Data Sheet

ADuCM3027/ADuCM3029

Rev. B | Page 37 of 39

Table 33. 21000017—SPI Read Command Mode Does Not Work

Background

When the SPI master is enabled and uses the DMA mode with SPI_CNT = 1, the read command mode may not

function properly. Consider the following configurations: SPI_RD_CTL = 0x07, SPI_CNT = 1, and the Tx and Rx DMA

channels configured for one half-word. In this configuration, the read command sent in the first byte on the MOSI

output is repeated in the second byte (in the address slot); thus, the slave device responds on the MISO line with

whatever content is at the address equivalent to the read command value (for example, if the read command is 0xB,

the response is the data read from Slave Address 0xB).

Issue

SPI read command mode does not work properly when SPI_CNT is 1 and DMA is enabled.

Workaround

Use the overlap mode to align the transmit/receive SPI operations and discard the junk bytes, as follows:

Set the OVERLAP bit in the SPI_RD_CTL register = 1 to enable overlap mode.

Set the TXBYTES bit in the SPI_RD_CTL register = 1 to configure a single transmit byte (8-bit address register).

Set SPI_CNT.VALUE = 3 to configure the transfer count: one byte for the address register, one byte for the command,

and one dummy byte to obtain the read value, and on the receive side, discard the first two bytes received during

the transfer of the address and command bytes before processing the actual read value in the third byte.

Do not use Tx DMA operation on the SPI transmit side, as follows: enable only the SPI Rx DMA requests, fill the SPI Tx

FIFO by using core accesses to write to the SPI_TX register, and perform a dummy read of the SPI_RX register to

start the SPI transfers.

Related Issues

None.