
PEB 20560
Functional Block Description
Semiconductor Group
2-144
2003-08
2.14.1.2
Programmable Baud Rate Generator (Divisors)
The UART contains a programmable baud rate generator. The output frequency of the
baud rate generator is 16
×
the baud rate [divisor = (61.44 MHz
÷
5)
÷
(baud rate
×
16)].
These divisor latches must be loaded during initialization to ensure proper operation of
the baud rate generator. Upon loading of the divisor latche, a 16-bit baud counter is
immediately loaded.
Table 2-35
provides decimal divisors to use with crystal frequency of 61.44 MHz.
Using a divisor of zero is not recommended.
STP
This bit is the stick parity bit. When bits 3, 4, and 5 are logic 1 the
parity bit is transmitted and checked as a logic 0. If bits 3 and 5 are 1
and bit 4 is logic 0 then the parity bit is transmitted and checked as a
logic 1. If bit 5 is a logic 0 stick parity is disabled.
This bit is the break control bit. It causes a break condition to be
transmitted by the UART. When it is set to a logic 1, the serial output
(SOUT) is forced to the spacing (logic 0) state. The break is disabled
by clearing bit 6 to a logic 0. The break control bit acts only on SOUT
and has no effect on the transmitter logic.
Note: This feature enables the
μ
P to alert a terminal in a computer
communications system. If the following sequence is used, no
erroneous or extraneous characters will be transmitted
because of the break
SBR
1. Load on all O’s pad character in response to THRE.
2. Set break after the next THRE.
3. Wait for the transmitter to be idle, (TEMT = 1) and clear break
when normal transmission is to be restored. During the break, the
transmitter can be used as a character timer to accurately
establish the break duration.
This bit is the divisor latch access bit. It must be set high (logic 1) to
access the divisor latches of the baud generator during a read or
write operation. It must be set low (logic 0) to access the receiver
buffer, the transmitter holding register, or the interrupt enable
register.
DLAB