Download  Print this page

HP 13255 Manual page 14

Memory controller module
Hide thumbs

Advertisement

13255
Memory Controller
13255-91252/12
REV JUN-23-81
3.9
3.9.1
3·9·3
MEMORY CYCLE GENERATOR
The memory
cycle generator
is a synchronous
sequntial
state machine
composed of several flip flops and gates.
The
purpose of this state
machine
is to
generate the row and column address strobes for
memory
cycle operations with the appropriate timing.
A memory
cycle begins when HEMGO goes low.
This is synchronized with
the leading edge of REQ by a 74s74 flip flop
(071).
This also accom-
plishes the rising edge
clock synchronization mentioned above in sec-
tion 3.6.2,
as REQ is synchronized with
the clock by
the processor.
The output of this flip flop (071-9) represents a memory cycle pending.
This signal is ANDed with RFSH to
prevent a memory cycle from starting
while a
refresh cycle is in progress.
The output of this gate(Ul5-6)
indicates an active memory cycle.
The leading
(negative) edge of this
cycle active signal
sets HEM BUSY to the true state,
thus
the
start of any refresh
cycle;
it also clocks
a 74LSl12
preventing
flip flop
which starts MEMRAS,
the row address strobe
(Ul2-12).
Two flip flops
(U13 and U25)
then maintain HEMRAS in the low state for 2 clock cycles
(408 nsec).
The output of the second flip flop
(U25-9)
is the column
address strobe, CAS.
It lasts for one clock cycle
(204 nsec) and fol-
lows the leading edge of HEMRAS by 204 nsec subject to logic delays.
As
with the refresh cycle generator,
two 74LS74 D flip flops (U21) are
used to extend the memory cycle to allow for
RAS
pre charge time.
The
output of the second of these
(U21-5)
resets
HEM BUSY
to
the false
state, thus ending the memory cycle.
MEMRAS is ted to a delay line (HP part I 1810-0384, 50 nsec delay, taps
at 10 nsec intervals)
where it is delayed 40 nsec
to become
R/C,
the
signal which
determines which halt of
the address is
fed through the
address multiplexers (see section 3.4.2).

Advertisement

Table of Contents
loading

  Also See for HP 13255

  Related Manuals for HP 13255