Z80 Datasheet, PDF (177/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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inactive, are caused by the activity on the BUSREQ and BAI lines, which is
explained later.
Search-Only
The standard timing for search-only operations is identical to the read cycles
of Figure 61 and Figure 62. Search-only is equivalent to read-only. Data is
read to a DMA register for comparison with the match byte.
Bus Requests
Figure 65 illustrates the bus request and acceptance timing. The RDY line,
which may be programmed active High or Low, is sampled on every rising
edge of CLK.
If the RDY line is active, and if the bus is not in use by any other device, the
following rising edge of CLK drives BUSREQ Low. After receiving BUS-
REQ the CPU acknowledges on its BUSACK, which is connected to the
DMA’s BAI input either directly or through a multiple-DMA daisy-chain.
The CPU checks its BUSREQ input one clock cycle before the end of each
CPU machine cycle. If the CPU detects a request, it releases the bus at the
end of that same machine cycle. The maximum time delay between the
CPU receiving BUSREQ and the response on its BUSACK line is one
machine cycle plus slightly less than one clock cycle. The CPU tristates all
its bus control lines when it acknowledges on the BUSACK line. M1 is not
tristated.
The RDY line, which has a specified setup time with respect to a rising
edge of CLK, must remain active until after the DMA becomes bus master
in Byte or Burst modes.
UM008101-0601
Direct Memory Access