12.0 Timers (Continued)
12.3 TIMER CONTROL FLAGS
the external event occurs. The capture register eliminates
the latency time, thereby allowing the applications program
to retrieve the timer value stored in the capture register.
The control bits and their functions are summarized below.
TxC3
TxC2
TxC1
TxC0
Timer mode control
Timer mode control
Timer mode control
In this mode, the timer Tx is constantly running at the fixed tC
or MCLK rate. The two registers, RxA and RxB, act as
capture registers. Each register also 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.
Timer Start/Stop control in Modes 1 and 2 (Pro-
cessor Independent PWM and External Event
Counter), where 1 = Start, 0 = Stop
Timer Underflow Interrupt Pending Flag in Mode
3 (Input Capture)
The timer value gets copied over into the register when a
trigger event occurs on its corresponding pin after synchro-
nization to the appropriate internal clock (tC or MCLK). Con-
trol bits, TxC3, TxC2 and TxC1, allow the trigger events to be
specified either as a positive or a negative edge. The trigger
condition for each input pin can be specified independently.
TxPNDA Timer Interrupt Pending Flag
TxENA Timer Interrupt Enable Flag
1 = Timer Interrupt Enabled
0 = Timer Interrupt Disabled
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.
TxPNDB Timer Interrupt Pending Flag
TxENB Timer Interrupt Enable Flag
1 = Timer Interrupt Enabled
0 = Timer Interrupt Disabled
The timer mode control bits (TxC3, TxC2 and TxC1) are
detailed in Table 15, Timer Operating Modes.
When the high speed timers are counting in high speed
mode, directly altering the contents of the timer upper or
lower registers, the PWM outputs or the reload registers is
not recommended. Bit operations can be particularly prob-
lematic. Since any of these six registers or the PWM outputs
can change as many as ten times in a single instruction
cycle, performing an SBIT or RBIT operation with the timer
running can produce unpredictable results. The recom-
mended procedure is to stop the timer, perform any changes
to the timer, the PWM outputs or reload register values, and
then re-start the timer. This warning does not apply to the
timer control register. Any type of read/write operation, in-
cluding SBIT and RBIT may be performed on this register in
any operating mode.
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-
sequently, the TxC0 control bit should be reset when enter-
ing the Input Capture mode. The timer underflow interrupt is
enabled with the TxENA control flag. When a TxA interrupt
occurs in the Input Capture mode, the user must check both
the TxPNDA and TxC0 pending flags in order to determine
whether a TxA input capture or a timer underflow (or both)
caused the interrupt.
Figure 17 shows a block diagram of the timer T1 in Input
Capture mode. T2 and T3 are identical to T1.
20006321
FIGURE 17. Timer in Input Capture Mode
33
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