COP8ACC Datasheet, PDF (15/43 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based and OTP Microcontrollers with 4k or 16k Memory and High Resolution A/D

Timers (Continued)
TIMER T1
The devicea have a powerful timer/counter block. The timer
consists of a 16-bit timer, T1, and two supporting 16-bit
autoreload/capture registers, R1A and R1B. The timer block
has two pins associated with it, T1A and T1B. The pin T1A
supports I/O required by the timer block, while the pin T1B is
an input to the timer block. The powerful and flexible timer
block allows the devices to easily perform all timer functions
with minimal software overhead. The timer block has three
operating modes: Processor Independent PWM mode, Ex-
ternal Event Counter mode, and Input Capture mode.
The control bits T1C3, T1C2, and T1C1 allow selection of the
different modes of operation.
Mode 1. Processor Independent PWM Mode
As the name suggests, this mode allows the devices to
generate a PWM signal with very minimal user intervention.
The user only has to define the parameters of the PWM
signal (ON time and OFF time). Once begun, the timer block
will continuously generate the PWM signal completely inde-
pendent of the microcontroller. The user software services
the timer block only when the PWM parameters require
updating.
In this mode the timer T1 counts down at a fixed rate of t_C.
Upon every underflow the timer is alternately reloaded with
the contents of supporting registers, R1A and R1B. The very
first underflow of the timer causes the timer to reload from
the register R1A. Subsequent underflows cause the timer to
be reloaded from the registers alternately beginning with the
register R1B.
The T1 Timer control bits, T1C3, T1C2 and T1C1 set up the
timer for PWM mode operation.
Figure 9 shows a block diagram of the timer in PWM mode.
The underflows can be programmed to toggle the T1A output
pin. The underflows can also be programmed to generate
interrupts.
Underflows from the timer are alternately latched into two
pending flags, T1PNDA and T1PNDB. The user must reset
these pending flags under software control. Two control
enable flags, T1ENA and T1ENB, allow the interrupts from
the timer underflow to be enabled or disabled. Setting the
timer enable flag T1ENA will cause an interrupt when a timer
underflow causes the R1A register to be reloaded into the
timer. Setting the timer enable flag T1ENB will cause an
interrupt when a timer underflow causes the R1B register to
be reloaded into the timer. Resetting the timer enable flags
will disable the associated interrupts.
Either or both of the timer underflow interrupts may be
enabled. This gives the user the flexibility of interrupting
once per PWM period on either the rising or falling edge of
the PWM output. Alternatively, the user may choose to inter-
rupt on both edges of the PWM output.
FIGURE 9. Timer in PWM Mode
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Mode 2. External Event Counter Mode
This mode is quite similar to the processor independent
PWM mode previously described. The main difference is that
the timer, T1, is clocked by the input signal from the T1A pin.
The T1 timer control bits, T1C3, T1C2 and T1C1 allow the
timer to be clocked either on a positive or negative edge from
the T1A pin. Underflows from the timer are latched into the
T1PNDA pending flag. Setting the T1ENA control flag will
cause an interrupt when the timer underflows.
In this mode the input pin T1B can be used as an indepen-
dent positive edge sensitive interrupt input if the T1ENB
control flag is set. The occurrence of a positive edge on the
T1B input pin is latched into the T1PNDB flag.
Figure 10 shows a block diagram of the timer in External
Event Counter mode.
Note: The PWM output is not available in this mode since the T1A pin is
being used as the counter input clock.
DS012865-58
FIGURE 10. Timer in External Event Counter Mode
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