14
FN2958.6
August 19, 2015
Transmitter
The serial transmitter section consists of a Transmitter
Holding Register (THR), Transmitter Shift Register (TSR),
and associated control logic. The Transmitter Holding
Register Empty (THRE) and Transmitter Shift Register
Empty (TEMT) are two bits in the Line Status Register which
indicate the status of THR and TSR. To transmit a 5-8 bit
word, the word is written through D0-D7 to the THR. The
microprocessor should perform a write operation only if
THRE is high. The THRE is set high when the word is
automatically transferred from the THR to the TSR during
the transmission of the start bit.
When the transmitter is idle, both THRE and TEMT are high.
The first word written causes THRE to be reset to 0. After
completion of the transfer, THRE returns high. TEMT
remains low for at least the duration of the transmission of
the data word. If a second character is transmitted to the
THR, the THRE is reset low. Since the data word cannot be
transferred from the THR to the TSR until the TSR is empty,
THRE remains low until the TSR has completed
transmission of the word. When the last word has been
transmitted out of the TSR, TEMT is set high. THRE is set
high one THR to TSR transfer time later.
Receiver
Serial asynchronous data is input into the SIN pin. The idle
state of the line providing the input into SIN is high. A start bit
detect circuit continually searches for a high to low transition
from the idle state. When the transition is detected, a counter
is reset, and counts the 16X clock to 7 1/2, which is the
center of the start bit. The start bit is valid if the SIN is still
low at the mid bit sample of the start bit. Verifying the start bit
prevents the receiver from assembling an incorrect data
character due to a low going noise spike on the SIN input.
The Line Control Register determines the number of data
bits in a character (LCR(0), LCR(1)), number of stop bits
LCR(2), if parity is used LCR(3), and the polarity of parity
LCR(4). Status information for the receiver is provided in the
Line Status Register. When a character is transferred from
the Receiver Shift Register to the Receiver Buffer Register,
the Data Received indication in LSR(0) is set high. The CPU
reads the Receiver Buffer Register through D0-D7. This read
resets LSR(0). If D0-D7 are not read prior to a new character
transfer from the RSR to the RBR, the overrun error status
indication is set in LSR(1). The parity check tests for even or
odd parity on the parity bit, which precedes the first stop bit.
If there is a parity error, the parity error is set in LSR (2).
There is circuitry which tests whether the stop bit is high. If it
is not, a framing error indication is generated in LSR(3).
The center of the start bit is defined as clock count 7 1/2. If
the data into SIN is a symmetrical square wave, the center of
the data cells will occur within 3.125% of the actual center,
providing an error margin of 46.875%. The start bit can begin
as much as one 16X clock cycle prior to being detected.
Baud Rate Generator (BRG)
The BRG generates the clocking for the UART function,
providing standard ANSI/CCITT bit rates. The oscillator
driving the BRG may be provided either with the addition of
an external crystal to the XTAL1 and XTAL2 inputs, or an
external clock into XTAL1. In either case, a buffered clock
output, BAUDOUT
, is provided for other system clocking. If
two 82C50As are used on the same board, one can use a
crystal, and the buffered clock output can be routed directly
into the XTAL1 of the second 82C50A.
The data rate is determined by the Divisor Latch registers
DLL and DLM and the external frequency or crystal input,
with the BAUDOUT
providing an output 16X the data rate.
The bit rate is selected by programming the two divisor
latches, Divisor Latch Most Significant Byte and Divisor
Latch Least Significant Byte. Setting DLL = 1 and DLM = 0
selects the divisor to divide by 1 (divide by 1 gives maximum
baud rate for a given input frequency at XTAL1). The on-chip
oscillator is optimized for a 10MHz crystal. Usually, higher
frequency are less expensive than lower frequency crystals.
The BRG can use any of three different popular crystals to
provide standard baud rates. The frequency of these three
common crystals on the market are 1.8432MHz,
2.4576MHz, and 3.072MHz. With these standard crystals,
standard bit rates from 50 to 38.5kbps are available. The
following tables illustrate the divisors needed to obtain
standard rates using these three crystal frequencies.
TABLE 4. BAUD RATES USING 1.8432MHz CRYSTAL
DESIRED
BAUD
RATE
DIVISOR USED TO
GENERATE
16 x CLOCK
PERCENT ERROR
DIFFERENCE BETWEEN
DESIRED AND ACTUAL
50 2304 -
75 1536 -
110 1047 0.026
134.5 857 0.058
150 768 -
300 384 -
600 192 -
1200 96 -
1800 64 -
2000 58 0.69
2400 48 -
3600 32 -
4800 24 -
7200 16 -
9600 12 -
19200 6 -
38400 3 -
56000 2 2.86
82C50A
