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SERIAL I/O

7721 Group User’s Manual

11–34

11.4 Clock asynchronous serial I/O (UART) mode

11.4.3 Method of transmission

Figure 11.4.3 shows an initial setting example for relevant registers when transmitting.

The difference due to selection of transfer data length (7 bits, 8 bits, or 9 bits) is only that data length.

When selecting a 7- or 8-bit data length, set the transmit data into the low-order byte of the UARTi transmit

buffer register. When selecting a 9-bit data length, set the transmit data into the low-order byte and bit 0

of the high-order byte.

Transmission is started when all of the following conditions ( to ) are satisfied:

Transmit is enabled (transmit enable bit = “1”).

Transmit data is present in the UARTi transmit buffer register (transmit buffer empty flag = “0”).

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The CTSi pin’s input is at “L” level (when the CTS function selected).

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Note: When the CTS function is not selected, condition is ignored.

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By connecting the RTSi pin (receiver side) and CTSi pin (transmitter side), the timing of transmission and

that of reception can be matched. For details, refer to section “11.4.6 Receive operation.”

When using interrupts, it is necessary to set the relevant registers to enable interrupts. For details, refer

to “CHAPTER 7. INTERRUPTS.”

Figure 11.4.4 shows writing data after start of transmission, and Figure 11.4.5 shows detection of transmit

completion.

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