ST92F124 datasheet(301/429 Pages) STMICROELECTRONICS

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J1850 Byte Level Protocol Decoder (JBLPD)

J1850 BYTE LEVEL PROTOCOL DECODER (Cont’d)

10.9.6.3 DMA Management in Reception Mode

The DMA in reception is performed when the

RDRF bit of the STATUS register is set (by hard-

ware). The RDRF bit is reset as soon as the DMA

cycle is finished.

To enable the DMA feature, the RXD_M bit of the

IMR register must be set (by software).

Each DMA request performs the transfer of a sin-

gle byte from the RXDATA register of the peripher-

al toward Register File or Memory Space (Figure

139).

Each DMA transfer consists of three operations

that are performed with minimum use of CPU time:

– A load from the JBLPD data register (RXDATA)

to a location of Register File/Memory addressed

through the DMA Address Register (or Register

pair);

– A post-increment of the DMA Address Register

(or Register pair);

– A post-decrement of the DMA transaction coun-

ter, which contains the number of transactions

that have still to be performed.

Note: When the REOBP pending bit is set (at the

end of the last DMA transfer), the reception DMA

enable bit (RXD_M) is automatically reset by hard-

ware. However, the DMA can be disabled by soft-

ware resetting the RXD_M bit.

Note: The DMA request acknowledge could de-

pend on the priority level stored in the PRLR regis-

ter.

Figure 139. DMA in Reception Mode

JBLPD peripheral

RXDATA

Memory space

Data received

Previous data

Current

Address

Pointer

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