UT9Q512K32E - Aeroflex Microelectronic Solutions

Standard Products
UT9Q512K32E 16 Megabit RadTolerant SRAM MCM
Data Sheet
June 28, 2011
INTRODUCTION
The UT9Q512K32E RadTol product is a high-performance 2M
byte (16Mbit) CMOS static RAM multi-chip module (MCM),
organized as four individual 524,288 x 8 bit SRAMs with a
common output enable. Memory expansion is provided by an
active LOW chip enable (En), an active LOW output enable (G),
and three-state drivers. This device has a power-down feature
that reduces power consumption by more than 90% when
deselected.
FEATURES
 25ns maximum (5 volt supply) address access time
 Asynchronous operation for compatible with industry
standard 512K x 8 SRAMs
 TTL compatible inputs and output levels, three-state
bidirectional data bus
 Operational environment:
- Total dose: 50 krads(Si)
- SEL Immune >110 MeV-cm2/mg
- LETTH(0.25) = >52 MeV-cm2/mg
- Saturated Cross Section (cm2) per bit, 2.8E-8
- <1.1E-9 errors/bit-day, Adams 90% geosynchronous
heavy ion
Writing to each memory is accomplished by taking chip enable
(En) input LOW and write enable (Wn) inputs LOW. Data on
the eight I/O pins (DQ0 through DQ7) is then written into the
location specified on the address pins (A0 through A18). Reading
from the device is accomplished by taking chip enable (En) and
output enable (G) LOW while forcing write enable (Wn) HIGH.
Under these conditions, the contents of the memory location
specified by the address pins will appear on the I/O pins.
 Packaging:
- 68-lead dual cavity ceramic quad flatpack (CQFP)
(11.0 grams)
 Standard Microcircuit Drawing 5962-01511
- QML Q and Vcompliant part
E3
W3
The input/output pins are placed in a high impedance state when
the device is deselected (En HIGH), the outputs are disabled (G
HIGH), or during a write operation (En LOW and Wn LOW).
Perform 8, 16, 24 or 32 bit accesses by making Wn along with
En a common input to any combination of the discrete memory
die.
E2
W2
E1
W1
W0
E0
A(18:0)
G
512K x 8
512K x 8
DQ(31:24)
or
DQ3(7:0)
DQ(23:16)
or
DQ2(7:0)
512K x 8
DQ(15:8)
or
DQ1(7:0)
Figure 1. UT9Q512K32E SRAM Block Diagram
1
512K x 8
DQ(7:0)
or
DQ0(7:0)
DEVICE OPERATION
NC
A0
A1
A2
A3
A4
A5
E2
VSS
E3
W0
A6
A7
A8
A9
A10
VDD
The UT9Q512K32E has three control inputs called Enable 1
(En), Write Enable (Wn), and Output Enable (G); 19 address
inputs, A(18:0); and eight bidirectional data lines, DQ(7:0). En
Device Enable controls device selection, active, and standby
modes. Asserting En enables the device, causes IDD to rise to its
active value, and decodes the 19 address inputs to select one of
524,288 words in the memory. Wn controls read and write
operations. During a read cycle, G must be asserted to enable
the outputs.
68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52
1
2
3
4
5
Top View
6
7
8
9
10
11
12
13
14
15
16
17
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
51
50
49
48
47
46
45
44
43
42
41
40
39
38
37
36
35
DQ0(2)
DQ1(2)
DQ2(2)
DQ3(2)
DQ4(2)
DQ5(2)
DQ6(2)
DQ7(2)
VSS
DQ0(3)
DQ1(3)
DQ2(3)
DQ3(3)
DQ4(3)
DQ5(3)
DQ6(3)
DQ7(3)
Table 1. Device Operation Truth Table
VDD
A11
A12
A13
A14
A15
A16
E0
G
E1
A17
W1
W2
W3
A18
NC
NC
DQ0(0)
DQ1(0)
DQ2(0)
DQ3(0)
DQ4(0)
DQ5(0)
DQ6(0)
DQ7(0)
VSS
DQ0(1)
DQ1(1)
DQ2(1)
DQ3(1)
DQ4(1)
DQ5(1)
DQ6(1)
DQ7(1)
G
Wn
En
I/O Mode
Mode
X1
X
1
3-state
Standby
X
0
0
Data in
Write
1
1
0
3-state
Read2
0
1
0
Data out
Read
Figure 2. 25ns SRAM Pinout (68)
Notes:
1. “X” is defined as a “don’t care” condition.
