C517A_99 Datasheet, PDF (156/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C517A
As data bits come from the right, 1’s shift out to the left. When the start bit arrives at the leftmost
position in the shift register (which is a 9-bit register), it flags the RX control block to do one last shift,
load S0BUF/S1BUF and RB80/ RB81, and set RI0/RI1. The signal to load S0BUF/S1BUF and
RB80/RB81, and to set RI0/RI1, will be generated if, and only if, the following conditions are met at
the time the final shift pulse is generated:
1) RI0/RI1 = 0, and
2) either SM20/SM21 = 0 or the received 9th data bit = 1
lf either one of these two conditions is not met, the received frame is irretrievably lost, and RI0/Rl1
is not set. lf both conditions are met, the received 9th data bit goes into RB80/RB81, the first 8 data
bits go into S0BUF/S1BUF. One bit time later, no matter whether the above conditions are met or
not, the unit goes back to look for a 1-to-0 transition at the RXD0/RXD1 input.
Note that the value of the received stop bit is irrelevant to S0BUF/S1BUF, RB80/RB81, or RI0/Rl1.
Semiconductor Group
6-90