HM-6514/883

FN2996 Rev.1.00

Page 7 of 9

March 1997

Timing Waveforms (Continued)

The write cycle is initiated by the falling edge of E (T = 0),

which latches the address information in the on-chip registers.

There are two basic types of write cycles, which differ in the

control of the common data-in/data-out bus.

Case 1: E falls before W falls

The output buffers may become enabled (reading) if E falls

before W falls. W is used to disable (three-state) the outputs so

input data can be applied. TWLDV must be met to allow the W

signal time to disable the outputs before applying input data.

Also, at the end of the cycle the outputs may become active if

W rises before E. The RAM outputs and all inputs will three-

state after E rises (TEHQZ). In this type of write cycle TWLEL

and TEHWH may be ignored.

Case 2: E falls equal to or after W falls, and E rises before

or equal to W rising

This E and W control timing will guarantee that the data out-

puts will stay disabled throughout the cycle, thus simplifying

the data input timing. TWLEL and TEHWH must be met, but

TWLDV becomes meaningless and can be ignored. In this

cycle TDVWH and TWHDX become TDVEH and TEHDX. In

other words, reference data setup and hold times to the E ris-

ing edge.

If a series of consecutive write cycles are to be performed, W

may be held low until all desired locations have been written

(an extension of Case 2).

TAVEL

TELWL

TEVEL

TELAX

TWHEH

HIGH Z

HIGH Z

TWHDZ

-1

TIME

01

2

3

4

5

REFERENCE

VALID ADD

NEXT ADD

TELEL

TEHEL

TELEH

TWLEH

TWLWH

TEHEL

TWLDV

VALID DATA INPUT

TDVWH

TELWH

W

DQ

E

A

FIGURE 2. WRITE CYCLE

TRUTH TABLE

TIME

REFERENCE

INPUTS

DQ

FUNCTION

E

W

A

-1

H

X

X

Z

Memory Disabled

0

X

V

Z

Cycle Begins, Addresses are Latched

1

L

L

X

Z

Write Period Begins

2

L

X

V

Data In is Written

3H

X

Z

Write Completed

4

H

X

X

Z

Prepare for Next Cycle (Same as -1)

5

X

V

Z

Cycle Ends, Next Cycle Begins (Same as 0)

IF

OBSERVE

IGNORE

Case 1

E falls before W

TWLDV

TWLEL

Case 2

E falls after W and

E rises before W

TWLEL

TEHWH

TWLDV

TWHDX