TL16C552A, TL16C552AM
DUAL ASYNCHRONOUS COMMUNICATIONS ELEMENT
WITH FIFO
SLLS189D – NOVEMBER 1994 – REVISED JANUARY 1999
PRINCIPLES OF OPERATION
parallel port registers
The TL16C552A parallel port can connect the device to a Centronic-style printer interface. When chip select 2
(CS2) is low, the parallel port is selected. Table 12 shows the registers associated with this parallel port. The
read or write function of the register is controlled by the state of the read (IOR) and write (IOW) terminals as
shown. The read data register allows the microprocessor to read the information on the parallel bus.
The read status register allows the microprocessor to read the status of the printer in the six most significant
bits. ThestatusbitsareprinterbusyBSY, acknowledge(ACK)(ahandshakefunction), paperempty(PE), printer
selected (SLCT), error (ERR), and printer interrupt (PRINT). The read control register allows the state of the
control lines to be read. The write control register sets the state of the control lines. They are direction (DIR),
interrupt enable (INT2 EN), select in (SLIN), initialize the printer (INIT), autofeed the paper (AFD), and strobe
(STB), which informs the printer of the presence of a valid byte on the parallel bus. The write data register allows
the microprocessor to write a byte to the parallel bus. The parallel port is completely compatible with the parallel
port implementation used in the IBM serial parallel adapter.
Table 12. Parallel Port Registers
REGISTER BITS
REGISTER
BIT 7
PD7
BSY
0
BIT 6
PD6
ACK
0
BIT 5
PD5
PE
BIT 4
BIT 3
BIT 2
PD2
BIT 1
PD1
1
BIT 0
PD0
1
Read data
PD4
PD3
Read status
Read control
Write data
SLCT
ERR
SLIN
PD3
PRINT
INIT
PEMD • DIR INT2 EN
AFD
PD1
AFD
STB
PD0
STB
PD7
0
PD6
0
PD5
DIR
PD4
PD2
Write control
INT2 EN
SLIN
INIT
Table 13. Parallel Port Register Select
CONTROL PINS
REGISTER SELECTED
IOR
L
IOW
H
H
H
H
L
CS2
L
A1
L
A0
L
Read data
Read status
Read control
Invalid
L
L
L
H
L
L
L
H
H
L
L
L
H
L
H
H
H
H
L
Write data
Invalid
L
L
L
H
L
L
L
H
H
Write control
Invalid
L
L
H
programmable baud rate generator
The ACE serial channel contains a programmable baud rate generator (BRG) that divides the clock (dc to
16
8 MHz) by any divisor from 1 to (2 –1). The output frequency of the baud generator is 16x the data rate [divisor
# = clock ÷ (baud rate x 16)], referred to in this document as RCLK. Two 8-bit divisor latch registers store the
divisor in a 16-bit binary format. These divisor latch registers must be loaded during initialization. Upon loading
either of the divisor latches, a 16-bit baud counter is immediately loaded. This prevents long counts on initial
load. The BRG can use any of three different popular frequencies to provide standard baud rates. These
frequencies are 1.8432 MHz, 3.072 MHz, and 8 MHz. With these frequencies, standard bit rates from 50 to
512 kbps are available. Tables 14, 15, 16, and 17 illustrate the divisors needed to obtain standard rates using
these three frequencies.
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