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Serial I/O
32180 Group User's Manual (Rev.1.0)
12.9 Precautions on Using UART Mode
• Settings of SIO Transmit/Receive Mode Register and SIO Baud Rate Register
The SIO Transmit/Receive Mode Register and SIO Baud Rate Register and the Transmit Control
Register’s BRG count source select bit must always be set before the serial I/O starts operating. If these
settings need to be changed after a transmit or receive operation has started, first check to see that
transmit and receive operations have finished and then clear the transmit and receive enable bits before
making changes.
• Settings of BRG (Baud Rate Register)
If f(BCLK) is selected with the BRG clock source select bit, make sure the value set in the BRG register
is equal to or greater than 7.
Writes to the SIO Baud Rate Register take effect in the next cycle after the BRG counter has finished
counting. However, if the register is accessed for write while transmission and reception are disabled,
the written value takes effect at the same time it is written.
• Transmission/reception using DMA
To transmit/receive data in DMA request mode, enable the DMAC to accept transfer requests (by setting
the DMA Mode Register) before serial communication starts.
• About overrun error
If all bits of the next received data have been set in the SIO Receive Shift Register before reading out the
SIO Receive Buffer Register (i.e., an overrun error occurred), the received data is not stored in the
receive buffer register, with the previous received data retained in it. Once an overrun error occurs,
although a receive operation continues, the subsequent received data is not stored in the receive buffer
register. Before normal receive operation can be restarted, the receive enable bit must be temporarily
cleared. And this is the only way that the overrun error flag can be cleared.
• Flags showing the status of UART receive operation
There are following flags that indicate the status of receive operation during UART mode:
• SIO Receive Control Register receive status bit
• SIO Receive Control Register reception finished bit
• SIO Receive Control Register receive error sum bit
• SIO Receive Control Register overrun error bit
• SIO Receive Control Register parity error bit
• SIO Receive Control Register framing error bit
The manner in which the reception finished bit and various error flags are cleared differs depending on
whether an overrun error occurred, as described below.
[When an overrun error did not occur]
Cleared by reading out the lower byte of the receive buffer register or by clearing the receive enable bit.
[When an overrun error occurred]
Cleared by only clearing the receive enable bit.
12.9 Precautions on Using UART Mode
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