ST STM8S Reference Manual

Universal asynchronous receiver transmitter (UART) RM0016
322/449 Doc ID 14587 Rev 8
Note: The lower the f
MASTER
frequency, 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.
22.3.5 Clock deviation tolerance of the UART receiver
The USART’s asynchronous receiver works correctly only if the total clock system deviation
is less 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 < UART receiver’s tolerance
The UART receiver’s tolerance to properly receive data is equal to the maximum tolerated
deviation and depends on the following choices:
● 10- or 11-bit character length defined by the M bit in the UART_CR1 register
● Use of fractional baud rate or not
Table 55. UART receiver tolerance when UART_DIV[3:0] is zero
Table 54. Baud rate programming and error calculation
Baud
rate
f
MASTER
= 10 MHz f
MASTER
= 16 MHz
in
kbps
Actual % Error
(1)
UART_DIV BRR1 BRR2 Actual % Error
(1)
UART_DIV BRR1 BRR2
2.4 2.399 −0.008% 0x1047 0x04 0x17 2.399 −0.005% 0x1A0B 0xA0 0x1B
9.6 9.596 −0.032% 0x0412 0x41 0x02 9.598 −0.020% 0x0693 0x68h 0x03
19.2 19.193 −0.032% 0x0209 0x20 0x09 19.208 −0.040% 0x0341 0x34 0x01
57.6 57.471 −0.224% 0x00AE 0x0A 0x0E 57.554 −0.080% 0x0116 0x11 0x06
115.2 114.942 −0.224% 0x0057 0x05 0x07 115.108 −0.080% 0x008B 0x08 0x0B
230.4 232.558 −0.937% 0x002B 0x02 0x0B 231.884 −0.644% 0x0045 0x04 0x05
460.8 454.545 −1.358% 0x0016 0x01 0x06 457.143 −0.794% 0x0023 0x02 0x03
921.6 NA NA NA NA NA 941.176 −2.124% 0x11 0x01 0x01
1. Error % = (Calculated - Desired) Baud Rate / Desired Baud Rate
M bit NF is an error NF is don’t care
0 3.75% 4.375%
1 3.41% 3.97%

ST STM8S Specifications

General

Brand	ST
Model	STM8S
Category	Microcontrollers
Language	English