ST92F124 datasheet(199/426 Pages) STMICROELECTRONICS

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MULTIFUNCTION TIMER (MFT)

MULTIFUNCTION TIMER (Cont’d)

Figure 105. Pointer Mapping for Register to

10.4.5.4 DMA Transaction Priorities

Each Timer DMA transaction is a 16-bit operation,

therefore two bytes must be transferred sequen-

tially, by means of two DMA transfers. In order to

speed up each word transfer, the second byte

transfer is executed by automatically forcing the

peripheral priority to the highest level (000), re-

gardless of the previously set level. It is then re-

stored to its original value after executing the

transfer. Thus, once a request is being serviced,

its hardware priority is kept at the highest level re-

gardless of the other Timer internal sources, i.e.

once a Comp0 request is being serviced, it main-

tains a higher priority, even if a Capt0 request oc-

curs between the two byte transfers.

10.4.5.5 DMA Swap Mode

After a complete data table transfer, the transac-

tion counter is reset and an End Of Block (EOB)

condition occurs, the block transfer is completed.

The End Of Block Interrupt routine must at this

point reload both address and counter pointers of

the channel referred to by the End Of Block inter-

rupt source, if the application requires a continu-

ous high speed data flow. This procedure causes

speed limitations because of the time required for

the reload routine.

The SWAP feature overcomes this drawback, al-

lowing high speed continuous transfers. Bit 2 of

the DMA Counter Pointer Register (DCPR) and of

the DMA Address Pointer Register (DAPR), tog-

gles after every End Of Block condition, alternately

providing odd and even address (D2-D7) for the

pair of pointers, thus pointing to an updated pair,

after a block has been completely transferred. This

allows the User to update or read the first block

and to update the pointer values while the second

is being transferred. These two toggle bits are soft-

ware writable and readable, mapped in DCPR bit 2

for the CM0 channel, and in DAPR bit 2 for the

CP0 channel (though a DMA event on a channel,

in Swap mode, modifies a field in DAPR and

DCPR common to both channels, the DAPR/

DCPR content used in the transfer is always the bit

related to the correct channel).

SWAP mode can be enabled by the SWEN bit in

the IDCR Register.

WARNING: Enabling SWAP mode affects both

channels (CM0 and CP0).

8 bit Counter

XXXXXX11

Compare 0

8 bit Addr Pointer

XXXXXX10

8 bit Counter

XXXXXX01

Capture 0

8 bit Addr Pointer

XXXXXX00

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