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ST92F124/F150/F250 - KNOWN LIMITATIONS

13.8.9 SCI-A wrong break duration

Description

A single break character is sent by setting and re-

setting the SBK bit in the SCICR2 register. In

some cases, the break character may have a

longer duration than expected:

- 20 bits instead of 10 bits if M=0

- 22 bits instead of 11 bits if M=1.

In the same way, as long as the SBK bit is set,

break characters are sent to the TDO pin. This

may lead to generate one break more than ex-

pected.

Occurrence

The occurrence of the problem is random and pro-

portional to the baudrate. With a transmit fre-

qu en cy of 1 9 2 0 0 ba ud ( f CPU =8MH z and

SCIBRR=0xC9), the wrong break duration occur-

rence is around 1%.

Workaround

If this wrong duration is not compliant with the

communication protocol in the application, soft-

ware can request that an Idle line be generated

before the break character. In this case, the break

duration is always correct assuming the applica-

tion is not doing anything between the idle and the

break. This can be ensured by temporarily disa-

bling interrupts.

The exact sequence is:

- Disable interrupts

- Reset and Set TE (IDLE request)

- Set and Reset SBK (Break Request)

- Re-enable interrupts

LIN mode (if available)

If the LINE bit in the SCICR3 is set and the M bit in

the SCICR1 register is reset, the SCI-A is in LIN

master mode. A single break character is sent by

setting and resetting the SBK bit in the SCICR2

have a longer duration than expected:

- 24 bits instead of 13 bits

ST92F124 Datasheet(PDF) 426 Page - STMicroelectronics