TL16C554A, TL16C554AI

ASYNCHRONOUS-COMMUNICATIONS ELEMENT

SLLS509E − AUGUST 2001 − REVISED JUNE 2010

29

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

modem-status register (MSR) (continued)

loop mode (MCR4 is set), MSR6 reflects the value of OUT1 in the MCR.

detect (DCD) input. When the channel is in the loop mode (MCR4 is set), MSR7 reflects the value of OUT2

in the MCR.

Reading the MSR clears the delta modem status indicators but has no effect on the other status bits. For LSR

and MSR, the setting of status bits is inhibited during status register read operations. If a status condition is

generated during a read IOR operation, the status bit is not set until the trailing edge of the read. When a status

bit is set during a read operation and the same status condition occurs, that status bit is cleared at the trailing

edge of the read instead of being set again. In the loopback mode, CTS, DSR, RI, and DCD inputs are ignored

when modem-status interrupts are enabled; however, a modem-status interrupt can still be generated by writing

to MCR3 −MCR0. Applications software should not write to the MSR.

Table 8. Modem-Status Register BIts

MSR BIT

MNEMONIC

DESCRIPTION

MSR0

ΔCTS

Delta clear to send

MSR1

ΔDSR

Delta data set ready

MSR2

TERI

Trailing edge of ring indicator

MSR3

ΔDCD

Delta data carrier detect

MSR4

CTS

Clear to send

MSR5

DSR

Data set ready

MSR6

RI

Ring indicator

MSR7

DCD

Data carrier detect

programming

The serial channel of the ACE is programmed by control registers LCR, IER, DLL, DLM, MCR, and FCR. These

control words define the character length, number of stop bits, parity, baud rate, and modem interface.

While the control registers can be written in any order, the IER should be written last because it controls the

interrupt enables. Once the serial channel is programmed and operational, these registers can be updated any

time the ACE serial channel is not transmitting or receiving data.

programmable baud-rate generator

The ACE serial channel contains a programmable baud-rate generator (BRG) that divides the clock (dc to

These divisor-latch registers must be loaded during initialization. A 16-bit baud counter is immediately loaded

upon loading of either of the divisor latches. 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, 8 MHz, and 16 MHz. With these frequencies, standard bit rates from 50 kbps to 512 kbps are

available. Tables 9, 10, 11, and 12 illustrate the divisors needed to obtain standard rates using these three

frequencies. The output frequency of the baud-rate generator is 16 times the data rate [divisor # = clock + (baud

rate

× 16)]. RCLK runs at this frequency.