STM32F101xx, STM32F102xx, STM32F103xx, STM32F105xx and ...

Universal synchronous asynchronous receiver transmitter (USART)
RM0008
The nearest real number is 0d16 = 0x10 => overflow of DIV_frac[3:0] => carry must be
added up to the mantissa
DIV_Mantissa = mantissa (0d50.990 + carry) = 0d51 = 0x33
Then, USART_BRR = 0x330 hence USARTDIV = 0d51.000
Table 174.
Error calculation for programmed baud rates
Baud rate f PCLK = 36 MHz f PCLK = 72 MHz
S.No
in
Kbps
Actual
Value
programmed
in the Baud
Rate register
% Error
=(Calculated -
Desired)B.Rate
/Desired B.Rate
Actual
Value
programmed
in the Baud
Rate register
% Error
1. 2.4 2.400 937.5 0% 2.4 1875 0%
2. 9.6 9.600 234.375 0% 9.6 468.75 0%
3. 19.2 19.2 117.1875 0% 19.2 234.375 0%
4. 57.6 57.6 39.0625 0% 57.6 78.125 0.%
5. 115.2 115.384 19.5 0.15% 115.2 39.0625 0%
6. 230.4 230.769 9.75 0.16% 230.769 19.5 0.16%
7. 460.8 461.538 4.875 0.16% 461.538 9.75 0.16%
8. 921.6 923.076 2.4375 0.16% 923.076 4.875 0.16%
9. 2250 2250 1 0% 2250 2 0%
10. 4500 NA NA NA 4500 1 0%
Note: 1 The lower the CPU clock the lower will be the accuracy for a particular Baud rate. The upper
limit of the achievable baud rate can be fixed with this data.
2 Only USART1 is clocked with PCLK2 (72 MHz Max). Other USARTs are clocked with
PCLK1 (36 MHz Max).
25.3.5 USART receiver’s tolerance to clock deviation
The USART’s asynchronous receiver works correctly only if the total clock system deviation
is smaller than the USART receiver’s tolerance. The causes which contribute to the total
deviation are:
● DTRA: Deviation due to the transmitter error (which also includes the deviation of the
transmitter’s local oscillator)
●
●
DQUANT: Error due to the baud rate quantization of the receiver
DREC: Deviation of the receiver’s local oscillator
● DTCL: Deviation due to the transmission line (generally due to the transceivers which
can introduce an asymmetry between the low-to-high transition timing and the high-tolow
transition timing)
DTRA + DQUANT + DREC + DTCL < USART receiver’s tolerance
666/995 Doc ID 13902 Rev 9