View source for PIC16f877a Timer ← PIC16f877a Timer

[[Category:PIC Tutorials]]
[[Category:Starter PIC16F877]]

In this tutorial we are going to discuss the Timer module of Pic16f877a.<br>
First we will see what are timers, their working and later we will configure the PIC16f877a timers  to generate delay of 100ms and 500ms respectively. At the end we will see how to use the ExploreEmdedded Timer library.
<br><br>

=Timer Basics=
As the name suggests these are used to measure the time or generate the accurate time delay. The microcontroller can also generate/measure the required time delays by running loops, but the timer/counter relieves the CPU from that redundant and repetitive task, allowing it to allocate maximum processing time for other tasks.<br><br>
Timer is nothing but a simple binary counter that can be configured to count clock pulses(Internal/External). Once it reaches the Max value, it will roll back to zero setting up an <b>OverFlow</b> flag and generates the interrupt if enabled.
<br><br>
[[FILE:timer.gif]]<br>

=PIC Timer Module=
Pic16f877a has three indepenndent timer which can be used as timer,Counters or for PWM generation.<br>
Below table provides the details of the three Timers.
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Timer || Size || Control Register || Count Register || Min Delay || Max Delay
|-
|TIMER0|| 8-bit || OPTION_REG || TMR0 || 0.2usec || 13.107ms
|-    
|TIMER1|| 16-bit || T1CON || TMR1H,TMR1L || 0.2usec || 104.857ms
|-
|TIMER2|| 8-bit || T2CON || TMR2 || 0.2usec || 819usec
|}<br>

=Timer Calculation=
PIC Oscillator frequency is divided by 4 and then fed to the controller, Now this this freq can be further divided by presacalar to generate the range of delays.<br>
Time to increment the Timer count by one(timer tick) can be determined as below. <br>
tick = (Prescalar/(Fosc/4)<br>
tick = (Prescalar/(20Mhz/4))<br>
$$tick = (Prescalar * 4)/Fosc$$

Now the Timer value for the required delay can be calculated as below.<br>
Delay =  TimerCount * tick<br>
Count = (Delay/tick)<br>
RegValue = TimerMax- Count <br>
RegValue = TimerMax-(Delay/tick) = TimerMax - (Delay/((Prescalar *4)/Fosc))<br>
$$RegValue = TimerMax-((Delay * Fosc)/(Prescalar*4))$$

Below table provides the formula for all the three Timers.
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Timer || Size || Formula for delay calculation
|-
|TIMER0|| 8-bit || $$RegValue =  256-((Delay * Fosc)/(Prescalar*4))$$
|-    
|TIMER1|| 16-bit || $$RegValue =  65536-((Delay * Fosc)/(Prescalar*4))$$
|-
|TIMER2|| 8-bit || $$RegValue =  256-((Delay * Fosc)/(Prescalar*4))$$
|}
----
<br><br>

=Timer 0=
The TMR0 module is an 8-bit timer/counter with the following features:
*8-bit timer/counter
*Readable and writable
*8-bit software programmable prescaler
*Internal or external clock select
*Interrupt on overflow from FFh to 00h
*Edge select for external clock

==Timer0 Registers==
The below table shows the registers associated with PIC16f877A Timer0 module.<br>
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Register || Description
|-
|OPTION_REG|| This registers is used to configure the TIMER0 Prescalar, Clock Source etc
|-    
|TMR0 || This register holds the timer count value which will be incremented depending on prescalar configuration
|-
|INTCON|| This register contains the Timer0 overflow flag(TMR0IF) and corresponding Inetrrupt Enable flag(TMR0IE).
|}<br>

{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|OPTION_REG 
|-
|7 || 6 ||   5 || 4 || 3 || 2 || 1 || 0  
|-
|RBPU || INTEDG || T0CS || T0SE || PSA || PS2 || PS1 || PS0
|}

<b>RBPU:</b> NA for Timers

<b>INTEDG:</b> NA for Timers

<b>T0CS:</b> TMR0 Clock Source Select bit<br>
1-Transition on T0CKI pin<br>
0-Internal instruction cycle clock (CLKO)

<b>T0SE:</b> TMR0 Source Edge Select bit<br>
1-Increment on high-to-low transition on T0CKI pin<br>
0-Increment on low-to-high transition on T0CKI pin

<b>PSA:</b> Prescaler Assignment bit<br>
1-Prescaler is assigned to the WDT<br>
0-Prescaler is assigned to the Timer0

<b>PS2:PS0:</b> Prescaler Rate Select bits
[[FILE:Timer0Prescalar.png]]
*<b>Note:</b>There is only one prescaler available which is mutually exclusively shared between the Timer0 module and the Watchdog Timer. A prescaler assignment for the Timer0 module means that there is no prescaler for the Watchdog Timer and vice versa. This prescaler is not accessible but can be configured using <b>PS2:PS0</b> bits of <b>OPTION_REG</b>.
<br><br>

