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CHAPTER 15 UART1
15.5.3
Baud rate by external clock
This section explains the setting when selecting the external clock as the transmit/
receive clock of the UART1.
I Baud rate by external clock
To select a baud rate by the external clock input, the following settings are essential:
• Set the CS2 to CS0 bits in the serial mode register (SMR1 register bit 5 to 3: CS2 to CS0) to "111
• Set the SCK1 pin as the input port in the port direction register (DDR).
• Set the serial dock I/O enable bit (SMR1 register bit 1: SCKE) to "0".
• Set the baud rate on the basis of the external clock input from the SCK1 pin.Since the internal division
ratio is fixed, the cycle of the external input clock must be changed when changing the baud rate.
G
Calculation expression for baud rate
Asynchronous baud rate = f/16 bps
Clock synchronous baud rate = φ bps
φ:External clock frequency (2 MHz max.)
460
Figure 15.5-4 Baud Rate Selector by External Clock
SMR1: CS2 to CS0 = "111
(Clock input source select bit)
SCK1
1/1 [Clock synchronous]
Pin
1/16 [Asynchronous]
(Operating mode select bit)
"
B
Clock selector
SMR1: MD1, MD0
Baud rate
".
B
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