Texas Instruments MSC1210 manual Using Timers as Event Counters

Using Timers as Event Counters

8.4 Using Timers as Event Counters

We have discussed how a timer can be used for the obvious purpose of keep- ing track of time. However, the MSC1210 also allows the use of timers to count events.

This can be useful in many applications. For example, a sensor is placed across a road that would send a pulse every time a car passes over it. This could be used to determine the volume of traffic on the road. The sensor is at- tached to one of the MSC1210 I/O lines and constantly monitored, detecting when it pulses high, and the counter incremented when it goes back to a low state. This is not terribly difficult, but requires some code. If the sensor is hooked to P1.0, the code to count passing cars would look something like this:

JNB P1.0,$	;If a	car hasn’t raised the signal,
	;keep	waiting
JB P1.0,$	;The line is high, car is on the sensor
	;right now
INC COUNTER	;The car has passed completely, so we count it

As shown, it is only three lines of code. However, what if other processing needs to be done at the same time? The program cannot be stuck in the JNB P1.0,$ loop waiting for a car to pass if it needs to be doing other things. What if the program is doing other things when a car passes over? It is possible that the car will raise the signal and the signal will fall low again before the program checks the line status; this would result in the car not being counted. Of course, there are ways to get around even this limitation, but the code quickly becomes big, complex, and ugly.

Luckily, the MSC1210 provides a way to use the timers to count events. It is painfully easy. Only one additional bit has to be configured.

Timer 0 can be used to count the number of cars that pass. In the bit table for the TCON SFR, there is a bit called C/T0—it is bit 2 (TCON.2). Reviewing the explanation of the bit, we see that if the bit is clear, Timer 0 will be incremented every instruction cycle. This is what has already been used to measure time.

If C/T0 is set, however, Timer 0 will monitor the P3.4 line. Instead of being in- cremented every machine cycle, Timer 0 will count events on the P3.4 line. So in this case, simply connect the sensor to P3.4 and let the 8052 do the work. Then, when the number of how many cars have passed is desired, just read the value of Timer 0—the value of Timer 0 will be the number of cars that have passed.

So what exactly is an event? What does Timer 0 actually count? Speaking at the electrical level, the MSC1210 counts 1−0 transitions on the P3.4 line. This means that when a car first runs over the sensor, it raises the input to a high

(1)condition. At that point, the MSC1210 does not count anything because this is a 0−1 transition. However, when the car has passed, the sensor falls back to a low (“0”) state. This is a 1−0 transition and at that instant the counter is incremented by 1.

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