COP888EB Datasheet, PDF (16/75 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 8k Memory, CAN Interface, 8-Bit A/D, and USART

Timers (Continued)
TxB. The pin TxA supports I/O required by the timer block,
while the pin TxB is an input to the timer block. The powerful
and flexible timer block allows the device to easily perform all
timer functions with minimal software overhead. The timer
block has three operating modes: Processor Independent
PWM mode, External Event Counter mode, and Input Cap-
ture mode.
The control bits TxC3, TxC2, and TxC1 allow selection of the
different modes of operation.
Mode 1. Processor Independent PWM Mode
As the name suggests, this mode allows the device to gen-
erate a PWM signal with very minimal user intervention.
The user only has to define the parameters of the PWM sig-
nal (ON time and OFF time). Once begun, the timer block will
continuously generate the PWM signal completely indepen-
dent of the microcontroller. The user software services the
timer block only when the PWM parameters require updat-
ing.
In this mode the timer Tx counts down at a fixed rate of t_c.
Upon every underflow the timer is alternately reloaded with
the contents of supporting registers, RxA and RxB. The very
first underflow of the timer causes the timer to reload from
the register RxA. Subsequent underflows cause the timer to
be reloaded from the registers alternately beginning with the
register RxB.
The Tx Timer control bits, TxC3, TxC2 and TxC1 set up the
timer for PWM mode operation.
Figure 9 shows a block diagram of the timer in PWM mode.
per PWM period on either the rising or falling edge of the
PWM output. Alternatively, the user may choose to interrupt
on both edges of the PWM output.
Mode 2. External Event Counter Mode
This mode is quite similar to the processor independent
PWM mode described above. The main difference is that the
timer, Tx, is clocked by the input signal from the TxA pin. The
Tx timer control bits, TxC3, TxC2 and TxC1 allow the timer to
be clocked either on a positive or negative edge from the
TxA pin. Underflows from the timer are latched into the TxP-
NDA pending flag. Setting the TxENA control flag will cause
an interrupt when the timer underflows.
In this mode the input pin TxB can be used as an indepen-
dent positive edge sensitive interrupt input if the TxENB con-
trol flag is set. The occurrence of the positive edge on the
TxB input pin is latched to the TxPNDB flag.
Figure 10 shows a block diagram of the timer in External
Event Counter mode.
FIGURE 9. Timer in PWM Mode
DS012837-10
The underflows can be programmed to toggle the TxA output
pin. The underflows can also be programmed to generate in-
terrupts.
Underflows from the timer are alternately latched into two
pending flags, TxPNDA and TxPNDB. The user must reset
these pending flags under software control. Two control en-
able flags, TxENA and TxENB, allow the interrupts from the
timer underflow to be enabled or disabled. Setting the timer
enable flag TxENA will cause an interrupt when a timer un-
derflow causes the RxA register to be reloaded into the timer.
Setting the timer enable flag TxENB will cause an interrupt
when a timer underflow causes the RxB register to be re-
loaded into the timer. Resetting the timer enable flags will
disable the associated interrupts.
Either or both of the timer underflow interrupts may be en-
abled. This gives the user the flexibility of interrupting once
DS012837-11
Note: The PWM output is not available in this mode since the TxA pin is
being used as the counter input clock.
FIGURE 10. Timer in External Event Counter Mode
Mode 3. Input Capture Mode
The device can precisely measure external frequencies or
time external events by placing the timer block, Tx, in the in-
put capture mode.
In this mode, the timer Tx is constantly running at the fixed t_c
rate. The two registers, RxA and RxB, act as capture regis-
ters. Each register acts in conjunction with a pin. The register
RxA acts in conjunction with the TxA pin and the register RxB
acts in conjunction with the TxB pin.
The timer value gets copied over into the register when a
trigger event occurs on its corresponding pin. Control bits,
TxC3, TxC2 and TxC1, allow the trigger events to be speci-
fied either as a positive or a negative edge. The trigger con-
dition for each input pin can be specified independently.
The trigger conditions can also be programmed to generate
interrupts. The occurrence of the specified trigger condition
on the TxA and TxB pins will be respectively latched into the
pending flags, TxPNDA and TxPNDB. The control flag
TxENA allows the interrupt on TxA to be either enabled or
disabled. Setting the TxENA flag enables interrupts to be
generated when the selected trigger condition occurs on the
TxA pin. Similarly, the flag TxENB controls the interrupts
from the TxB pin.
Underflows from the timer can also be programmed to gen-
erate interrupts. Underflows are latched into the timer TxC0
pending flag (the TxC0 control bit serves as the timer under-
flow interrupt pending flag in the Input Capture mode). Con-
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