AN2354

ST10 ASC peripherals

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For asynchronous operation, the baud rate generator provides a clock with 16 times the rate

of the established baud rate. Every received bit is sampled at the 7th, 8th and 9th cycle of

this clock. The baud rate for asynchronous operation of serial channels and the required

reload value for a given baud rate can be determined by the following formulas:

(SxBRL) represents the content of the reload register, taken as unsigned 13-bit integer and

(SxBRS) represents the value of bit SxBRS (‘0’ or ‘1’), taken as integer.

Using the above equation, the maximum baud rate can be calculated for any given clock

speed. It follows a table of values for baud rate versus reload register value for SxBRS = 0

and SxBRS = 1.

multiple (use of the internal PLL) of an external input clock which is generally different from

the ones used in a PC to generate the UART clock. Therefore, there is a deviation error

between the standard baud rate (reference being the PC) and the baud rate of the ST10

ASCx. This clock mismatch may lead to a reduction of tolerance as discussed in more detail

in

Section 4.1: Arithmetic error in baud rate setting.

Table 2.

Common baud rate settings and deviation error

Baud rate

Deviation error

Reload value

Deviation error

Reload value

2 Mbaud

0%

--

500 Kbaud

0%

--

115.2 Kbaud

+2.12% / -3.55%

+5.22% / -3.55%

57.6 Kbaud

+2.12% / -0.79%

+0.64% / -3.55%

38.4 Kbaud

+0.16% / -1.73%

+2.12% / -0.79%

19.2 Kbaud

+0.16% / -0.79%

+0.64% / -0.79%

9600 baud

+0.16% / -0.32%

+0.64% / -0.08%

4800 baud

+0.16% / -0.08%

+0.28% / -0.08%

2400 baud

+0.04% / -0.08%

+0.10% / -0.08%

1200 baud

+0.04% / -0.02%

+0.01% / -0.08%

600 baud

+0.01% / -0.02%

+0.01% / -0.03%

244 baud

-0.06%

+0.01% / -0.01%

16 x [2 + (SxBRS)] x [(SxBRL) + 1]

SxBRL = (

) - 1