2. Device active; outputs disabled.
PIN NAMES
READ CYCLE
A(18:0)
DQn(7:0)
En
Address
Data Input/Output
Enable
Wn
Write Enable
G
Output Enable
VDD
Power
VSS
Ground
A combination of Wn greater than VIH (min) and En less than
VIL (max) defines a read cycle. Read access time is measured
from the latter of Device Enable, Output Enable, or valid address
to valid data output.
SRAM Read Cycle 1, the Address Access in figure 3a, is
initiated by a change in address inputs while the chip is enabled
with G asserted and Wn deasserted. Valid data appears on data
outputs DQ(7:0) after the specified tAVQV is satisfied. Outputs
remain active throughout the entire cycle. As long as Device
Enable and Output Enable are active, the address inputs may
change at a rate equal to the minimum read cycle time (tAVAV).
SRAM read Cycle 2, the Chip Enable - Controlled Access in
figure 3b, is initiated by En going active while G remains
asserted, Wn remains deasserted, and the addresses remain
stable for the entire cycle. After the specified tETQV is satisfied,
the eight-bit word addressed by A(18:0) is accessed and appears
at the data outputs DQ(7:0).
SRAM read Cycle 3, the Output Enable - Controlled Access in
figure 3c, is initiated by G going active while En is asserted, Wn
is deasserted, and the addresses are stable. Read access time is
tGLQV unless tAVQV or tETQV have not been satisfied.
2
WRITE CYCLE
OPERATIONAL ENVIRONMENT
A combination of Wn less than VIL(max) and En less than
VIL(max) defines a write cycle. The state of G is a “don’t care”
for a write cycle. The outputs are placed in the high-impedance
state when either G is greater than VIH(min), or when Wn is less
than VIL(max).
The UT9Q512K32E SRAM incorporates features which allows
operation in a limited environment.
Table 2. Operational Environment
Design Specifications1
Write Cycle 1, the Write Enable-controlled Access is defined by
a write terminated by Wn going high, with En still active. The
write pulse width is defined by tWLWH when the write is initiated
by Wn, and by tETWH when the write is initiated by En. Unless
the outputs have been previously placed in the high-impedance
state by G, the user must wait tWLQZ before applying data to the
nine bidirectional pins DQ(7:0) to avoid bus contention.
Total Dose
50
krad(Si)
Heavy Ion
Error Rate2
<1.1E-9
Errors/Bit-Day
Notes:
1. The SRAM will not latchup during radiation exposure under recommended
operating conditions.
2. 90% worst case particle environment, Geosynchronous orbit, 100 mils of
Aluminum.
Write Cycle 2, the Chip Enable-controlled Access is defined by
a write terminated by the latter of En going inactive. The write
pulse width is defined by tWLEF when the write is initiated by
Wn, and by tETEF when the write is initiated by the En going
active. For the Wn initiated write, unless the outputs have been
previously placed in the high-impedance state by G, the user
must wait tWLQZ before applying data to the eight bidirectional
pins DQ(7:0) to avoid bus contention.
3
ABSOLUTE MAXIMUM RATINGS1
(Referenced to VSS)
SYMBOL
PARAMETER
LIMITS
VDD
DC supply voltage
-0.5 to 7.0V
VI/O
Voltage on any pin
-0.5 to 7.0V
TSTG
Storage temperature
-65 to +150C
PD
Maximum power dissipation
TJ
Maximum junction temperature2
+150C
Thermal resistance, junction-to-case3
10C/W
DC input current
±10 mA
JC
II
1.0W (per byte)
Notes:
1. Stresses outside the listed absolute maximum ratings may cause permanent damage to the device. This is a stress rating only, and functional operation of the device
at these or any other conditions beyond limits indicated in the operational sections of this specification is not recommended. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability and performance.
