Slave Select Synchronization

The Slave Select can also be used to synchronize communication. The Slave Select line is held high until the master device is ready to communicate. When the Slave Select line is pulled low, the slave knows that a new transmission is starting.

If the slave fails to receive the communication properly, it will be reset at the end of the transmission, when the Slave Select line returns to a high state. The slave is then ready to receive a new transmission when the Slave Select line is pulled low again. If the Slave Select line is not used, there is a risk that the slave will eventually become out of sync with the master. If the slave misses a bit, it will always be one bit off in future transmissions. Use of the Slave Select line allows the slave and master to align themselves at the beginning of each transmission.

The SS pin allows a Synchronous Slave mode. The SPI must be in Slave mode with SS pin control enabled (SSPM = 0100).

When the SS pin is low, transmission and reception are enabled and the SDO pin is driven.

When the SS pin goes high, the SDO pin is no longer driven, even if in the middle of a transmitted byte and becomes a floating output. External pull-up/pull-down resistors may be desirable depending on the application.

  1. 1.When the SPI is in Slave mode with SS pin control enabled (SSPM = 0100), the SPI module will reset if the SS pin is set to VDD.
  2. 2.When the SPI is used in Slave mode with CKE set; the user must enable SS pin control.
  3. 3.While operated in SPI Slave mode the SMP bit must remain clear.

When the SPI module resets, the bit counter is forced to ‘0’. This can be done by either forcing the SS pin to a high level or clearing the SSPEN bit.

Figure 1. Slave Select Synchronous Waveform
Figure 2. SPI Mode Waveform (Slave Mode with CKE = 0)
Figure 3. SPI Mode Waveform (Slave Mode with CKE = 1)