Z80 Datasheet, PDF (178/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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CLK
Active
RDY
Inactive
BUSREQ
BAI
Note: RDY is detected as a level, not an edge
DMA
Inactive
Figure 65. Bus Request and Acceptance Timing
DMA
Active
Continuous mode is the only instance when a pulse on RDY allows the
DMA to become bus master.In this event, the DMA becomes bus master
but does not begin operations.
When the DMA detects a Low on BAI for two consecutive rising edges of
CLK, the DMA begins transferring data on the next rising edge of CLK.
In Byte mode, after each byte is transferred, the DMA waits until its BAI
line goes inactive before requesting the bus again on BUSREQ for the next
byte transfer. This feature allows a minimum of one CPU machine cycle to
occur between each byte transferred.
Bus Release Byte-at-a-Time
In Byte mode, BUSREQ is brought High on the rising edge of CLK prior to
the end of each read cycle (search-only) or write cycle (transfer and trans-
fer/search) as illustrated in Figure 66. This action occurs regardless of the
state of RDY. There is no possibility of confusion when a Z80 CPU is used
because the CPU cannot begin an operation until the following clock cycle.
Nor does this condition affect most other CPUs. The result of timing is one-
clock-cycle less time needed for a byte transfer.
UM008101-0601
Direct Memory Access