ST92F124 datasheet(124/429 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - ON-CHIP DIRECT MEMORY ACCESS (DMA)

DMA TRANSACTIONS (Cont’d)

6.4 DMA CYCLE TIME

The interrupt and DMA arbitration protocol func-

tions completely asynchronously from instruction

flow.

Requests are sampled every 5 CPUCLK cycles.

DMA transactions are executed if their priority al-

lows it.

A DMA transfer with the Register file requires 8

CPUCLK cycles.

A DMA transfer with memory requires 16 CPUCLK

cycles, plus any required wait states.

6.5 SWAP MODE

An extra feature which may be found on the DMA

channels of some peripherals (e.g. the MultiFunc-

tion Timer) is the Swap mode. This feature allows

transfer from two DMA tables alternatively. All the

DMA descriptors in the Register File are thus dou-

bled. Two DMA transaction counters and two DMA

address pointers allow the definition of two fully in-

dependent tables (they only have to belong to the

same space, Register File or Memory). The DMA

transaction is programmed to start on one of the

two tables (say table 0) and, at the end of the

block, the DMA controller automatically swaps to

the other table (table 1) by pointing to the other

DMA descriptors. In this case, the DMA mask (DM

bit) control bit is not cleared, but the End Of Block

interrupt request is generated to allow the optional

updating of the first data table (table 0).

Until the swap mode is disabled, the DMA control-

ler will continue to swap between DMA Table 0

and DMA Table 1.

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