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GD32F403xx User Manual
401
STB[1:0]
stop bit length (bit)
usage description
10
2
normal USART and single-wire modes
11
1.5
Smartcard mode for transmitting and receiving
In an idle frame, all the frame bits are logic 1. The frame length is equal to the normal USART
frame.
The break frame structure is a number of low bits followed by the configured number of stop
bits. The transfer speed of a USART frame depends on the frequency of the UCLK and the
configuration of the baud rate generator.
17.3.2.
Baud rate generation
The baud-rate divider is a 16-bit number which consists of a 12-bit integer and a 4-bit
fractional part. The number formed by these two values is used by the baud rate generator to
determine the bit period. Having a fractional baud-rate divider allows the USART to generate
all the standard baud rates.
When oversampled by 16, the baud-rate divider (USARTDIV) has the following relationship
with the UCLK:
USARTDIV=
UCLK
16×Baud Rate
(16-1)
For example, when oversampled by 16:
1.
Get USARTDIV by caculating the value of USART_BUAD:
If USART_BUAD=0x21D, then INTDIV=33 (0x21), FRADIV=13 (0xD).
USARTDIV=33+13/16=33.81.
2.
Get the value of USART_BUAD by calculating the value of USARTDIV:
If USARTDIV=30.37, then INTDIV=30 (0x1E).
16*0.37=5.92, the nearest integer is 6, so FRADIV=6 (0x6).
USART_BUAD=0x1E6.
Note:
If the roundness of FRADIV is 16 (overflow), the carry must be added to the integer
part.
17.3.3.
USART transmitter
If the transmit enable bit (TEN) in USART_CTL0 register is set, when the transmit data buffer
is not empty, the transmitter shifts out the transmit data frame through the TX pin. The polarity
of the TX pin can be configured by the TINV bit in the USART_CTL3 register. Clock pulses
can be output through the CK pin.
After the TEN bit is set, an idle frame will be sent. The TEN bit should not be cleared while
the transmission is ongoing.
After power on, the TBE bit is high by default. Data can be written to the USART_DATA when