Common Features Description
2-97 Atmel 8051 Microcontrollers Hardware Manual
4316B–8051–02/04
Figure 2-22. Timer 2 in Baud Rate Generator Mode.
The baud rate generator mode is similar to the auto-reload mode, in that a rollover in
TH2 causes the Timer 2 registers to be reloaded with the 16-bit value in registers
RCAP2H and RCAP2L, which are preset by software.
Now, the baud rates in Modes 1 and 3 are determined by Timer 2’s overflow rate as
follows:
The Timer can be configured for either “timer” or “counter” operation. In the most typical
applications, it is configured for “timer” operation (C/T2
= 0). “Timer” operation is a little dif-
ferent for Timer 2 when it’s being used as a baud rate generator. Separately as a timer it would
increment every machine cycle (thus at 1/12 the oscillator frequency). As a baud rate generator,
however, it increment every state time (thus at 1/2 the oscillator frequency).
Timer 2 as a baud rate generator is shown in Figure 2-22. This Figure is valid only if
RCLK + TCLK = 1 in T2CON. Note that a rollover in TH2 does not set TF2, and will not
generate an interrupt. Therefore, the Timer 2 interrupt does not have to be disabled
when Timer 2 is in the baud rate generator mode. Note too, that if EXEN2 is set, a 1-to-
0 transition in T2EX will set EXF2 but will not cause a reload from (RCAP2H, RCAP2L)
to (TH2, TL2). Thus when Timer 2 is in use as a baud rate generator, T2EX can be used
as an extra external interrupt, if desired.
It should be noted that when Timer 2 is running (TR2 = 1) in “Timer” function in the baud
rate generator mode, one should not try to read or write TH2 or TL2. Under these condi-
tions the Timer is being incremented every state time, and the results of a read or write
may not be accurate. The RCAP registers may be read, but shouldn’t be written to,
because a write might overlap a reload and cause write and/or reload errors. In this
case, turn the Timer off (clear TR2) before accessing the Timer 2 or RCAP registers.
Periph
Clock
1
To Next Page
Loading...
Need help?
General
