ST20-GP6 datasheet(85/123 Pages) STMICROELECTRONICS

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ST20-GP6

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The most signiﬁcant bit of each rxﬁfo entry (rxﬁfo[x][9]) records whether or not there was a frame

error when that entry was received (i.e. one of the effective stop bit values was ’0’).

ASCSta-

tus(FrameError) is set when at least one of the valid entries in the rxﬁfo has its MSB set.

If the mode is one where a parity bit is expected, then the next bit, rxﬁfo[x][8], records whether

there was a parity error when that entry was received. Note, it does not contain the parity bit that

was received.

ASCStatus(ParityError) is set when at least one of the valid entries in the rxﬁfo has

bit 8 set.

After changing the ﬁfoenable bit, it is important to reset the ﬁfo to empty (by writing to the

ASCRxReset register), since the state of the ﬁfo pointers may be garbage.

Reception is stopped by clearing the

ASCControl(RxEnable) bit. A currently received frame is

completed including the generation of the receive status ﬂags. Start bits that follow this frame will

not be recognized.

15.2 Timeout mechanism

The ASC contains an 8-bit timeout counter. This reloads from

ASCTimeout whenever one or more

of the following is true

•

ASCRxBuffer is read

• The ASC is in the middle of receiving a character

•

ASCTimeout is written to

If none of these conditions hold, the counter decrements towards 0 at every baud rate tick.

ASCStatus(TimeoutNotEmpty) is’1’ exactly whenever the rxﬁfo is not empty and the timeout

counter is zero.

ASCStatus(TimeoutIdle) is ‘1’ exactly whenever the rxﬁfo is empty and the timeout counter is

zero.

The effect of this is that whenever the rxﬁfo has got something in it, the timeout counter will decre-

ment until something happens to the rxﬁfo. If nothing happens, and the timeout counter reaches

zero, the

ASCStatus(TimeoutNotEmpty) ﬂag will be set.

When the software has emptied the rxﬁfo, the timeout counter will reset and start decrementing. If

no more characters arrive, when the counter reaches zero the

ASCStatus(TimeoutIdle) ﬂag will

be set.

15.3 Baud rate generation

The baud rate generator provides a clock at 16 times the baud rate, called the oversampling clock.

This clock only ticks if

ASCControl(Run) is set to’1’. Setting this bit to 0 will immediately freeze the

state of the ASCs transmitter and receiver. This should only be done when the ASC is idle.

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