December 2004
Copyright Alliance Semiconductor. All rights reserved.
AS7C251MNTD18A
12/23/04, v.2.2
Alliance Semiconductor
P. 1 of 18
2.5V 1M x 18 Pipelined SRAM with NTD
TM
Features
Organization: 1,048,576 words 18 bits
NTD
TM
architecture for efficient bus operation
Fast clock speeds to 166 MHz
Fast clock to data access: 3.5/3.8 ns
Fast OE access time: 3.5/3.8 ns
Fully synchronous operation
Asynchronous output enable control
Available in 100-pin TQFP package
Individual byte write and global write
Clock enable for operation hold
Multiple chip enables for easy expansion
2.5V core power supply
Self-timed write cycles
Interleaved or linear burst modes
Snooze mode for standby operation
Logic block diagram
Wr
i
t
e
B
u
ff
e
r
Address
D
Q
CLK
register
Output
register
DQ [a,b]
20
20
CLK
CE0
CE1
CE2
A[19:0]
OE
CLK
CEN
Control
CLK
logic
Data
D
Q
CLK
input
register
18
18
OE
1M x 18
SRAM
array
R/W
DQ [a,b]
BWa
BWb
CLK
Q
D
ADV / LD
LBO
burst logic
addr. registers
Write delay
20
ZZ
CLK
18
18
18
18
Selection guide
-166
-133
Units
Minimum cycle time
6
7.5
ns
Maximum clock frequency
166
133
MHz
Maximum clock access time
3.5
3.8
ns
Maximum operating current
290
270
mA
Maximum standby current
85
75
mA
Maximum CMOS standby current (DC)
40
40
mA
AS7C251MNTD18A
12/23/04, v.2.2
Alliance Semiconductor
P. 2 of 18
16 Mb 2.5V Synchronous SRAM products list
1,2
1 Core Power Supply: VDD = 2.5V + 0.125V
2 I/O Supply Voltage: VDDQ = 2.5V + 0.125V
PL-SCD
:
Pipelined Burst Synchronous SRAM - Single Cycle Deselect
PL-DCD
:
Pipelined Burst Synchronous SRAM - Double Cycle Deselect
FT
:
Flow-through Burst Synchronous SRAM
NTD
1
-PL
:
Pipelined Burst Synchronous SRAM with NTD
TM
NTD-FT
:
Flow-through Burst Synchronous SRAM with NTD
TM
Org
Part Number
Mode
Speed
1MX18
AS7C251MPFS18A
PL-SCD
166/133 MHz
512KX32
AS7C25512PFS32A
PL-SCD
166/133 MHz
512KX36
AS7C25512PFS36A
PL-SCD
166/133 MHz
1MX18
AS7C251MPFD18A
PL-DCD
166/133 MHz
512KX32
AS7C25512PFD32A
PL-DCD
166/133 MHz
512KX36
AS7C25512PFD36A
PL-DCD
166/133 MHz
1MX18
AS7C251MFT18A
FT
7.5/8.5/10 ns
512KX32
AS7C25512FT32A
FT
7.5/8.5/10 ns
512KX36
AS7C25512FT36A
FT
7.5/8.5/10 ns
1MX18
AS7C251MNTD18A
NTD-PL
166/133 MHz
512KX32
AS7C25512NTD32A
NTD-PL
166/133 MHz
512KX36
AS7C25512NTD36A
NTD-PL
166/133 MHz
1MX18
AS7C251MNTF18A
NTD-FT
7.5/8.5/10 ns
512KX32
AS7C25512NTF32A
NTD-FT
7.5/8.5/10 ns
512KX36
AS7C25512NTF36A
NTD-FT
7.5/8.5/10 ns
1. NTD: No Turnaround Delay. NTD
TM
is a trademark of Alliance Semiconductor Corporation. All trademarks mentioned in this document are the property
of their respective owners.
AS7C251MNTD18A
12/23/04, v.2.2
Alliance Semiconductor
P. 3 of 18
Pin assignment
100-pin TQFP - top view
1
2
3
4
5
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
80
79
78
77
76
75
74
73
72
71
70
69
68
67
66
65
64
63
62
61
60
59
58
57
56
55
54
53
52
51
LBO
A
A
A
A
A1 A0
NC
NC
V
SS
V
DD
NC
NC
A
A
A
A
A
A
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
10
0
99
98
97
96
95 94
93
92
91
90
89 88
87
86
85
84
83 82
81
A A
CE0
CE1
NC
NC
BWb
BW
a
CE2
V
DD
V
SS
CLK
R/W
CEN
OE
ADV/LD
A
A
A
TQFP 14 x 20mm
A
NC
NC
NC
V
DDQ
V
SSQ
NC
NC
DQb0
DQb1
V
SSQ
V
DDQ
DQb2
DQb3
NC
V
DD
NC
V
SS
DQb4
DQb5
V
DDQ
V
SSQ
DQb6
DQb7
DQPb
NC
V
SSQ
V
DDQ
NC
NC
NC
A
NC
NC
V
DDQ
V
SSQ
NC
DQPa
DQa7
DQa6
V
SSQ
V
DDQ
DQa5
DQa4
V
SS
ZZ
DQa3
DQa2
V
DDQ
V
SSQ
DQa1
DQa0
NC
NC
V
SSQ
V
DDQ
NC
NC
NC
V
DD
NC
A
AS7C251MNTD18A
12/23/04, v.2.2
Alliance Semiconductor
P. 4 of 18
Functional description
The AS7C251MNTD18A family is a high performance CMOS 16-Mbit synchronous Static Random Access Memory (SRAM) organized as
1,048,576 words 18 bits and incorporates a LATE LATE Write.
