Slave Mode

Slave Mode

Observations:

The ESP32 does not stretch SCL. When the peripherals are configured for SLAVE Mode, data for transmission must be in the fifo before the Master device requests it. If a Master device requests 4 bytes of data from a ESP32 that is configured as SLAVE, and there are only 3 bytes in the txFifo, the ESP32 will send the three bytes of valid data then continue to send 0xFF as long as the master clocks SCL.

Interrupts will be generated in the SLAVE Mode ESP32 both:

• int_status.tx_send_empty whenever the contents of txFifo falls Below fifo_conf.nonfifo_tx_thres
• int_status.tx_fifo_empty whenever the contents of txFifo is Equal or Below fifo_conf.tx_fifo_empty_thrhd

But, neither of these interrupt will cause SCL to be stretched. So data must be added to the txFifo before it is needed else 0xFF with be emitted.

I can envision a couple of methods to work with this limitation:

• Trigger the onRequest() after the Master Read request, to fill the fifo for the next request. This would require a dummy read, then a full read of valid data. The dummy read equivalent to Wire.requestFrom(id,0); would trigger the SLAVE to flush it's txFifo, reload by calling onRequest().
• Coordinate between a Master Write (Wire.onReceive()) and the Master Read (Wire.onRequest()). The Master Write would be considered a command that would trigger onRequest() to fill the txFifo with the 'requested' data. This method is similar to how most I2C devices work. A Write() command is issued to set an 'internal' device pointer. After the 'Write()' command completes, Wire.onReceive() then process the address pointer adjustment, then executes Wire.onRequest() which reloads the txFifo. This pre-staged data is now waiting for the 'Read()' operation. 'Read()' operations could be of infinite length, each time the txFifo reached its threshold values the onRequest() call-back would be executed to refill the txfifo.
• Leave the txFifo empty which cause the peripheral to emit 0xFF, call onRequest(), hope the call returns soon enough to satisfy the Master, Personally I think this last idea is bonkers.