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7470/7471/7477/7478 GROUP USER’S MANUAL
HARDWARE
1.13 Serial I/O
1.13B.2 Pins
The 7477/7478 group uses 4 pins for data transmit, data receive, shift clock transmit/receive and serial
I/O transfer ready signal output. All these pins are used in common with P1. A function selection is made
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by the serial I/O enable bit (bit 7) and the SRDY output enable bit (bit 2) of the Serial I/O control register.
The function of each pin is explained below.
(1) Data transmit pin[TxD]
Transmit data is output bit by bit. This pin is used in common with P15. When the transfer enable
bit and the serial I/O enable bit of the Serial I/O control register is set to “1,” this pin becomes a serial
I/O data output pin.
(2) Data receive pin [RxD]
Data is input bit by bit. This pin is used in common with P14. When the receive enable bit and the
serial I/O enable bit of serial I/O control register are set to “1,” this pin becomes a serial I/O data
input pin.
(3) Shift clock transmit/receive pin [SCLK]
2 Clock synchronous
This pin inputs (receives from the outside) or outputs (supplies to the outside) the synchronous
clock for data transmit/receive.
When the serial I/O synchronous clock selection bit (bit 1) of the Serial I/O control register is
cleared to “0” (use of internal clock), the synchronous clock is output.
When the same bit is set to “1” (use of internal clock), the synchronous clock is input from the
outside.
2 Clock asynchronous (UART)
When the serial I/O synchronous clock selection bit (bit 1) of the Serial I/O control register is set
to “1” (use of external clock), the synchronous clock is supplied from the outside.
When the same bit is cleared to “0” (use of internal clock), this pin does not function.
Note: When the internal clock is selected, SCLK pin can be used as port P16.
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(4) Serial transfer enable signal output pin [SRDY]
This pin informs the outside of a receive enable state in the clock synchronous serial I/O.
In case of the UART, this pin does not function.
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q SRDY signal output enable bit (bit 2) of Serial I/O control register is set to “1.”
q Transmit enable bit (bit 4) of Serial I/O control register is set to “1.”
When the above 2 conditions are satisfied, the level of the pin changes from “H” to “L” at the timing
at which data was written into the Transmit buffer register, informing the outside of a serial transfer
enable state.