component required is the
crystal. The compensation network is optimized for operation
with certain crystal parameters which are common in many
of the surface mount or tuning-fork crystals available today.
Table 4 summarizes these parameters.
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Application Note 154
TABLE 4. CRYSTAL PARAMETERS REQUIRED FOR INTERSIL RTC’S
PARAMETER
MIN
Frequency
TYP
MAX
UNITS
32.768
Freq. Tolerance
Turnover Temperature
20
Operating Temperature Range
-40
kHz
±100
ppm
30
deg C
85
deg C
25
Parallel Load Capacitance
NOTES
12.5
Equivalent Series Resistance
Down to 20ppm if desired
Typically the value used for most crystals
pF
50
kΩ
For best oscillator performance
TABLE 5. CRYSTAL MANUFACTURERS
PART NUMBER
TEMP RANGE
+25 DEG C FREQ TOLER.
Citizen
CM201, CM202, CM200S
-40 to +85 deg C
+/-20ppm
Epson
MC-405, MC-406
-40 to +85 deg C
+/-20ppm
Raltron
RSM-200S-A or B
-40 to +85 deg C
+/-20ppm
SaRonix
32S12A or B
-40 to +85 deg C
+/-20ppm
Ecliptek
ECPSM29T-32.768K
-10 to +60 deg C
+/-20ppm
ECS
ECX-306/ECX-306I
-10 to +60 deg C
+/-20ppm
Fox
FSM-327
-40 to +85 deg C
+/-20ppm
Table 5 contains some crystal manufacturers and part numbers
that meet the requirements for the Intersil RTC products.
The turnover temperature in Table 4 describes the
temperature where the apex of the of the drift vs. temperature
curve occurs. This curve is parabolic with the drift increasing
as (T-T0)2(see figure 2). For an Epson MC-405 device, for
example, the turnover temperature is typically 25 deg C, and a
peak drift of >110ppm occurs at the temperature extremes of
-40 and +85 deg C. It is possible to address this variable drift
by adjusting the load capacitance of the crystal, which will
result in predictable change to the crystal frequency. The
Intersilintersil RTC family allows this adjustment over
temperature since the devices include on-chip load capacitor
trimming. This control is handled by the Analog Trimming
Register, or ATR, which has 6 bits of control . The load
capacitance range covered by the ATR circuit is
approximately 3.25pF to 18.75pF, in 0.25pf increments. Note
that actual capacitance would also include about 2pF of
package related capacitance. In-circuit tests with
commercially available crystals demonstrate that this range of
capacitance allows frequency control from +116ppm to 37ppm, using a 12.5pF load crystal.
In addition to the analog compensation afforded by the
adjustable load capacitance, a digital compensation feature is
available for the Intersil RTC family. There are three bits known
as the Digital Trimming Register or DTR, and they operate by
adding or skipping pulses in the clock signal. The range
provided is ±30ppm in increments of 10ppm. The default setting
is 0ppm. The DTR control can be used for coarse adjustments
of frequency drift over temperature or for crystal initial accuracy
correction.
3
A final application for the ATR control is in-circuit
calibration for high accuracy applications, along with a
temperature sensor chip. Once the RTC circuit is powered up
with battery backup, the PHZ output is set at 32.768kHz and
frequency drift is measured. The ATR control is then adjusted to
a setting which minimizes drift. Once adjusted at a particular
temperature, it is possible to adjust at other discrete
temperatures for minimal overall drift, and store the resulting
settings in the EEPROM. Extremely low overall temperature
drift is possible with this method. The Intersil evaluation board
contains the circuitry necessary to implement this control.
