Timer 2 as a baud rate generator is shown in Figure 4. This figure is valid only if RCLK
or TCLK = 1 in T2CON. Note that a rollover in TH2 does not set TF2 and will not gener-
ate an interrupt. 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.
Note that when Timer 2 is running (TR2 = 1) as a timer in the baud rate generator mode,
TH2 or TL2 should not be read from or written to. Under these conditions, the Timer is
incremented every state time, and the results of a read or write may not be accurate.
The RCAP2 registers may be read but should not be written to, because a write might
overlap a reload and cause write and/or reload errors. The timer should be turned off
(clear TR2) before accessing the Timer 2 or RCAP2 registers.
Programmable
Clock Out
A 50% duty cycle clock can be programmed to come out on P1.0, as shown in Figure 5.
This pin, besides being a regular I/0 pin, has two alternate functions. It can be pro-
grammed to input the external clock for Timer/Counter 2 or to output a 50% duty cycle
clock ranging from 61 Hz to 4 MHz (for a 16-MHz operating frequency).
To configure the Timer/Counter 2 as a clock generator, bit C/T2 (T2CON.1) must be
cleared and bit T2OE (T2MOD.1) must be set. Bit TR2 (T2CON.2) starts and stops the
timer.
The clock-out frequency depends on the oscillator frequency and the reload value of
Timer 2 capture registers (RCAP2H, RCAP2L), as shown in the following equation.
Oscillator Frequency
Clock Out Frequency = ------------------------------------------------------------------------------------------
4 × [65536 – (RCAP2H,RCAP2L)]
In the clock-out mode, Timer 2 rollovers will not generate an interrupt. This behavior is
similar to when Timer 2 is used as a baud-rate generator. It is possible to use Timer 2 as
a baud-rate generator and a clock generator simultaneously. Note, however, that the
baud-rate and clock-out frequencies cannot be determined independently from one
another since they both use RCAP2H and RCAP2L.
16
AT89S8252
0401G–MICRO–3/06
ATMEL [ ATMEL ]
