Fractional Baud Rate Generation - ST STM32F101 series Reference Manual

Universal synchronous asynchronous receiver transmitter (USART)
Configurable stop bits during reception
The number of stop bits to be received can be configured through the control bits of Control
Register 2 - it can be either 1 or 2 in normal mode and 0.5 or 1.5 in Smartcard mode.
1. 0.5 stop bit (reception in Smartcard mode): No sampling is done for 0.5 stop bit. As
a consequence, no framing error and no break frame can be detected when 0.5 stop bit
is selected.
2. 1 stop bit: Sampling for 1 stop Bit is done on the 8th, 9th and 10th samples.
3. 1.5 stop bits (Smartcard mode): When transmitting in smartcard mode, the device
must check that the data is correctly sent. Thus the receiver block must be enabled (RE
=1 in the USART_CR1 register) and the stop bit is checked to test if the smartcard has
detected a parity error. In the event of a parity error, the smartcard forces the data
signal low during the sampling - NACK signal-, which is flagged as a framing error.
Then, the FE flag is set with the RXNE at the end of the 1.5 stop bit. Sampling for 1.5
stop bits is done on the 16th, 17th and 18th samples (1 baud clock period after the
beginning of the stop bit). The 1.5 stop bit can be decomposed into 2 parts: one 0.5
baud clock period during which nothing happens, followed by 1 normal stop bit period
during which sampling occurs halfway through. Refer to
page 801
4. 2 stop bits: Sampling for 2 stop bits is done on the 8th, 9th and 10th samples of the
first stop bit. If a framing error is detected during the first stop bit the framing error flag
will be set. The second stop bit is not checked for framing error. The RXNE flag will be
set at the end of the first stop bit.
27.3.4

Fractional baud rate generation

The baud rate for the receiver and transmitter (Rx and Tx) are both set to the same value as
programmed in the Mantissa and Fraction values of USARTDIV.
USARTDIV is an unsigned fixed point number that is coded on the USART_BRR register.
Note: The baud counters are updated with the new value of the Baud registers after a write to
USART_BRR. Hence the Baud rate register value should not be changed during
communication.
How to derive USARTDIV from USART_BRR register values
Example 1:
If DIV_Mantissa = 0d27 and DIV_Fraction = 0d12 (USART_BRR = 0x1BC), then
Mantissa (USARTDIV) = 0d27
Fraction (USARTDIV) = 12/16 = 0d0.75
Therefore USARTDIV = 0d27.75
791/1128
for more details.
f
Tx/ Rx baud =
(16*USARTDIV)
legend: f - Input clock to the peripheral (PCLK1 for USART2, 3, 4, 5 or PCLK2 for USART1)
CK
DocID13902 Rev 15
CK
Section 27.3.11: Smartcard on
RM0008