DSP56857 datasheet(279/662 Pages) FREESCALE

279 / 662 page

Operating Modes

Serial Peripheral Interface (SPI), Rev. 4

Freescale Semiconductor

11-9

When the master SPI starts a transmission, the data in the slave Shift register begins shifting out

on the MISO pin. The slave can load its Shift register with new data for the next transmission by

writing to its Transmit Data register. The slave must write to its Transmit Data register at least

one bus cycle before the master begins the next transmission. Otherwise, the data already in the

slave Shift register shifts out on the MISO pin. Data written to the Slave Shift register during a

transmission remains in a buffer until the end of the transmission.

When the CPHA bit is set, the first edge of SCLK starts a transmission. When CPHA is cleared,

the falling edge of SS starts a transmission.

Note:

SCLK must be in the proper idle state before the slave is enabled to prevent SCLK

from appearing as a clock edge.

11.6.3

DMA Mode

When the TDMAEN or RDMAEM bits are set, the SPI operates in DMA mode. Normal SPTE

and/or SPRF interrupts are suppressed. Instead DMA requests are generated, signaling the DMA

controllers to read the SPDRR or SPDTR as needed. The DMA requests are suppressed in the

case of an OVRF or MODF interrupt, illustrated in Figure 11-4. If the SPI is being used to send

and receive data, both TDMAEN and RDMAEN should be enabled/disabled at the same time. If

data is only being received or transmitted TDMAEN or RDMAEN may be left inactive as

appropriate, however in this case ERRIE must not be set to prevent the DMA request being

masked by a mode-fault error or overflow error. DMA mode must be used carefully. In the case

where CPHA=0, the SS signal to the slave is toggled by software between words. In DMA mode

there is no way of generating this pulse easily, so the DMA should not be used when CPHA=0.

Figure 11-4. SPI DMA Request Generation

Receive DMA Request

Transmit DMA Request

SPTE

SPRIE

MODF

OVRF

TDMAEN

MODF

OVRF

ERRIE

SPRF

RDMAEN

DSP56857 Datasheet(PDF) 279 Page - Freescale Semiconductor, Inc