C515A_9708 Datasheet, PDF (107/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
6.3.6
Details about Modes 2 and 3
Eleven bits are transmitted (through TXD), or received (through RXD): a start bit (0), 8 data bits
(LSB first), a programmable 9th data bit, and a stop bit (1). On transmit, the 9th data bit (TB8) can
be assigned the value of 0 or 1. On receive, the 9th data bit goes into RB8 in SCON. The baud rate
is generated by either using timer 1 or the internal baud rate generator.
Figure 6-29 shows a functional diagram of the serial port in modes 2 and 3. The receive portion is
exactly the same as in mode 1. The transmit portion differs from mode 1 only in the 9th bit of the
transmit shift register. The associated timings for transmit/receive are illustrated in figure 6-30.
Transmission is initiated by any instruction that uses SBUF as a destination register. The "Write-to-
SBUF" signal also loads TB8 into the 9th bit position of the transmit shift register and flags the TX
control unit that a transmission is requested. Transmission starts at the next rollover in the divide-
by-16 counter. (Thus, the bit times are synchronized to the divide-by-16 counter, not to the "Write-
to-SBUF" signal.)
The transmission begins with activation of SEND, which puts the start bit at TXD. One bit time later,
DATA is activated, which enables the output bit of the transmit shift register to TXD. The first shift
pulse occurs one bit time after that. The first shift clocks a 1 (the stop bit) into the 9th bit position of
the shift register. Thereafter, only zeroes are clocked in. Thus, as data bits shift out to the right,
zeroes are clocked in from the left. When TB8 is at the output position of the shift register, then the
stop bit is just to the left of TB8, and all positions to the left of that contain zeroes. This condition
flags the TX control unit to do one last shift and then deactivate SEND and set TI. This occurs at
the 11th divide-by-16 rollover after "Write-to-SBUF".
Reception is initiated by a detected 1-to-0 transition at RXD. For this purpose RXD is sampled at a
rate of 16 times whatever baud rate has been established. When a transition is detected, the divide-
by-16 counter is immediately reset, and 1FFH is written to the input shift register.
At the 7th, 8th and 9th counter states of each bit time, the bit detector samples the value of RXD.
The value accepted is the value that was seen in at least 2 of the 3 samples. If the value accepted
during the first bit time is not 0, the receive circuits are reset and the unit goes back to looking for
another 1-to-0 transition. If the start bit proves valid, it is shifted into the input shift register, and
reception of the rest of the frame will proceed.
As data bit come from the right, 1s shift out to the left. When the start bit arrives at the leftmost
position in the shift register (which in modes 2 and 3 is a 9-bit register), it flags the RX control block
to do one last shift, load SBUF and RB8, and to set RI. The signal to load SBUF and RB8, and to
set RI, will be generated if, and only if, the following conditions are met at the time the final shift
pulse is generated:
1) RI = 0, and
2) Either SM2 = 0 or the received 9th data bit = 1
If either of these conditions is not met, the received frame is irretrievably lost, and RI is not set. If
both conditions are met, the received 9th data bit goes into RB8, and the first 8 data bit goes into
SBUF. One bit time later, whether the above conditions were met or not, the unit goes back to
looking for a 1-to-0 transition at the RXD input.
Note that the value of the received stop bit is irrelevant to SBUF, RB8 or RI.
Semiconductor Group
6-56
1997-08-01