2. Maximum junction temperature may be increased to +175C during burn-in and steady-static life.
3. Test per MIL-STD-883, Method 1012.
RECOMMENDED OPERATING CONDITIONS
SYMBOL
PARAMETER
LIMITS
VDD
Positive supply voltage
4.5 to 5.5V
TC
Case temperature range
(W) Screen - 40C to 105C
VIN
DC input voltage
0V to VDD
4
DC ELECTRICAL CHARACTERISTICS (Pre/Post-Radiation)*
-40C to +105C (VDD = 5.0V + 10% for (W) screening)
SYMBOL
PARAMETER
CONDITION
MIN
MAX
2.0
UNIT
VIH
High-level input voltage
(TTL)
V
VIL
Low-level input voltage
(TTL)
0.8
V
VOL1
Low-level output voltage
IOL = 8mA, VDD =4.5V (TTL)
0.4
V
VOL2
Low-level output voltage
IOL = 200A,VDD =4.5V (CMOS)
0.08
V
VOH1
High-level output voltage
IOH = -4mA,VDD =4.5V (TTL)
2.4
V
VOH2
High-level output voltage
IOH = 200A,VDD =4.5V (CMOS)
3.0
V
CIN1
Input capacitance
 = 1MHz @ 0V
45
pF
CIO1
Bidirectional I/O capacitance
 = 1MHz @ 0V
25
pF
IIN
Input leakage current
VIN = VDD and VSS, VDD = VDD (max)
-2
2
A
IOZ
Three-state output leakage current
VO = VDD and VSS
-2
2
A
-90
90
mA
VDD = VDD (max)
G = VDD (max)
IOS2, 3
Short-circuit output current
VDD = VDD (max), VO = VDD
VDD = VDD (max), VO = 0V
IDD(OP)
Supply current operating
@ 1MHz
(per byte)
Inputs: VIL = 0.8V,
VIH = 2.0V
IOUT = 0mA
VDD = VDD (max)
40
mA
IDD1(OP)
Supply current operating
@40MHz
(per byte)
Inputs: VIL = 0.8V,
70
mA
Supply current standby
@0MHz
(per byte)
-40C and
Inputs: VIL = VSS
25C
IOUT = 0mA
E1 = VDD - 0.5, VDD = VDD (max)
105C
VIH = VDD - 0.5V
9
mA
24
mA
IDD2(SB)
VIH = 2.0V
IOUT = 0mA
VDD = VDD (max)
Notes:
* Post-radiation performance guaranteed at 25C per MIL-STD-883 Method 1019.
1. Measured only for initial qualification and after process or design changes that could affect input/output capacitance.
2. Supplied as a design limit but not guaranteed or tested.
3. Not more than one output may be shorted at a time for maximum duration of one second.
5
AC CHARACTERISTICS READ CYCLE (Pre/Post-Radiation)*
-40C to +105C (VDD = 5.0V + 10% for (W) screening)
SYMBOL
PARAMETER
MAX
MIN
UNIT
tAVAV1
Read cycle time
tAVQV
Read access time
tAXQX2
Output hold time
3
ns
tGLQX2
G-controlled Output Enable time
0
ns
tGLQV
G-controlled Output Enable time (Read Cycle 3)
10
ns
tGHQZ2
G-controlled output three-state time
10
ns
tETQX2,3
En-controlled Output Enable time
tETQV3
tEFQZ1,2,4
25
ns
25
3
ns
ns
En-controlled access time
25
ns
En-controlled output three-state time
10
ns
Notes: * Post-radiation performance guaranteed at 25C per MIL-STD-883 Method 1019.
