• The HCS08 has a double-buffered receiver; the M68HC08 does not.
• The M68HC08 Family has an interruptible error flag for receiver overruns (when a new byte is received before the previous byte has been read). The HCS08 Family has a double-buffered receiver and therefore does not require a flag or interrupt for this condition.
Because the transmitter and receiver are double buffered, a second byte, in addition to the byte currently being shifted out, can be queued into the transmit data buffer, and a previously received character can be in the receive data buffer while a new character is being shifted in. The SPTEF flag indicates when the transmit buffer has room for a new character. The SPRF flag indicates when a received character is available in the receive data buffer. The received character must be read out of the receive buffer (read SPID) before the next transfer is finished or a receive overrun error results. In the case of a receive overrun, the new data is lost because the receive buffer still held the previous character and was not ready to accept the new data. There is no indication for such an overrun condition so the application system designer must ensure that previous data has been read from the receive buffer before a new transfer is initiated.
I would be interested to hear if others agree or disagree with my analysis, and whether I have correctly interpreted the data sheet information.
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