Intel 80C186XL, User Manual

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DIRECT MEMORY ACCESS UNIT
10-20
10.2.2 Suspension of DMA Transfers
Whenever the CPU receives an NMI, all DMA activity is suspended at the end of the current
transfer. The CPU suspends DMA activity by setting the DHLT bit in the Interrupt Status Regis-
ter (Figure 8-14 on page 8-23). When an IRET instruction is executed, the CPU clears the DHLT
bit and DMA transfers are allowed to resume. Software can read and write the DHLT bit.
NOTE
Do not write to the DHLT bit while Timer/Counter Unit interrupts are enabled.
A conflict with the internal use of the register may cause incorrect processing
of timer interrupts.
The DHLT bit does not function when the interrupt controller is in slave mode.
10.2.3 Initializing the DMA Unit
Use the following sequence when programming the DMA Unit:
1. Program the source and destination pointers for all used channels.
2. Program the DMA Control Registers in order of highest-priority channel to lowest-
priority channel.
10.3 HARDWARE CONSIDERATIONS AND THE DMA UNIT
This section covers hardware interfacing and performance factors for the DMA Unit.
10.3.1 DRQ Pin Timing Requirements
The DRQ pins are sampled on the falling edge of CLKOUT. The DRQ pins must be set up a min-
imum of T
INVCL
before CLKOUT falling, to guarantee recognition at a specific clock edge. Refer
to the data sheet for specific values.
The DRQ pins have an internal synchronizer. Violating the setup time can cause only a missed
DMA request, not a processor malfunction.