{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|INTCON 
|-
|7 || 6 ||   5 || 4 || 3 || 2 || 1 || 0  
|-
|GIE || PIE || TMR0IE || INTE || RBIE || TMR0IF || INTF || RBIF
|}

<b>GIE:</b> Global Interrupt Enable bit<br>
1-Enables all unmasked interrupts<br>
0-Disables all interrupts

<b>PIE:</b> Peripheral Interrupt Enable bit<br>
1-Enables all unmasked peripheral interrupts<br>
0-Disables all peripheral interrupts

<b>TMR0IE:</b> TMR0 Overflow Interrupt Enable bit<br>
1-Enables the TMR0 interrupt<br>
0-Disables the TMR0 interrupt

<b>INTE:</b> NA for Timers

<b>RBIE:</b> NA for Timers

<b>TMR0IF:</b> TMR0 Overflow Interrupt Flag bit<br>
1-TMR0 register has overflowed (must be cleared in software)<br>
0-TMR0 register did not overflow
   
<b>INTF:</b> NA for Timers

<b>RBIF:</b> NA for Timers
<br>
[[FILE:Timer0_Block_Diagram.png]]



Delay Calculations for 1ms @20Mhz with Prescalar as 32:<br>
'''RegValue = 256-(Delay * Fosc)/(Prescalar*4)) = 256-((1ms * 20Mhz)/(32*4)) = 256-156=100'''

==Code==
Below are the steps for configuring and using the Timer0 for delay generation:
#Calculate the Timer Count for the required delay.
#Set the Presaclar bits in <b>OPTION_REG</b> as per the delay calculations.
#Clear the  <b>PSA</b>bit for using the prescalar.
#Select the Clock Source Internal/External using <b>TOCS</b> bit.
#Load the timer value into <b>TMRO</b> register.
#Enable the Timer0 Interrupt by setting <b>TMR0IE</b> bit
#Enable the Global and Peripheral interrupts by setting <b>GIE</b> and <b>PIE</b> bits

Below is the sample code to blink the LEDs with '''1ms''' delay.
<html>
<script src="https://gist.github.com/SaheblalBagwan/319888d39a82ea1ebbb1a0e29ed0754d.js"></script>
</html>
Delay measured using CRO:
[[FILE:Timer0 Cro.png]]
<br>
-----
<br><br>

=Timer 1=
The timer TMR1 module is an 16-bit timer/counter with the following features:
*16-bit timer/counter with two 8-Bit register TMR1H/TMR1L
*Readable and writable
*software programmable prescaler upto 1:8
*Internal or external clock select
*Interrupt on overflow from FFFFh to 00h
*Edge select for external clock

==Timer1 Registers==
The below table shows the registers associated with PIC16f877A Timer1 module.<br>
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Register || Description
|-
|T1CON:|| This registers is used to configure the TIMER1 Prescalar, Clock Source etc
|-    
|TMRIH || This register holds the higher 8-bits of timer value. TMR1H and TMR1L are used in pair to increment from 0000 - FFFFh
|-    
|TMRIL || This register holds the lower 8-bits of timer value. TMR1H and TMR1L are used in pair to increment from 0000 - FFFFh
|-
|PIR1|| This register contains the Timer1 overflow flag(TMR1IF).
|-
|PIE1|| This register contains the Timer1 Interrupt Enable flag(TMR1IE).
|}<br>

{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|T1CON
|-
|7 || 6 ||   5 || 4 || 3 || 2 || 1 || 0  
|-
|— || — || T1CKPS1 || T1CKPS0 || T1OSCEN || T1SYNC || TMR1CS || TMR1ON
|}

<b>T1CKPS1:T1CKPS0:</b>Timer1 Input Clock Prescale Select bits<br>
11 = 1:8 prescale value<br>
10 = 1:4 prescale value<br>
01 = 1:2 prescale value<br>
00 = 1:1 prescale value

<b>T1OSCEN:</b> Timer1 Oscillator Enable Control bit<br>
1-Oscillator is enabled<br>
0-Oscillator is shut-off

<b>T1SYNC:</b> Timer1 External Clock Input Synchronization Control bit<br>
1-Do not synchronize external clock input<br>
0-Synchronize external clock input

<b>TMR1CS:</b> Timer1 Clock Source Select bit<br>
1-External clock from pin RC0/T1OSO/T1CKI (on the rising edge)<br>
0-Internal clock (FOSC/4)

<b>TMR1ON:</b> Timer1 On bit<br>
1-Enables Timer1<br>
0-Stops Timer1
<br><br>

[[FILE:Timer1_Block_Diagram.png]]