1. Functional test.
2. Three-state is defined as a 500mV change from steady-state output voltage.
3. The ET (enable true) notation refers to the falling edge of En. SEU immunity does not affect the read parameters.
4. The EF (enable false) notation refers to the rising edge of En. SEU immunity does not affect the read parameters.
High Z to Active Levels
Active to High Z Levels
VH - 500mV
VLOAD + 500mV
}
VLOAD
{
{
}
VLOAD - 500mV
VL + 500mV
Figure 3. 5-Volt SRAM Loading
6
tAVAV
A(18:0)
DQn(7:0)
Previous Valid Data
Valid Data
tAVQV
tAXQX
Assumptions:
1. En and G < VIL (max) and Wn > VIH (min)
Figure 4a. SRAM Read Cycle 1: Address Access
A(18:0)
En
tETQV
tETQX
tEFQZ
DQn(7:0)
DATA VALID
Assumptions:
1. G < VIL (max) and Wn > VIH (min)
Figure 4b. SRAM Read Cycle 2: Chip Enable-Controlled Access
tAVQV
A(18:0)
tGHQZ
tGLQX
DATA VALID
Qn(7:0)
tGLQV
ssumptions:
En < VIL (max) and Wn > VIH (min)
Figure 4c. SRAM Read Cycle 3: Output Enable-Controlled Access
7
AC CHARACTERISTICS WRITE CYCLE (Pre/Post-Radiation)*
-40C to +105C (VDD = 5.0V + 10% for (W) screening)
SYMBOL
PARAMETER
MIN
MAX
UNIT
tAVAV1
Write cycle time
25
ns
tETWH
Device Enable to end of write
20
ns
tAVET
Address setup time for write (En - controlled)
1
ns
tAVWL
Address setup time for write (Wn - controlled)
0
ns
tWLWH
Write pulse width
20
ns
tWHAX
Address hold time for write (Wn - controlled)
0
ns
tEFAX
Address hold time for Device Enable (En - controlled)
0
ns
tWLQZ2
Wn - controlled three-state time
10
tWHQX2
Wn - controlled Output Enable time
5
ns
tETEF
Device Enable pulse width (En - controlled)
20
ns
tDVWH
Data setup time
15
ns
tWHDX
Data hold time
2
ns
tWLEF
Device Enable controlled write pulse width
20
ns
tDVEF
Data setup time
15
ns
tEFDX
Data hold time
2
ns
tAVWH
Address valid to end of write
20
ns
tWHWL1
Write disable time
5
ns
Notes:
* Post-radiation performance guaranteed at 25C per MIL-STD-883 Method 1019.
1. Functional test performed with outputs disabled (G high).
2. Three-state is defined as 500mV change from steady-state output voltage.
8
ns
A(18:0)
tAVAV2
En
tAVWH
tETWH
tWHWL
Wn
tWLWH
tAVWL
tWHAX
Qn(7:0)
tWLQZ
Dn(7:0)
tWHQX
APPLIED DATA
Assumptions:
1. G < VIL (max). If G > VIH (min) then Qn(7:0) will be
in three-state for the entire cycle.
2. G high for tAVAV cycle.
tDVWH
tWHDX
Figure 5a. SRAM Write Cycle 1: Write Enable - Controlled Access
9
tAVAV3
A(18:0)
tETEF
tAVET
tEFAX
En
or
tAVET
En
tETEF
tEFAX
tWLEF
Wn
Dn(7:0)
APPLIED DATA
tWLQZ
tDVEF
Qn(7:0)
tEFDX
Assumptions & Notes:
1. G < VIL (max). If G > VIH (min) then Qn(7:0) will be in three-state for the entire cycle.
2. Either En scenario above can occur.
3. G high for tAVAV cycle.
Figure 5b. SRAM Write Cycle 2: Chip Enable - Controlled Access
CMOS
90%
VDD-0.05V
300 ohms
10%
0.5V
VLOAD = 1.55V
10%
< 5ns
50pF
< 5ns
Input Pulses
Notes:
1. 50pF including scope probe and test socket capacitance.
2. Measurement of data output occurs at the low to high or high to low transition mid-point
(i.e., CMOS input = VDD/2).
Figure 6. AC Test Loads and Input Waveforms
10
DATA RETENTION MODE
VDR > 2.5V
VDD
4.5V
4.5V
tR
tEFR
EN
VDD = VDR
Figure 7. Low VDD Data Retention Waveform
DATA RETENTION CHARACTERISTICS (Pre-Radiation) *(VDD2 = VDD2 (min), 1 Sec DR
Pulse)
SYMBOL
PARAMETER
TEMP
MINIMUM
VDR
IDDR 1
tEFR1
tR1
VDD1 for data retention
MAXIMUM
UNIT
--
2.5
--
V
-40oC
--
9
mA
25oC
--
9
mA
105oC
--
24
mA
Chip deselect to data retention time
--
0
--
ns
Operation recovery time
--
tAVAV
--
ns
Data retention current
(per byte)
Notes:
*Post-radiation performance guaranteed at 25oC per MILSTD-883 Method 1019.
