Clock Phase and Polarity Controls

13.3 Clock Phase and Polarity Controls

Software can select one of four combinations of serial clock phase and polarity using two bits in the SPI control register (SPICON 9AH). The clock polarity is specified by the CPOL control bit, which selects an active high or active low clock, and has no significant effect on the transfer format.

The clock phase (CPHA) control bit selects one of two different transfer for- mats. The clock phase and polarity should be identical for the master SPI de- vice and the communicating slave device. In some cases, the phase and polar- ity are changed between transfers to allow a master device to communicate with peripheral slaves having different requirements.

When CPHA = 0, the SPI standard defines that the SS line must be negated and reasserted between each successive serial byte. This is more difficult when using the FIFO to transmit the bytes and cannot be done at higher clock speeds.

When CPHA = 1, the SS line can remain low between successive transfers.

13-4

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Texas Instruments MSC1210 manual Clock Phase and Polarity Controls