0
-20
Delta Frequency (PPM)
MANUFACTURER
-40
-60
-80
-100
-120
-140
-160
-180
-40 -30 -20 -10
0
10
20
30
40
50
60
70
80
Temperature (°C)
FIGURE 2. CRYSTAL FREQUENCY DEVIATION vs
TEMPERATURE
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Application Note 154
Layout Considerations
The crystal input at X1 has a very high impedance and will
pick up high frequency signals from other circuits on the
board. Since the X2 pin is tied to the other side of the crystal,
it is also a sensitive node. These signals can couple into the
oscillator circuit and produce double clocking or misclocking, seriously affecting the accuracy of the RTC. Care
needs to be taken in layout of the RTC circuit to avoid noise
pickup. Below in Figure 3 is a suggested layout for the
X1226 or X1227 devices.
FIGURE 3. SUGGESTED LAYOUT FOR INTERSIL RTC IN SO-8
The X1 and X2 connections to the crystal are to be kept as
short as possible. A thick ground trace around the crystal is
advised to minimize noise intrusion, but ground near the X1
and X2 pins should be avoided as it will add to the load
capacitance at those pins. Keep in mind these guidelines for
other PCB layers in the vicinity of the RTC device. A small
decoupling capacitor at the Vcc pin of the chip is mandatory,
with a solid connection to ground.
The X1226 product has a special consideration. The
PHZ/IRQ- pin on the 8-lead SOIC package is located next to
the X2 pin. When this pin is used as a frequency output
(PHZ) and is set to 32.768kHz output frequency, noise can
couple to the X1 or X2 pins and cause double-clocking. The
layout in figure 1 can help minimize this by running the PHZ
output away from the X1 and X2 pins. Also, minimizing the
switching current at this pin by careful selection of the pullup
resistor value will reduce noise. Intersil suggests a minimum
value of 5.1K for 32.768kHz, and higher values for lower
frequency PHZ outputs.
For other RTC products, the same rules stated above should
be observed, but adjusted slightly since the packages and
pinouts are slightly different.
Assembly
inserted in final assembly, then there are no issues with
operation of the RTC. If the battery is soldered to the board
directly, then the RTC device Vback pin will see some
transient upset from either soldering tools or intermittent
battery connections which can stop the circuit from
oscillating. Once the battery is soldered to the board, the
only way to assure the circuit will start up is to momentarily
(very short period of time!) short the Vback pin to ground and
the circuit will begin to oscillate.
Oscillator Measurements
When a proper crystal is selected and the layout guidelines
above are observed, the oscillator should start up in most
circuits in less than one second. Some circuits may take
slightly longer, but startup should definitely occur in less than
5 seconds. When testing RTC circuits, the most common
impulse is to apply a scope probe to the circuit at the X2 pin
(oscillator output) and observe the waveform. DO NOT DO
THIS! Although in some cases you may see a useable
waveform, due to the parasitics (usually 10pF to ground)
applied with the scope probe, there will be no useful
information in that waveform other than the fact that the
circuit is oscillating. The X2 output is sensitive to capacitive
impedance so the voltage levels and the frequency will be
affected by the parasitic elements in the scope probe.
Applying a scope probe can possibly cause a faulty oscillator
to start up, hiding other issues (although in the Intersil
RTC’s, the internal circuitry assures startup when using the
proper crystal and layout).
The best way to analyze the RTC circuit is to power it up and
read the real time clock as time advances, or if the chip has
the PHZ output, look at the output of that pin on an
oscilloscope (after enabling it with the control register, and
using a pullup resistor for an open-drain output).
Alternaltively, the X1226/1286 devices have an IRQ- output
which can be checked by setting an alarm for each minute.
Using the pulse interrupt mode setting, the once-per-minute
interrupt functions as an indication of proper oscillation.
Backup Battery Operation
Many types of batteries can be used with the Intersil RTC
products. 3.0V or 3.6V Lithium batteries are appropriate, and
sizes are available that can power a Intersil RTC device for
up to 10 years. Another option is to use a supercapacitor for
applications where Vcc may disappear intermittently for
short periods of time. Depending on the value of
supercapacitor used, backup time can last from a few days
to two weeks (with >1F). A simple silicon or Schottky barrier
diode can be used in series with Vcc to charge the
supercapacitor, which is connected to the Vback pin. Do not
use the diode to charge a battery (especially lithium
batteries!).