1. E n= VDD all other inputs = VDD or VSS
11
PACKAGING
Figure 8. 68-Lead Ceramic Quad Flatpack
12
ORDERING INFORMATION
512K32 16Megabit SRAM MCM:
UT9Q512K32E -* * *
*
Lead Finish: (Note 1)
(C) = Gold
Screening: (Notes 2 & 3)
(P) = Prototype flow
(W) = -40oC to +105oC
Package Type:
(S) = 68-lead dual cavity CQFP
Device Type:
- = 25ns access time, 5.0V operation
Aeroflex UTMC Core Part Number
Notes:
1. Gold lead finish only.
2. Prototype flow per Aeroflex Colorado Springs Manufacturing Flows Document. Devices are tested at 25oC. Radiation neither tested nor guaranteed.
3. Extended Industrial Temperature Range flow per Aeroflex Colorado Springs Manufacturing Flows Document. Devices are tested at -40C to +105C.
Radiation neither tested nor guaranteed.
13
512K32E 16Megabit SRAM MCM: SMD
5962 - 01511 **
* * *
Lead Finish: (Note 1)
(C) = Gold
Case Outline:
(Y) = 68-lead dual cavity CQFP
Class Designator:
(Q) = QML Class Q
(V) = QML Class V
Device Type
02 = 25 ns access time, 5.0V operation, (-40oC to +105oC)
Drawing Number: 01511
Total Dose: (Note 2)
(D) = 1E4 (10krad(Si))
(P) = 3E4 (30krad(Si))
(L) = 5E4 (50krad(Si))
Federal Stock Class Designator: No Options
Notes:
1. Lead finish is "C" (Gold) only..
2. Total dose radiation must be specified when ordering.
14
Aeroflex Colorado Springs - Datasheet Definition
Advanced Datasheet - Product In Development
Preliminary Datasheet - Shipping Prototype
Datasheet - Shipping QML & Reduced HiRel
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Aeroflex Colorado Springs, Inc., reserves the right to make
changes to any products and services herein at any time
without notice. Consult Aeroflex or an authorized sales
representative to verify that the information in this data sheet
is current before using this product. Aeroflex does not assume
any responsibility or liability arising out of the application or
use of any product or service described herein, except as
expressly agreed to in writing by Aeroflex; nor does the
purchase, lease, or use of a product or service from Aeroflex
convey a license under any patent rights, copyrights,
trademark rights, or any other of the intellectual rights of
Aeroflex or of third parties.
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15
Aeroflex Colorado Springs Application Note
AN-MEM-002
Low Power SRAM Read Operations
Table 1: Cross Reference of Applicable Products
Manufacturer
Part Number
SMD #
Device Type
Internal PIC
Number:*
4M Asynchronous SRAM
UT8R128K32
5962-03236
01 & 02
WC03
4M Asynchronous SRAM
UT8R512K8
5962-03235
01 & 02
WC01
16M Asynchronous SRAM
UT8CR512K32
5962-04227
01 & 02
MQ08
16M Asynchronous SRAM
UT8ER512K32
5962-06261
05 & 06
WC04/05
4M Asynchronous SRAM
UT8Q512E
5962-99607
05 & 06
WJ02
4M Asynchronous SRAM
UT9Q512E
5962-00536
05 & 06
WJ01
16M Asynchronous SRAM
UT8Q512K32E
5962-01533
02 & 03
QS04
16M Asynchronous SRAM
UT9Q512K32E
5962-01511
02 & 03
QS03
32M Asynchronous SRAM
UT8ER1M32
5962-10202
01 - 04
QS16/17
64M Asynchronous SRAM
UT8ER2M32
5962-10203
01 - 04
QS09/10
128M Asynchronous SRAM
UT8ER4M32
5962-10204
01 - 04
QS11/12
40M Asynchronous SRAM
UT8R1M39
5962-10205
01 & 02
QS13
80M Asynchronous SRAM
UT8R2M39
5962-10206
01 & 02
QS14
160M Asynchronous SRAM
UT8R4M39
5962-10207
01 & 02
QS15
Product Name:
* PIC = Aeroflex’s internal Product Identification Code
1.0 Overview
The purpose of this application note is to discuss the Aeroflex SRAMs low power read architecture and to inform users of the
affects associated with the low power read operations.
