SWCU193A April 2023 – August 2024 CC2340R2 , CC2340R5 , CC2340R5-Q1
The SPI module is compatible with the Texas Instruments Synchronous Serial frame format.
Figure 20-6 shows the TI synchronous serial frame format for a single and continuous transmitted frame.
SCLK and CS are forced low and the transmit data line PICO is put in tristate whenever the SPI is idle. When the TX FIFO contains data, CS is pulsed high for one SCLK period. The transmitted value is also transferred from the TX FIFO to the serial shift register of the transmit logic. On the next rising edge of SCLK, the MSB of the 4- to 16-bit data frame is shifted out on the PICO pin. Likewise, the MSB of the received data is shifted onto the POCI pin by the off-chip serial peripheral device. Both the SPI and the off-chip serial peripheral device then clock each data bit into their serial shifter on each falling edge of SCLK. The received data is transferred from the serial shifter to the RX FIFO on the first rising edge of SCLK after the least significant bit (LSB) is latched.
The serial clock (SCLK) is held inactive while the SPI is idle and SCLK transitions at the programmed frequency only during active transmission or reception of data. The IDLE state of SCLK provides a receive time-out indication that occurs when the RX FIFO still contains data after a time-out period.
When configured as a peripheral in TI Synchronous frame format, the off-chip controller device has to ensure that spurious pulses are not driven on the SCLK when CS is high.
This can result in the first data frame getting transmitted incorrectly by the peripheral.