NEC PD17062 - Manual (Page 213)

213
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PD17062
(2) For transmission in two-wire bus mode (SIO0TX = 1)
In this case, the contents of an acknowledgement received from the receiver side are set in the SBACK
flag. This means that the acknowledge state of the receiver side can be determined simply by reading
the contents of the SBACK flag.
This examining of the SBACK flag must be done after the 9th bit of 1-byte is set but before the 9th bit of
the next data is set. Normally, waiting is instigated at the falling edge of the 9th bit. Therefore, the SBACK
flag must be read at this time.
Data can be written into the SBACK flag by executing a POKE instruction, even during transmission.
(3) In serial I/O mode
In this case, the contents of the SBACK flag are not influenced by the shift clock. In other words, the SBACK
flag is completely isolated from the serial interface. Hence, SBACK can be used as a 1-bit flag for data
storage.