2.0 Low Power Read Architecture
The aforementioned Aeroflex designed SRAMs all employ an architecture which reduces power consumption during read
accesses. The architecture internally senses data only when new data is requested. A request for new data occurs anytime the
chip enable device pin is asserted, or any of the device address inputs transition states while the chip enable is asserted. A trigger is generated and sent to the sensing circuit anytime a request for new data is observed. Since several triggers could occur
simultaneously, these triggers are wire-ORed to result in a single sense amplifier activity for the read request. This design
method results in less power consumption than designs that continually sense data. Aeroflex’s low power SRAMs listed above
activate the sensing circuit for approximately 5ns whenever and access is requested, thereby, significantly reducing active
power.
Creation Date: 8/19/11
Page 1 of 5
Modification Date: 4/24/13
Aeroflex Colorado Springs Application Note
AN-MEM-002
2.1 The SRAM Read Cycles.
The data sheets for all the devices noted in Table #1 discuss three methods for performing a read operation. The two most common methods for reading data are an Address Access and a Chip Enabled-Controlled Access. The third access discussed is the
Output Enable-Controlled Access. The sequence at which control lines and address inputs are toggled determines which cycle
is considered relevant. As discussed in section 2.0, an assertion of chip enable or any address transition while chip enable is
asserted, initiates a read cycle. If the device chip enable is asserted prior to any address input transitions, then the read access
is considered an Address Access. By keeping the device enabled and repeatedly switching address locations, the user retrieves
all data of interest. A Chip Enable-Controlled Access occurs when the address signals are stable prior to asserting the chip
enable. The Output Enabled-Controlled Access requires that either an Address Access or Chip Enable-Controlled Access has
already been performed and the data is waiting for the Output Enable pin to assert, driving data to the device I/O pins.
The subsequent read cycle verbiage and diagrams are based on the Aeroflex UT8R512K8 data sheet. The number of control,
input, and I/O pins will vary across the products listed in Table 1. The basic design family functionality for read operations is
common among all the devices.
2.1.0 Address Access Read Cycle
The Address Access is initiated by a change in address inputs while the chip is enabled with G asserted and W deasserted. Valid
data appears on data outputs DQ(7:0) after the specified tAVQV is satisfied. Outputs remain active throughout the entire cycle.
As long as chip enable and output enable are active, the address inputs may change at a rate equal to the minimum read cycle
time (tAVAV).
tAVAV
A(18:0)
DQ(7:0)
Previous Valid Data
Valid Data
tAVQV
Assumptions:
1. E1 and G < VIL (max) and E2 and W > VIH (min)
tAXQX
Note: No time references are relevant with respect to Chip Enable(s). Chip Enable(s) is assumed to be asserted.
SRAM Read Cycle 1: Address Access
Creation Date: 8/19/11
Page 2 of 5
Modification Date: 4/24/13
Aeroflex Colorado Springs Application Note
AN-MEM-002
2.1.1 Chip Enable-Controlled Read Cycle
The Chip Enable-controlled Access is initiated by E1 and E2 going active while G remains asserted, W remains deasserted, and
the addresses remain stable for the entire cycle. After the specified tETQV is satisfied, the eight-bit word addressed by A(18:0)
is accessed and appears at the data outputs DQ(7:0).
A(18:0)
E1 low or
E2 high
tETQV
tETQX
DQ(7:0)
tEFQZ
DATA VALID
Assumptions:
1. G < VIL (max) and W > VIH (min)
Note: No specification is given for address set-up time with respect to chip enable assertion. The read cycle description states that
addresses are to remain stable for the entire cycle. Address set-up time relative to chip enable is assumed to be 0ns minimum.
