TMS44400, TMS44400P, TMS46400, TMS46400P

1048576-WORD BY 4-BIT

DYNAMIC RANDOM-ACCESS MEMORIES

SMHS562C – MAY 1995 – REVISED NOVEMBER 1996

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operation

enhanced page mode

Enhanced-page-mode operation allows faster memory access by keeping the same row address while

selecting random column addresses. The time for row-address setup and hold and address multiplex is

eliminated. The maximum number of columns that can be accessed is determined by the maximum RAS low

time and the CAS page cycle time used. With minimum CAS page cycle time, all 1024 columns specified by

column addresses A0 through A9 can be accessed without intervening RAS cycles.

Unlike conventional page-mode DRAMs, the column-address buffers in this device are activated on the falling

edge of RAS. The buffers act as transparent or flow-through latches while CAS is high. The falling edge of CAS

latches the column addresses. This feature allows the TMS4x400 to operate at a higher data bandwidth than

conventional page-mode parts because data retrieval begins as soon as the column address is valid rather than

when CAS transitions low. This performance improvement is referred to as enhanced page mode. A valid

column address can be presented immediately after row-address hold time has been satisfied, usually well in

for the next cycle are valid at the time CAS goes high, access time for the next cycle is determined by the later

address (A0 – A9)

Twenty address bits are required to decode any one of the 1 048 576 storage-cell locations. Ten row-address

bits are set up on inputs A0 through A9 and latched onto the chip by RAS. The ten column-address bits are set

up on A0 through A9 and latched onto the chip by CAS. All addresses must be stable on or before the falling

edges of RAS and CAS. RAS is similar to a chip enable because it activates the sense amplifiers as well as the

row decoder. CAS is used as a chip select, activating the output buffer, as well as latching the address bits into

the column-address buffer.

write enable (W)

The read or write mode is selected through W input. A logic high on W selects the read mode and a logic low

selects the write mode. W can be driven from standard TTL circuits ( TMS44400 / P) or low voltage TTL circuits

( TMS46400 / P) without a pullup resistor. The data input is disabled when the read mode is selected. When W

goes low prior to CAS (early write), data out remains in the high-impedance state for the entire cycle, permitting

a write operation independent of the state of OE. This permits early-write operation to complete with OE

grounded.

data in / out (DQ1 – DQ4)

Data out is the same polarity as data in. The output is in the high-impedance (floating) state until CAS and OE

are brought low. In a read cycle, the output becomes valid after all access times are satisfied. The output remains

valid while CAS and OE are low. CAS or OE going high returns the output to a high-impedance state. This is

output enable (OE)

OE controls the impedance of the output buffers. When OE is high, the buffers remain in the high-impedance

state. Bringing OE low during a normal cycle activates the output buffers, putting them in the low-impedance

state. It is necessary for both RAS and CAS to be brought low for the output buffers to go into the low-impedance

state. They remain in the low-impedance state until either OE or CAS is brought high.