Delay Calculations for 100ms @20Mhz with Prescalar as 8:<br>
'''RegValue = 65536-(Delay * Fosc)/(Prescalar*4)) = 65536-((100ms * 20Mhz)/(8*4)) = 3036 = 0x0BDC
'''

==Code==
Below are the steps for configuring and using the Timer1 for delay generation:
#Calculate the Timer Count for the required delay.
#Set the Presaclar bits in <b>T1CON</b> as per the delay calculations.
#Select the Clock Source Internal/External using <b>TMR1CS</b> bit.
#Load the timer value into <b>TMR1H,TMR1L</b> register.
#Enable the Timer1 Interrupt by setting <b>TMRIE</b> bit
#Enable the Global and Peripheral interrupts by setting <b>GIE</b> and <b>PIE</b> bits
#Finally start the timer by setting <b>TMR1ON</b> bit 

Below is the sample code to blink the LEDs with '''100ms''' delay.
<html>
<script <script src="https://gist.github.com/SaheblalBagwan/ca5d37cb51af0a6777be8b0dd81a4348.js"></script>
</html>
Delay measured using CRO:
[[FILE:Timer 1 Cro.png]]
<br>
-----
<br><br>

=Timer 2=
The TImer2 module is an 8-bit timer/counter with the following features:
*8-bit timer/counter
*Readable and writable
*Software programmable prescaler/PostScaler upto 1:16
*Interrupt on overflow from FFh to 00h


==Timer2 Registers==
The below table shows the registers associated with PIC16f877A Timer0 module.<br>
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Register || Description
|-
|T2CON|| This registers is used to configure the TIMER2 Prescalar, Clock Source etc
|-    
|TMR2 || This register holds the timer count value which will be incremented depending on prescalar configuration
|-
|PIR1|| This register contains the Timer2 overflow flag(TMR2IF).
|-
|PIE1|| This register contains the Timer2 Interrupt Enable flag(TMR2IE).
|}<br>

{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|T2CON 
|-
|7 || 6 || 5 || 4 || 3 || 2 || 1 || 0  
|-
|— || TOUTPS3 || TOUTPS2 || TOUTPS1 || TOUTPS0 || TMR2ON || T2CKPS1 || T2CKPS0
|}

<b>TOUTPS3:TOUTPS0:</b> Timer2 Output Postscale Select bits<br>
0000 = 1:1 postscale<br>
0001 = 1:2 postscale<br>
0010 = 1:3 postscale<br>
•<br>
•<br>
•<br>
1111 = 1:16 postscale

<b>TMR2ON:</b> Timer2 On bit<br>
1-Timer2 is on<br>
0-Timer2 is off

<b>T2CKPS1:T2CKPS0:</b> Timer2 Clock Prescale Select bits<br>
00 = Prescaler is 1<br>
01 = Prescaler is 4<br>
1x = Prescaler is 16
<br>

[[FILE:Timer2_Block_Diagram.png]]


<b>Generating 1sec delay using Timer2:</b><br>
As the timer2 is 8-bit and supports 1:16 prescalar, it is not possible to directly generate the delay of 1sec. The max delay with 1:16 prescaler will be:<br>
'''Delay = 256 * (Prescaler*4)/Fosc = 256 * 16*4/20Mhz=819us'''

Now 500us can be generated using timers which will be used to increment a counter 2000 times to get 1sec delay.<br>  
Delay Calculations for 500usec @20Mhz with Prescalar as 16:<br>
'''RegValue = 256-(Delay * Fosc)/(Prescalar*4)) = 256-((500us* 20Mhz)/(16*4)) = 256-156=100'''

==Code==
Below are the steps for configuring and using the Timer2 for delay generation:
#Calculate the Timer Count for the required delay.
#Set the Prescaler bits in <b>T2CON</b> as per the delay calculations.
#Load the timer value into <b>TMR2</b> register.
#Enable the Timer2 Interrupt by setting <b>TMR2IE</b> bit
#Enable the Global and Peripheral interrupts by setting <b>GIE</b> and <b>PIE</b> bits
#Finally start the Timer2 by setting <b>TMR2ON</b> bit<br> 

Below is the sample code to blink the LEDs with '''1sec''' delay.
<html>
<script src="https://gist.github.com/SaheblalBagwan/93556fb952ef8f199fb302db27207230.js"></script>
</html>
Delay measured using CRO:
[[FILE:Timer 2 Cro.png]]
<br><br>

[[File:00_Lpc1768_Timer.jpg]]
<br><br>

=Downloads=
Download the complete project folder from the below link:<br>
[https://github.com/ExploreEmbedded/Pic16f877a_ExploreUltraPicDevKit/archive/master.zip Hardware design Files and Code Library]


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