Motorola DSP56305 User Manual download pdf (Page 58)

Using the DMA

Servicing a Peripheral

MOTOROLA Optimizing DSP56300/DSP56600 Applications 4-9

Mx), and the MIPS required to process a fast interrupt for

every 24-bit word transfer.

• Using the DMA 3-D Addressing mode to increment the

source address after the basic 3-word transfer—The core is

interrupted only after N words are transmitted to fill the

buffer again. The “cost” is one DMA channel and four offset

registers.

• A second DMA transfer defined in another channel

triggered by the end of the basic 3-word transfer—The

second transfer copies data from a buffer to a fixed address

used by the first channel. This option basically splits the

previous option between two DMA channels, thus enabling

the use of simpler addressing modes and freeing common

offset registers. The “cost” is two DMA channels and one

offset register.

The choice of a solution depends on the availability of the relevant

resources in the specific application. This example implements the

second option (using one DMA channel), for feeding the SCI

transmitter. Words for transmission are arranged in a data buffer.

As shown in Figure 4-1 on page 4-10, each word in the buffer

should be written 3 times to the SCI transmit registers so that all 3

bytes are transmitted. After all the buffer is transmitted, the core is

interrupted. This addressing is defined in the 3-D Addressing

mode. The first dimension counter is null (DCOL value 0), so that an

offset 0 (from DOR0) is added after each transfer. The second

dimension is used to increment the address by 1 after 3 transfers

(DOR1). The third dimension is used to count the words in the

buffer.

Note: An offset register is needed for zero offset since the

no-update mode is only for linear counting. Transfer

destination (SCI side) is addressed as a circular 3-word

buffer—after three words, the address is decreased by 3.

Usually a two-dimensional addressing mode is enough for

such addressing, but this example should also be

implemented using 3-D addressing. The reason is the need to

align the offsets needed for the destination with the counter

value as was defined for the source.

1 2 ... 53 54 55 56 57 58 59 60 61 62 63 ... 111 112

No comments

Motorola DSP56305 User Manual Page 58