ST9 datasheet(94/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

tual counter and the pointer, they point to where you want to locate them in the register file.

This double pointer mechanism is similar to that described for interrupts.

Here there can be two cases: the transfer involves the memory or the register file.

If the transfer involves the memory, it’s standard procedure. The DCPR points to the 16-bit

Counter register, and the DAPR points to the 16-bit Address register. These two register pairs

may reside anywhere there is room for them in the register file. Since register pairs are used,

their addresses are even. The DCPR must be even. To reach the DMA segment the DMASR

(or ISR depending on DAPR bit 0) MMU register contains the segment number.

If the transfer involves the register file, things are a bit different. Both the transaction counter

and the address register are 8-bit values. They are supposed to be adjacent in the register file,

i.e. they occupy two successive registers: the first one, even numbered, is the address reg-

ister. The second one, odd numbered, is the transaction counter. To select this mode, the

least significant bit of the DCPR must be one. Its value is the address of the address register

(which is even) plus one. The DAPR is not used.

The whole mechanism described above applies for both the transmitter side and the receiver

side. Thus, there are twice as many registers as mentioned: the TDCPR and the TDAPR for

the transmitter side, the RDCPR and the RDAPR for the receiver side. The registers you have

located in the register file are also twice as many. So, the TDCPR points to the transmitter

transaction counter and the TDAPR points to the transmitter address register (with the excep-

tion mentioned above if the transaction involves the register file). The RDCPR points to the re-

ceiver transaction counter and the RDAPR points to the receiver address register.

When a DMA transfer is in progress, the processor is not even aware of it (except that it is

slightly slowed down by the stolen clock cycles). So nothing special has to be done as far as

the program is concerned. The question is: how to start and end a DMA transfer? Here, the

processor is involved, and some code is needed. Depending on which side is concerned, the

processes are different.

4.6.1.2.1 Transmitter Side

To start a DMA transmission, you need to do the following operations, in order:

– Set the DMA segment register DMASR (or ISR) to point to the selected segment of data

memory.

– Set the transmit address register to the address of the second word to be sent (not the

TDAPR, which is only set once at initialisation, and points permanently to the transmit ad-

dress register).

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