CP3CN17 Datasheet, PDF (158/220 Pages) National Semiconductor (TI) – Reprogrammable Connectivity Processor with CAN Interface

22.2.4 Mode 4: Input Capture Plus Timer
Mode 4 is the Single Input Capture and Single Timer mode,
which provides one external event counter and one system
timer.
Figure 81 is a block diagram of the Multi-Function Timer
configured to operate in Mode 4. This mode offers a combi-
nation of Mode 3 and Mode 2 functions. Timer/Counter 1 is
used as a system timer as in Mode 3 and Timer/Counter 2
is used as a capture timer as in Mode 2, but with a single
input rather than two inputs.
Timer 1
Clock
Timer 2
Clock
Reload A
TCRA
Underflow
Timer/Counter 1
TCNT1
Capture B
TCRB
Preset
Timer/Counter 2
TnCNT2
TBEN
TAPND
TAIEN
TAEN
TBIEN
TBPND
Timer
Interrupt 1
TA
Timer
Interrupt 1
TB
TDPND
TDIEN
Timer
Interrupt 2
DS087
Figure 81. Input Capture Plus Timer Mode
Timer/Counter 1 (TCNT1) operates the same as in Mode 3.
It counts down at the rate of the selected clock. On under-
flow, it is reloaded from the TCRA register and counting pro-
ceeds down from the reloaded value. The TA pin is toggled
on each underflow, when this function is enabled by the
TAEN bit. When the TA pin is toggled from low to high, it sets
the TCPND interrupt pending bit and also generates an in-
terrupt if the interrupt is enabled by the TAIEN bit. A 50%
duty cycle PWM signal can be generated on TA without any
further action from the CPU.
Timer/Counter 2 (TCNT1) counts down at the rate of the se-
lected clock. The TB pin functions as the capture input. A
transition received on TB transfers the timer contents to the
TCRB register. The input pin can be configured to sense ei-
ther rising or falling edges.
The TB input can be configured to preset the counter to
FFFFh on reception of a valid capture event. In this case,
the current value of the counter is transferred to the capture
register and then the counter is preset to FFFFh.
The values captured in the TCRB register at different times
reflect the elapsed time between transitions on the TA pin.
The input signal on TB must have a pulse width equal to or
greater than one System Clock cycle.
Neither Timer/Counter 1 (TCNT1) nor Timer/Counter 2
(TCNT2) can be configured to operate as an external event
counter or to operate in the pulse-accumulate mode be-
cause the TB input is used as a capture input. Attempting to
select one of these configurations will cause one or both
counters to stop. In this mode, Timer/Counter 2 must be en-
abled at all times.
22.3 TIMER INTERRUPTS
The Multi-Function Timer unit has four interrupt sources,
designated A, B, C, and D. Interrupt sources A, B, and C are
mapped into a single system interrupt called Timer Interrupt
1, while interrupt source D is mapped into a system interrupt
called Timer Interrupt 2. Each of the four interrupt sources
has its own enable bit and pending bit. The enable bits are
named TAIEN, TBIEN, TCIEN, and TDIEN. The pending
bits are named TAPND, TBPND, TCPND, and TDPND.
Timer Interrupts 1 and 2 are system interrupts TA and TB
(IRQ14 and IRQ13), respectively.
Table 63 shows the events that trigger interrupts A, B, C,
and D in each of the four operating modes. Note that some
interrupt sources are not used in some operating modes.
There are two separate interrupts associated with the cap-
ture timer, each with its own enable bit and pending bit. The
two interrupt events are reception of a transition on TB and
underflow of the TCNT2 counter. The enable bits for these
events are TBIEN and TDIEN, respectively.
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