6.17 Serial Peripheral Interface Ports (SPI0, SPI1)

Figure 6-36 is a block diagram of the SPI module, which is a simple shift register and buffer plus control logic. Data is written to the shift register before transmission occurs and is read from the buffer at the end of transmission. The SPI can operate either as a master, in which case, it initiates a transfer and drives the SPIx_CLK pin, or as a slave. Four clock phase and polarity options are supported as well as many data formatting options.

Figure 6-36 Block Diagram of SPI Module

The SPI supports 3-, 4-, and 5-pin operation with three basic pins (SPIx_CLK, SPIx_SIMO, and SPIx_SOMI) and two optional pins (SPIx_SCS, SPIx_ENA).

The optional SPIx_SCS (Slave Chip Select) pin is most useful to enable in slave mode when there are other slave devices on the same SPI port. The device will only shift data and drive the SPIx_SOMI pin when SPIx_SCS is held low.

In slave mode, SPIx_ENA is an optional output. The SPIx_ENA output provides the status of the internal transmit buffer (SPIDAT0/1 registers). In four-pin mode with the enable option, SPIx_ENA is asserted only when the transmit buffer is full, indicating that the slave is ready to begin another transfer. In five-pin mode, the SPIx_ENA is additionally qualified by SPIx_SCS being asserted. This allows a single handshake line to be shared by multiple slaves on the same SPI bus.

In master mode, the SPIx_ENA pin is an optional input and the master can be configured to delay the start of the next transfer until the slave asserts SPIx_ENA. The addition of this handshake signal simplifies SPI communications and, on average, increases SPI bus throughput since the master does not need to delay each transfer long enough to allow for the worst-case latency of the slave device. Instead, each transfer can begin as soon as both the master and slave have actually serviced the previous SPI transfer.

Figure 6-37 Illustration of SPI Master-to-SPI Slave Connection