Usart Receiver Tolerance To Clock Deviation - STMicroelectronics STM32F405 Reference Manual

RM0090
Table 142. Error calculation for programmed baud rates at f
Baud rate
S.No Desired
9. 460.8 KBps
10. 921.6 KBps
11. 1.792 MBps
12. 1.8432 MBps
13. 3.584 MBps
14. 3.6864 MBps
15. 7.168 MBps
16. 7.3728 MBps
18. 9 MBps
20. 10.5 MBps
1. The lower the CPU clock the lower the accuracy for a particular baud rate. The upper limit of the achievable baud rate can
be fixed with these data.
2. Only USART1 and USART6 are clocked with PCLK2. Other USARTs are clocked with PCLK1. Refer to the device
datasheets for the maximum values for PCLK1 and PCLK2.
30.3.5

USART receiver tolerance to clock deviation

The USART 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-to-
low transition timing)
DTRA + DQUANT + DREC + DTCL < USART receiver's tolerance
Universal synchronous asynchronous receiver transmitter (USART)
oversampling by 8
Oversampling by 8 (OVER8=1)
f = 42 MHz
PCLK
Value
programme
Actual d in the
baud rate
register
461.538 KBps 11.375
913.043 KBps 5.75
1.826 MBps 2.875
1.826 MBps 2.875
3.5 MBps 1.5
3.82 MBps 1.375
N.A N.A
N.A N.A
N.A N.A
N.A N.A
DocID018909 Rev 11
= 42 MHz or f
PCLK
(1)(2)
(continued)
% Error =
(Calculated -
Actual
Desired)B.Rate
/Desired B.Rate
0.16 461.538 KBps
0.93 923.076 KBps
1.9 1.787Mbps
0.93 1.826 MBps
2.34 3.652 MBps
3.57 3.652 MBps
N.A 7 MBps
N.A 7.636 MBps
N.A 9.333 MBps
N.A 10.5 MBps
= 84 MHz,
PCLK
f = 84 MHz
PCLK
Value
programmed %
in the baud Error
rate register
22.75 0.16
11.375 0.93
5.875 0.27
5.75 0.93
2.875 1.9
2.875 0.93
1.5 2.34
1.375 3.57
1.125 3.7
1 0
981/1731
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