Most electronic circuits do not have to deal with assembly
issues, but with the RTC devices assembly includes
insertion or soldering of a live battery into an unpowered
circuit. If a socket is soldered to the board, and a battery is
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Application Note 154
2.7-5.5V
VCC
timekeeping circuitry. The fact that the chip can be powered
from Vback is not necessarily an issue since standby current for
the RTC devices is <2µA for this mode (called “main
timekeeping current” in the data sheet). Only when the serial
interface is active is there an increase in supply current, and
with Vcc powered down, the serial interface will most likely be
inactive.
Vback
Supercapacitor
VSS
One way to prevent operation in battery backup mode above
the Vtrip level is to add a diode drop (silicon diode preferred) to
the battery to insure it is below Vtrip. This will also provide
reverse leakage protection which may be needed to get safety
agency approval.
FIGURE 4. SUPERCAPACITOR CHARGING CIRCUIT
Since the battery switchover occurs at Vcc = Vback-0.1V
(See figure 1), the battery voltage must always be lower than
the Vcc voltage during normal operation or the battery will be
drained. A second consideration is the trip point setting for
the system RESET- function, known as Vtrip. Vtrip is set at
the factory at levels for systems with either Vcc = 5V or 3.3V
operation, with the following standard options (except for the
X1226 which has no RESET- function):
One mode that should always be avoided is the operation of
the RTC device with Vback greater than both Vcc and Vtrip
(Condition 2d in Table 5). This will cause the battery to drain
quickly as serial bus communication and non-volatile writes will
require higher supplier current.
VTRIP = 4.63V ± 3%
VTRIP = 4.38V ± 3%
VTRIP = 2.85V ± 3%
VTRIP = 2.65V ± 3%
Summary
The Intersil RTC product family integrates the clock/calendar
function, alarms, battery backup circuit,
precision crystal compensation, CPU supervisor and EEPROM
into a single device. The device also draws very low battery
current insuring long life in remote applications. This functional
integration is crucial to applications where clock accuracy, nonvolatile storage and long field life are needed, such as utility
meters, security surveillance systems and network equipment.
The small packages offered along with the low parts count also
make the devices ideal for handheld applications.
The summary of conditions for backup battery operation is
given in Table 6.
Referring to example 1 in Table 6, Vtrip applies to the “Internal
Vcc” node which powers the entire device. This means that if
Vcc is powered down and the battery voltage at Vback is higher
than the Vtrip voltage, then the entire chip will be running from
the battery. If Vback falls to lower than Vtrip, then the chip shuts
down and all outputs are disabled except for the oscillator and
TABLE 6. BATTERY BACKUP OPERATION
1. EXAMPLE APPLICATION, VCC=5V, VBACK=3.0V
CONDITION
VCC
VBACK
VTRIP
IBACK
RESET
NOTES
a. Normal Operation
5.00
3.00
4.38
<<1µA
H
b. Vcc on with no battery
5.00
0
4.38
0
H
c. Backup Mode
0-1.8
1.8-3.0
4.38
<2µA
L
VCC
VBACK
VTRIP
IBACK
RESET
a. Normal Operation
3.30
3.00
2.65
<<1µA
H
b. Vcc on with no battery
3.30
0
2.65
0
H
c. Backup Mode
0-1.8
1.8-3.0*
2.65
<2µA*
L
Timekeeping only
2.65 - 3.30
> Vcc
2.65
up to 3mA
H
Internal Vcc=Vback
Timekeeping only
2. EXAMPLE APPLICATION, VCC=3.3V,VBACK=3.0V
CONDITION
d. UNWANTED - Vcc ON, Vback powering
* since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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