ST16C552 Exar Corporation, ST16C552 Datasheet - Page 17

• Manufacturer Part Number: ST16C552
• Description: DUAL UART WITH 16-BYTE FIFO AND PARALLEL PRINTER PORT
• Manufacturer: Exar Corporation

Note 1* The General Register set is accessible only when CS A or CS B is a logic 0.
Note 2* The Baud Rate register set is accessible only when CS A or CS B is a logic 0 and LCR bit-7 is a logic 1.
Note 3*: Printer Port Register set is accessible only when -CSP is a logic 0 in conjunction with the states of the
interface signal BIDEN and Printer Control Register bit-5 or IOSEL register.
Note 5* The value between the square brackets represents the register’s initialized HEX value, X =N/A.
MODEM (UART) REGISTER DESCRIPTIONS
Transmit (THR) and Receive (RHR) Holding Reg-
isters
The serial transmitter section consists of an 8-bit
Transmit Hold Register (THR) and Transmit Shift
Register (TSR). The status of the THR is provided in
the Line Status Register (LSR). Writing to the THR
transfers the contents of the data bus (D7-D0) to the
THR, providing that the THR or TSR is empty. The
THR empty flag in the LSR register will be set to a logic
1 when the transmitter is empty or when data is
transferred to the TSR. Note that a write operation can
be performed when the transmit holding register
empty flag is set (logic 0 = FIFO full, logic 1= at least
Rev. 3.40
A2 A1 A0
[X] 0
[X] 0
[X] 0
[X] 0
[X] 1
[X] 1
Printer Port Register Set: Note 3*
0
0
1
1
0
0
Register
[Default]
COM[E0]
CON[00]
Note 5*
SR[4F]
PR[00]
PR[00]
IOSEL
BIT-7
-Busy
bit-7
bit-7
bit-7
logic
“1”
[X]
BIT-6
-ACK
logic
bit-6
bit-6
bit-6
“1”
[X]
IN/OUT
PD 0-7
BIT-5
bit-5
bit-5
bit-5
logic
PE
“1”
17
one FIFO location available).
The serial receive section also contains an 8-bit
Receive Holding Register, RHR. Receive data is
removed from the 552/552A and receive FIFO by
reading the RHR register. The receive section pro-
vides a mechanism to prevent false starts. On the
falling edge of a start or false start bit, an internal
receiver counter starts counting clocks at the 16x
clock rate. After 7 1/2 clocks the start bit time should
be shifted to the center of the start bit. At this time the
start bit is sampled and if it is still a logic 0 it is
validated. Evaluating the start bit in this manner
prevents the receiver from assembling a false charac-
ter. Receiver status codes will be posted in the LSR.
Enable
Enable
BIT-4
SLCT
-INTP
-INTP
bit-4
bit-4
bit-4
-SLCTIN
-SLCTIN
ST16C552/552A
BIT-3
Error
State
bit-3
bit-3
bit-3
BIT-2
-IRQ
bit-2
bit-2
bit-2
INIT
INIT
BIT-1
FDXT
FDXT
-Auto
-Auto
bit-1
bit-1
logic
bit-1
“1”
-STROBE
-STROBE
BIT-0
logic
bit-0
bit-0
bit-0
“1”