UARTE in semi-duplex mode

Question

Hi all, 
 I would like to have a semi-duplex communication link over one pin (+GND). The nRF will act as a pure slave device which responds to, and acknowledges, commands from a "master".Thus: 
 1. I set UARTE in RX mode. PSEL.RXD is set to the (shared) pin. 
 2. A command is successfully received from the master. 
 3. PSEL.RXD = 0xffffffff, PSEL.TXD s set to the (shared) pin. 
 4. A reply is successfully sent to the master. 
 5. Back to 1 
 The problem is that this only works two times. Then I get an "overrun" error. I've tried "EVERYTHING" to prevent this from happening; delays in various places, different sequences of pin role swithing (when the UARTE module is disabled) and trying to run STOPRX (according to the 35.4 UARTE-Reception section of the device datasheet) in order to properly empty the RX buffer. 
 
 Any ideas ? Please help ! 
 
 Cheers 
 Eric

hmolesworth · Answer

Hey Eric, just to clarify I didn't mean use a separate pin for the Rx data, just a separate pin to "park" Rx in a input-high-via-pull-up state while transmitting on Tx. Any input pin will do even if used for something else as long as you can avoid things happening during that Rx interval. For example you might try the 32kHz crystal output drive pin P0.1; I haven't tried this yet but considered it for a similar issue a while ago. It all hinges whether the pull-up is applied to the pin-side or input buffer-side of the input FET switch. 
 Ok I just looked at the crystal pins as that might be a special case as it doesn't seem to work, but other pins look ok, eg SDA just avoid I2C transfers during Tx activity while Rx is parked 
 It works!! 
 // Park Rx pin somewhere safe with a known '1' input level
 NRF_UARTE0->PSEL.RXD = PIN_I2C_SCL;

// Transmit some data, be sure last tx byte has cleared the pin

 if (mTxDone)
 {
 // Revert Parked Rx pin here or in interrupt (not both)
 NRF_UARTE0->PSEL.RXD = PIN_UART_RX;
 mTxDone = false;
 } 
 To be sure the last byte has cleared the Tx pin before turning on Rx again: 
 case APP_UART_TX_EMPTY: // Event UART has completed transmission of all available data in the TX FIFO
 // Revert Parked Rx pin
 NRF_UARTE0->PSEL.RXD = PIN_UART_RX;
or set a flag:
 mTxDone = true;
 break;

UARTE in semi-duplex mode

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