1.9.2 Double Byte Transmit

Under certain conditions, a byte written to the TXREG register can be transmitted twice. This happens when a byte is written to TXREG just as the TSR register becomes empty. This new byte is immediately transferred to the TSR register, but also remains in the TXREG register until the completion of the current instruction cycle. If the new byte in the TSR register is transmitted before this instruction cycle has completed, the duplicate in the TXREG register will subsequently be transferred to the TSR register on the following instruction clock cycle and transmitted.

Work around

Method 1:

Monitor the Transmit Interrupt Flag (TXIF) bit. Writes to the TXREG register can be performed once the TXIF bit is set, indicating that the TXREG register is empty. If using this method, ensure that the second byte is filled in the TXREG before bit 6 of the first byte is transmitted. If the delay is more than six bit times, there is a possibility of double byte transmission.

Method 2:

Monitor the TMRT bit of the TXxSTA register. Writes to the TXREG register can be performed once the TMRT bit is set, indicating that the Transmit Shift Register (TSR) is empty. This work around can be applied if back-to-back transmissions are not necessary.

Affected Silicon Revisions