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CHAPTER 8 SERIAL INTERFACE FUNCTION
8.2.5 Operation
(1) Data format
Full-duplex serial data is transmitted/received.
One data frame of the transmit/receive data consists of a start bit, character bits, parity bit, and stop bit, as
shown in Figure 8-2.
The length of the character bit, parity, and the length of the stop bit in one data frame are specified by the
asynchronous serial interface mode registers (ASIMn0, ASIMn1) (n = 0, 1).
Figure 8-2. Format of Transmit/Receive Data of Asynchronous Serial Interface
Start bit...................... 1 bit
Character bit ............. 7/8 bits
Parity/extension bit ... Even/odd/0/none/extended bit
Stop bit ...................... 1/2 bits
(2) Transmission
Transmission is started when data is written to the transmit shift registers (TXSn or TXSnL). The next data
is written to the TXSn or TXSnL registers by the service routine of the transmission completion interrupt
processing routine (INTSTn).
(a) Transmission enabled status
Set using the TXEn bit of the ASIMn0 register.
TXEn = 1 : Transmission enabled status
TXEn = 0 : Transmission disabled status
However, to set the transmission enabled status, set both the CTXEn and CRXEn bits of the clocked serial
interface mode register (CSIMn) to “0”.
Because the UARTn does not have a CTS (transmission enabled signal) input pin, use a general input
port when checking whether the other is in the reception enabled status.
(b) Starting transmission
In the transmission enabled status, transmission starts when data is written to the transmission shift
register (TXSn or TXSnL). The transmit data is transferred starting from the start bit with the LSB first.
The start bit, parity/extension bit, and stop bit are automatically appended.
Data cannot be written to the transmission shift register in the transmission disabled status, and values
written are ignored.
D0
D1
D2
D3
D4
D5
D6
D7
Parity/
extension
bit
Stop
bit
Start
bit
Character bit
1 data frame