UPSD3254 STMICROELECTRONICS [STMicroelectronics], UPSD3254 Datasheet - Page 65

• Manufacturer Part Number: UPSD3254
• Description: Flash Programmable System Devices with 8032 Microcontroller Core
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

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, zeros 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 zeros. This condition flags the
TX Control unit to do one last shift and then deac-
tivate SEND and set TI. This occurs at the 10th di-
vide-by-16 rollover after “WRITE to SBUF.”
Reception is initiated by a detected 1-to-0 transi-
tion at RxD. For this purpose RxD is sampled at a
rate of 16 times whatever baud rate has been es-
tablished. When a transition is detected, the di-
vide-by-16 counter is immediately reset, and 1FFH
is written into the input shift register. Resetting the
divide-by-16 counter aligns its roll-overs with the
boundaries of the incoming bit times.
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 least 2 of the 3 samples. This is done for
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 an-other 1-to-
0 transition. This is to provide rejection of false
start bits. If the start bit proves valid, it is shifted
into the input shift register, and reception of the re-
set 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 left-most
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 generat-
ed:
1. R1 = 0, and
2. Either SM2 = 0, or the received Stop Bit = 1.
If either of these two conditions is not met, the re-
ceived frame is irretrievably lost. If both conditions
are met, the Stop Bit goes into RB8, the 8 data bits
go 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.
More About Modes 2 and 3. Eleven
transmitted (through TxD), or received (through
RxD): a Start Bit (0), 8 data bits (LSB first), a pro-
grammable 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 data bit goes
into RB8 in SCON. The baud rate is programma-
UPSD3254A, UPSD3254BV, UPSD3253B, UPSD3253BV
bits
are
ble to either 1/16 or 1/32 the CPU clock frequency
in Mode 2. Mode 3 may have a variable baud rate
generated from Timer 1.
Figure 31, page 68 and Figure 33, page 69 show
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.
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 po-
sition of the transmit shift register and flags the TX
Control unit that a transmission is requested.
Transmission commences at S1P1 of the machine
cycle following the next roll-over 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. There-after, only zeros are
clocked in. Thus, as data bits shift out to the right,
zeros are clocked in from the left. When TB8 is at
the out-put 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 zeros. This condition flags
the TX Control unit to do one last shift and then de-
activate SEND and set TI. This occurs at the 11th
divide-by 16 rollover after “WRITE to SUBF.”
Reception is initiated by a detected 1-to-0 transi-
tion at RxD. For this purpose RxD is sampled at a
rate of 16 times whatever baud rate has been es-
tablished. When a transition is detected, the di-
vide-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 R-D.
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 an-
other 1-to-0 transition. If the Start Bit proves valid,
it is shifted into the input shift register, and recep-
tion 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 left-most
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 set
RI.
The signal to load SBUF and RB8, and to set RI,
will be generated if, and only if, the following con-
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