8052 Microcontroller Tutorial
Timer 2 in Capture Mode

Author: Craig Steiner

Book: The 8051/8052 Microcontroller:
         Architecture, Assembly Language, And Hardware Interfacing (Paperback)

TIMER 2 IN CAPTURE MODE

A new mode specific to Timer 2 is called "Capture Mode." As the name implies, this mode captures the value of Timer 2 (TH2 and TL2) into the capture SFRs (RCAP2H and RCAP2L). To put Timer 2 in capture mode, CP/RL2 (T2CON.0) must be set, as must be EXEN2 (T2CON.3).

When configured as mentioned above, a capture will occur whenever a 1-0 transition is detected on T2EX (P1.1). At the moment the transition is detected, the current values of TH2 and TL2 will be copied into RCAP2H and RCAP2L, respectively. At the same time, the EXF2 (T2CON.6) bit will be set, which will trigger an interrupt if Timer 2 interrupt is enabled.

NOTE 1: Note that even in capture mode, an overflow of Timer 2 will result in TF2 being set and an interrupt being triggered.

NOTE 2: Capture mode is an efficient way to measure the time between events. At the moment that an event occurs, the current value of Timer 2 will be copied into RCAP2H/L. However, Timer 2 will not stop and an interrupt will be triggered. Thus your interrupt routine may copy the value of RCAP2H/L to a temporary holding variable without having to stop Timer 2. When another capture occurs, your interrupt can take the difference of the two values to determine the time transpired. Again, the main advantage is that you don't have to stop timer 2 to read its value, as is the case with timer 0 and timer 1.

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