C501_1 Datasheet, PDF (73/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
6.3.5 Details about Mode 1
Ten bits are transmitted (through TxD), or received (through RxD): a start bit (0), 8 data bits (LSB
first), and a stop bit (1). On receive, the stop bit goes into RB8 in SCON. The baud rate is
determined either by the timer 1 overflow rate, or the timer 2 overflow rate, or both (one for transmit
and the other for receive).
Figure 6-21 shows a simplified functional diagram of the serial port in mode 1. The assiociated
timings for transmit receive are illustrated in figure 6-22.
Transmission is initiated by an instruction that uses SBUF as a destination register. The “Write to
SBUF” signal also loads a 1 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.
As data bits shift out to the right, zeroes are clocked in from the left. When the MSB of the data byte
is at the output position of the shift register, then the 1 that was initially loaded into the 9th position
is just to the left of the MSB, 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 10th
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 into the input shift register, and reception
of the rest of the frame will proceed.
The 16 states of the counter divide each bit time into 16ths. 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 latest 2 of the 3 samples. This is done for the noise rejection. 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. This is to provide rejection or false start bits. 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 bits come in from the right, 1s shift out to the left. When the start bit arrives at the leftmost
position in the shift register, (which in mode 1 is a 9-bit register), it flags the RX control block to do
one last shift, load SBUF and RB8, and 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 stop bit = 1
If either of these two condtions is not met, the received frame is irretrievably lost. If both conditions
are met, the stop bit goes into RB8, the 8 data bit goes into SBUF, and RI is activated. At this time,
whether the above conditions are met or not, the unit goes back to looking for a 1-to-0 transition in
RxD.
Semiconductor Group
6-39