Z80 Datasheet, PDF (212/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Only port lines whose mask bit is zero are monitored for generating an
interrupt. When IORQ is high, the forced state of Ready prevents input
register data from changing while the CPU is reading the PIO. Ready goes
High again after the trailing edge of the IORQ, as previously described
TIMING
Output Mode (Mode 0)
Figure 7 illustrates the timing associated with Mode 0 operation. An output
cycle is always started by the execution of an output instruction by the
CPU. A WR^* pulse is generated by the PIO during a CPU I/O write opera-
tion and is used to latch the data from the CPU data bus to the addressed
ports (A or B) output register. The rising edge of the WR^* pulse then raises
the Ready flag after the next falling edge of Φ, indicating that data is avail-
able for the peripheral device. In most systems, the rising edge of the Ready
signal can be used as a latching signal in the peripheral device. The Ready
signal remains active until: (1) a positive edge is received from the strobe
line, indicating that the peripheral has taken the data, or (2) if already
active, Ready is forced low for one and one-half Φ cycles after the leading
edge of IORQ, but only if the port’s output register is written to. Ready
returns High on the first falling edge of Φ after the trailing edge of IORQ.
This guarantees that Ready is low when port data is changing. The Ready
signal does not go inactive until a falling edge occurs on the clock (Φ) line.
The purpose of delaying the negative transition of the Ready signal until
after a negative clock transition is to allow a simple generation scheme for
the strobe pulse. By connecting the Ready line to the Strobe line, a strobe
with a duration of one clock period is generated with no other logic
required. The positive edge of the strobe pulse automatically generates an
INT request when the interrupt enable flip-flop has been set and this device
is the highest priority device requesting an interrupt.
UM008101-0601
Parallel Input/Output