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UM008101-0601 Direct Memory Access

the Z80 bus, to function. These functions are described in the following

sections, and design solutions are offered:

•

Bus request/release mechanisms

•

Bus characteristics

•

Interrupt request, acknowledge, and return

Bus Request/Release Mechanisms

The most fundamental characteristic that distinguishes the Z80 DMA from

other monolithic DMACs is its full control of the system bus during its

active state. Therefore, processors using the DMA must be able to give up

control of the system bus, including address, data, and the control lines

MREQ

,IORQ,RD,andWR(or their equivalents). Some processors have

no mechanism for freeing the bus. Others, including the 6800, have rudi-

mentary bus control facilities, but because of their internal dynamic logic

implementations, cannot relinquish control for indefinite periods of time.

This makes them difficult to interface to the DMA.

Many popular microprocessor CPUs, however, do have adequate bus

control facilities, and some are very similar to the Z80 BUSREQ

and

BUSACK signals. For instance, the 8080, 8085, and 8086 signals HOLD

and HLDA are very close approximations.

The active levels of HOLD and HLDA are positive rather than negative,

and variations exist in timing. But the use of HOLD and HLDA allows the

address and data bus drivers to be put into their high-impedance states. In

8080 systems using an 8238 to demultiplex commands, the MEMW

MEMR,IOW,and IOR control lines can be floated using the BUSEN

input. With the 8085, a tristate decoder allows decode or disable corre-

sponding signals. The 8086 and its support chips also tristate their control

signals when HLDA is active.

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Zilog Z08470 Manual do Utilizador Página 162