RM0312 datasheet(236/275 Pages) STMICROELECTRONICS

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Universal synchronous/asynchronous receiver transmitter (USART)

RM0312

236/275

DocID022352 Rev 3

Note:

The lower the fMASTER frequency, the lower the accuracy for a particular baud rate.The upper

limit of the achievable baud rate can be fixed with the above data.

21.3.5

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-to-

low transition timing)

DTRA + DQUANT + DREC + DTCL < USART receiver’s tolerance

The USART 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 USART_CR1 register

•

Use of fractional baud rate or not

Table 46. Baud rate programming and error calculation

Baud rate

fMASTER = 16 MHz

In bps

Actual

% Error(1)

1. Error % = (Calculated - Desired) Baud Rate / Desired Baud Rate

USART_DIV

BRR1

BRR2

2.4

0.0

2710h

71h

20h

9.6

0.0

09C4h

9Ch

04h

19.2

0.0

04E2

4Eh

02h

57.6

57.554

-0.08

01A1h

1Ah

01h

115.2

115.385

0.16

00D0h

0Dh

00h

230.4

230.769

0.16

0068h

06h

08h

460.8

461.538

0.16

0034h

03h

04h

921.6

923.077

0.16

001Ah

01h

0Ah

Table 47. USART receiver’s tolerance when USART_DIV[3:0] is 0

M bit

NF is an error

NF is don’t care

3.75%

4.375%

3.41%

3.97%

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