Many times, we plan and build systems that perform various processes that depend on time.
Simple example of this process is the digital wristwatch. The role of this electronic system is to display time in a very precise manner and change the display every second (for seconds), every minute (for minutes) and so on.
To perform the steps we've listed, the system must use a timer, which needs to be very accurate in order to take necessary actions.The clock is actually a core of any electronic system.
In this PIC timer module tutorial we will study the existing PIC timer modules. The microcontroller PIC16F877 has 3 different timers:
We can use these timers for various important purposes. So far we used delay procedure to implement some delay in the program, that was counting up to a specific value, before the program could be continued. "Delay procedure" had two disadvantages:
Now, using Timers we can build a very precise time delays which will be based on the system clock and allow us to achieve our desired time delay well-known in advance.
In order for us to know how to work with these timers, we need to learn some things about each one of them. We will study each one separately.
The Timer2 module, timer/counter, has the following features:
Lets explain the features of PIC Timer2 we have listed above:
Timer2 has 2 count registers: TMR2 and PR2. The size of each registers is 8-bit in which we can write numbers from 0 to 255. The TMR2 register is readable and writable and is cleared on any device Reset. PR2 is a readable and writable register and initialized to FFh upon Reset.
Register TMR2 is used to store the "initial" count value (the value from which it begins to count). Register PR2 is used to store the "ending" count value (the maximum value we need/want to reach). ie: using Timer2 we can determine the started count value, the final count value, and the count will be between these two values. The Timer2 increments from 00h until it matches PR2 and then resets to 00h on the next increment cycle.
Prescaler and Postscaler - Timer2 is an 8-bit timer with a prescaler and a postscaler. Each allows to make additional division of the frequency clock source.
Prescaler divides the frequency clock source BEFORE the counting take place at the register TMR2, thus the counting inside the TMR2 register is performed based on the divided frequency clock source by the Prescaler
The match output of TMR2 goes through a 4-bit postscaler (which gives a 1:1 to 1:16 scaling inclusive) to generate a TMR2 interrupt (latched in flag bit, TMR2IF (PIR1<1>)).
Postscaler divides the frequency that comes out of the Comparator again for the last time.
TIMER2 Prescaler and Postscaler
TMR2IF - TMR2 to PR2 Match Interrupt Flag bit.
Comparator Compares the value of the register TMR2 and the maximum value of the register PR2.
TMR2 The register in which the "initial" count value is written.
PR2 The register in which the final or the maximum count value is written.
We perform all the necessary settings with T2CON Register The structure of the T2CON register:
T2CKPS1:T2CKPS0: Timer2 Clock Prescale Select bits
The input clock (FOSC/4) has a prescale option of 1:1, 1:4 or 1:16, selected by control bits T2CKPS1:T2CKPS0 (T2CON<1:0>).
00 = Prescaler is 1
01 = Prescaler is 4
1x = Prescaler is 16
TMR2ON: Timer2 On bit
Timer2 can be shut-off by clearing control bit, TMR2ON (T2CON<2>), to minimize power consumption.
1 = Timer2 is on
0 = Timer2 is off
TOUTPS3:TOUTPS0: Timer2 Output Postscale Select bits
The match output of TMR2 goes through a 4-bit postscaler (which gives a 1:1 to 1:16 scaling inclusive selected by control bits TOUTPS3:TOUTPS0 (T2CON<6:3>).
0000 = 1:1 postscale
0001 = 1:2 postscale
0010 = 1:3 postscale
1111 = 1:16 postscale
The following is an example how we can initialize the T2CON register:
1. TMR2ON=1; // the timer is enable
2. T2CKPS0=0; // Prescaler 1:1
4. TOUTPS0=1; // Postscaler 1:16
Or you can set all the T2CON register at once as follows:
TIMER2 BLOCK DIAGRAM
How to calculate the required values of the TIMER2:
Fout The output frequency after the division.
Tout The Cycle Time after the division.
4 - The division of the original clock (4 MHz) by 4, when using internal crystal as clock (and not external oscillator).
Count - A numeric value to be placed to obtain the desired output frequency - fout.
(PR2 TMR2) - The number of times the counter will count.
Simple example and calculation of how to use TIMER2:
Suppose we want to create a delay of 1 second in the our program using Timer2. What is the value of Count?
First, let's assume that the frequency division by the Prescaler will be 1:1 and Postscaler will be 1:16. Second, let's set TMR1=0 and PR2=255. Thus:
The following simple C language program creates a delay of 1 sec using TIMER2: