35.6.2.6.2 Client

In Client mode (CTRLA.MODE=0x2), the SPI interface will remain inactive with the MISO line tri-stated as long as the SS pin is pulled high. Software may update the contents of Tx Buffer at any time as long as the Data Register Empty flag in the Interrupt Status and Clear register (INTFLAG.DRE) is set.

When SS is pulled low and SCK is running, the client will sample and shift out data according to the transaction mode set.

If FIFO is disabled, when the content of DATA has been loaded into the Shift register, INTFLAG.DRE will be set, and new data can be written to DATA. If the FIFO is enabled, INTFLAG.DRE will be set when at least FIFO threshold (CTRLC.TXTRHOLD) locations in TX FIFO are empty. When DATA or FIFO threshold locations are empty respectively, it takes three CLK_SERCOM_APB cycles for INTFLAG.DRE to be set.

Similar to the host, the client will receive one character for each character transmitted. A character will be transferred into the receive buffer within the same clock cycle its last data bit is received. The received character can be retrieved from DATA when the Receive Complete interrupt flag (INTFLAG.RXC) is set. If FIFO is disabled, the RXC is set when a character reception is complete. If the FIFO is enabled, the RXC is set when at least RX threshold (CTRLC.RXTRHOLD) data are received.

When the host pulls the SS line high, the transaction is done and the Transmit Complete Interrupt flag in the Interrupt Flag Status and Clear register (INTFLAG.TXC) will be set.

After DATA is written it takes up to three SCK clock cycles until the content of DATA is ready to be loaded into the shift register on the next character boundary. As a consequence, the first character transferred in a SPI transaction will not be the content of DATA. This can be avoided by using the preloading feature. Refer to Preloading the Client Shift Register.

When transmitting several characters in one SPI transaction, the data has to be written into DATA register with at least three SCK clock cycles left in the current character transmission. If this criteria is not met, the previously received character will be transmitted.