SRAM Read Cycle 2: Chip Enable Access
2.1.1 Output Enabled-Controlled Read Cycle
The Output Enable-controlled Access is initiated by G going active while E1 and E2 are asserted, W is deasserted, and the
addresses are stable. Read access time is tGLQV unless tAVQV or tETQV have not been satisfied.
tAVQV
A(18:0)
G
tGHQZ
tGLQX
DATA VALID
DQ(7:0)
tGLQV
Assumptions:
1. E1 < VIL (max) , E2 > and W > VIH (min)
SRAM Read Cycle 3: Output Enable Access
3.0 Low Power Read Architecture Timing Consideration
The low power read architecture employed by Aeroflex designed SRAMs results in significant power reduction, especially in
applications with longer than minimum read cycle times. However, this type of architecture is responsive to excessive input
signal skew when device addressing and chip enable assertion occur simultaneously. Signal skew of greater than 4-5ns
between all of the read triggering activities is sufficient to start another read cycle.
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3.1 Simultaneous Control and Address Switching
Simultaneous switching of controls and address pins, alone, is not a problem; excessive skew between them is the concern.
Consider the application where several SRAM devices are connected to the same memory bus. The address bus is commonly
connected to all the devices, but the chip enable pin is singularly connected to each individual SRAM. This configuration
results in a loading difference between the address inputs and the chip enable. This lightly loaded chip enable propagates to the
memory more quickly than the heavily loaded address lines. The oscilloscope capture of Figure #1 is the actual timing of an
application which had intermittent data errors due to address transitions lagging chip enable.
Address Signal (Ax)
Chip Enable (/E)

Timing shown from VIL (yellow trace /CS) and VIH (pink for address signal) as delta X =
6ns. Even at actual internal gate switching point (~ VDD/2), the skew is still around 6ns.
Figure #1 SRAM Signal Capture
The signal transitions in the scope plot of Figure #1 appear to be fairly coincidental. A closer look however, reveals the chip
enable signal actually starts and reaches VIL approximately 6ns before the address signal reaches VIH. Even at one half VDD
(closer to actual logical gate switching of the inputs), the delta in signal times is still approximately 6ns.
Simultaneous switching of controls and address inputs is not recommended for a couple of reasons. The first is the previously
described signal skew sensitivity between controls and/or address inputs. The second reason is that activating all the controls
and address inputs simultaneously results in peak instantaneous current consumption. This condition causes maximum strain
to the power decoupling. Chip Enable activates address decoding circuits, address switching introduces input buffer switching
current, and output enable assertion turns on all the device output drivers. Peforming all three simultaneously results in worst
case transient current demand by the memory.
3.1.0 Technical Overview of Skew Sensitivity
Recall from section 2.0 that any activity requesting new data causes a read trigger. The triggers are wire-ORed together. In
order to meet the faster access times demanded by today’s applications, the ORed trigger only exists during the first 4-5ns of
the read cycle. Since the slowest of the address transitions occurs more than 5ns after the initiation of the read activity, a second read activity is initiated. The sensing circuit does not have time to normalize before the second read activity has started.
For this reason a Chip Enable-Controlled read cycle requires that address inputs remain stable for the entire cycle. Infrequent
and random sensing errors can result if the bit columns are continually pulled to one state then quickly requested to sense the
opposite state. Another effect of the low power read architecture that differs from previous generation designs (those that continually sense for data) is that the bit line will not be sensed again until another read triggering event occurs. If another read
trigger event (chip enable assertion and/or address change) does no occur for a particular address, the incorrect data remains at
the outputs.
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4.0 Summary and Conclusion
The Aeroflex SRAMs in Table #1 all employ a low power consumption read architecture. Power is conserved by sensing data
only when new data is requested. A request occurs anytime chip enable is asserted or any address input signal transitions while
chip enable is asserted. The data sheets for the SRAMs listed in Table #1 do not explicitly define the case of simultaneous
switching of address and control signals during read operations. Data sheet read cycle descriptions indicate that control inputs
are established prior to address changes, and address inputs are stable prior to control assertions. Simultaneous switching of
addresses and controls is tolerable, when the skew between all input signals is < 4ns. For designs that must employ the simultaneous activation of address and control signals, two important issues should be considered by the designer. The first is the
input signal skew sensitivity of the low power read architecture discussed by this application note. The second is the instantaneous current consumption that results from simultaneous access methods. Aeroflex recommends the use of only one read
access method at a time. If multiple read accesses (simultaneous chip enable assertion and address switching) cannot be
avoided, then Aeroflex recommends that the chip enable signal be delayed until all addresses have completed transitions.
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