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CONDITIONS LEADING TO CYCLE START
1. ·The INSTRUCTION-FETCH flip-flop is set by the leading edge of
SMI.
For a Z80 processor, the instruction fetch presents the
tightest access time requirement (even though SMI precedes
SMEMR) .
2. For a Z80 processor, the FETCH flip-flop is set by the leading
edge of SMEMR.
3. For an 8080 processor, the FETCH flip-flop is set by the AND
of PSYNC, PHI
1,
and D7 which is an anticipation of MEMR
status.
4. Note that it is possible to have INSTRUCTION-FETCH and FETCH
set simultaneously, a condition which is redundant but
harmless.
5. The STORE flip-flop is set by the leading edge of PWR unless
blocked by SOUTo
STORE causes a CYCLE-START and a WE.
6. A control panel deposit is detected as the AND of MWRITE and
SMEMR, which causes first a DEP-RQ and then a DEPOSIT-CY.
The
deposit cycle is delayed if it would otherwise immediately
follow a waiting refresh cycle.
7. The RUNNING-REFRESH flip-flop is set by the trailing edge of
SMI and provides refresh cycles when the processor is running.
As long as there are 64 instructions executed in any 2ms
period, this type of cycle will satisfy the refresh
requirements.
8. Whenever 20us (approximately) elapses with no cycles of any
kind, the retriggerable one-shot RECENT goes false, allowing a
WAITING-REFRESH cycle.
This type of cycle therefore maintains
refresh activity when the reset switch is depressed, during
long wait states (control panel or North Star disk
controller), or when an 8080 is halted.
See the next section
for more information.
NOTES ON WAITING-REFRESH
1. The WAITING-REFRESH flip-flop is clocked on the trailing edge
of the PHI 2 clock, and is true for only one clock period.
2. The refresh cycle is delayed by a DEP-REO if a control panel
deposit cycle is in progress or will start on the next clock
cycle.
A refresh and a deposit cycle cannot occur on the same
or sequential clock cycles.
3. Since some of its input conditions are asynchronous to the PHI
2 clock, the setup and hold times of the WAITING-REFRESH flip-
North Star 16K RAM Board
28
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