COP688CL datasheet(17/40 Pages) NSC

17 / 40 page

Timers (Continued)

Mode 3 Input Capture Mode

The device can precisely measure external frequencies or

time external events by placing the timer block Tx in the

input capture mode

In this mode the timer Tx is constantly running at the fixed

tc rate The two registers RxA and RxB act as capture

registers Each register acts in conjunction with a pin The

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 TxE-

NA allows the interrupt on TxA to be either enabled or dis-

abled Setting the TxENA flag enables interrupts to be gen-

erated 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-

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 9 shows a block diagram of the timer in Input Capture

mode

TIMER CONTROL FLAGS

The timers T1 and T2 have indentical control structures

The control bits and their functions are summarized below

TxC0

Timer StartStop control in Modes 1 and 2

(Processor Independent PWM and External

Event Counter) where 1 e Start 0 e Stop

Timer Underflow Interrupt Pending Flag in

Mode 3 (Input Capture)

TxPNDA Timer Interrupt Pending Flag

TxPNDB Timer Interrupt Pending Flag

TxENA

Timer Interrupt Enable Flag

TxENB

Timer Interrupt Enable Flag

1 e Timer Interrupt Enabled

0 e Timer Interrupt Disabled

TxC3

Timer mode control

TxC2

Timer mode control

TxC1

Timer mode control

TLDD9766 – 15

FIGURE 9 Timer in Input Capture Mode

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