This variation of the 16Mb+ synchronous SRAM uses the No Turnaround Delay (NTD
TM
) architecture, featuring an enhanced write
operation that improves bandwidth over pipelined burst devices. In a normal pipelined burst device, the write data, command, and address
are all applied to the device on the same clock edge. If a read command follows this write command, the system must wait for two 'dead'
cycles for valid data to become available. These dead cycles can significantly reduce overall bandwidth for applications requiring random
access or read-modify-write operations.
NTD
TM
devices use the memory bus more efficiently by introducing a write latency which matches the two-cycle pipelined or one-cycle
flow-through read latency. Write data is applied two cycles after the write command and address, allowing the read pipeline to clear. With
NTD
TM
, write and read operations can be used in any order without producing dead bus cycles.
Assert R/W low to perform write cycles. Byte write enable controls write access to specific bytes, or can be tied low for full 18 bit writes.
Write enable signals, along with the write address, are registered on a rising edge of the clock. Write data is applied to the device two clock
cycles later. Unlike some asynchronous SRAMs, output enable OE does not need to be toggled for write operations; it can be tied low for
normal operations. Outputs go to a high impedance state when the device is de-selected by any of the three chip enable inputs. In pipelined
mode, a two cycle deselect latency allows pending read or write operations to be completed.
Use the ADV (burst advance) input to perform burst read, write and deselect operations. When ADV is high, external addresses, chip select, R/W
pins are ignored, and internal address counters increment in the count sequence specified by the LBO control. Any device operations, including
burst, can be stalled using the CEN=1, the clock enable input.
The AS7C251MNTD18A operates with a 2.5V 5% power supply for the device core (V
DD
). These devices are available in a 100-pin
TQFP package.
TQFP capacitance
* Guaranteed not tested
TQFP thermal resistance
Parameter
Symbol
Test conditions
Min
Max
Unit
Input capacitance
C
IN
*
V
IN
= 0V
-
5
pF
I/O capacitance
C
I/O
*
V
OUT
= 0V
-
7
pF
Description
Conditions
Symbol
Typical
Units
Thermal resistance
(junction to ambient)
1
1 This parameter is sampled
Test conditions follow standard test methods
and procedures for measuring thermal
impedance, per EIA/JESD51
1layer
JA
40
C/W
4layer
JA
22
C/W
Thermal resistance
(junction to top of case)
1
JC
8
C/W
AS7C251MNTD18A
12/23/04, v.2.2
Alliance Semiconductor
P. 5 of 18
Signal descriptions
Snooze Mode
SNOOZE MODE is a low current, power-down mode in which the device is deselected and current is reduced to I
SB2
. The duration of
SNOOZE MODE is dictated by the length of time the ZZ is in a High state.
The ZZ pin is an asynchronous, active high input that causes the device to enter SNOOZE MODE.
When the ZZ pin becomes a logic High, I
SB2
is guaranteed after the time t
ZZI
is met. After entering SNOOZE MODE, all inputs except ZZ
is disabled and all outputs go to High-Z. Any operation pending when entering SNOOZE MODE is not guaranteed to successfully complete.
Therefore, SNOOZE MODE (READ or WRITE) must not be initiated until valid pending operations are completed. Similarly, when exiting
SNOOZE MODE during t
PUS
, only a DESELECT or READ cycle should be given while the SRAM is transitioning out of SNOOZE
MODE.
Signal
I/O
Properties Description
CLK
I
CLOCK
Clock. All inputs except OE, LBO, and ZZ are synchronous to this clock.
CEN
I
SYNC
Clock enable. When de-asserted high, the clock input signal is masked.
A, A0, A1
I
SYNC
Address. Sampled when all chip enables are active and ADV/LD is asserted.
DQ[a,b]
I/O
SYNC
Data. Driven as output when the chip is enabled and OE is active.
CE0, CE1,
CE2
I
SYNC
Synchronous chip enables. Sampled at the rising edge of CLK, when ADV/LD is asserted.
Are ignored when ADV/LD is high.
ADV/LD
I
SYNC
Advance or Load. When sampled high, the internal burst address counter will increment in
the order defined by the LBO input value. When low, a new address is loaded.
R/W
I
SYNC
A high during LOAD initiates a READ operation. A low during LOAD initiates a WRITE
operation. Is ignored when ADV/LD is high.
BW[a,b]
I
SYNC
Byte write enables. Used to control write on individual bytes. Sampled along with WRITE
command and BURST WRITE.
OE
I
ASYNC
Asynchronous output enable. I/O pins are not driven when OE is inactive.
LBO
I
STATIC
Selects Burst mode. When tied to V
DD
or left floating, device follows interleaved Burst order. When
driven Low, device follows linear Burst order. This signal is internally pulled High.
ZZ
I
ASYNC
Snooze. Places device in low power mode; data is retained. Connect to GND if unused.
NC
-
-
No connects.
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