View source for LPC1768: SysTick Timer ← LPC1768: SysTick Timer

[[category: LPC1768 Tutorials]]
=Objective=
In this tutorial we will discuss how to configure lpc1768 systick timer for 1ms tick.<br>
At the end of the tutorial we will see how to use the ExploreEmbedded sysTick libraries.
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=Systick Module=
The System Tick Timer is an integral part of the Cortex-M3. The System Tick Timer is intended to generate a fixed 10 millisecond(user configurable) interrupt for use by an operating system or other system management software.<br>

The System Tick Timer is a 24-bit timer that counts down to zero and generates an interrupt. The intent is to provide a fixed time interval between interrupts.<br>

In order to generate recurring interrupts at a specific interval, the STRELOAD register must be initialized with the correct value for the desired interval.<br>
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=Registers=
{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
!Register || Address || Description
|-
|STCTRL|| 0xE000 E010  ||System Timer Control and status register
|-class="active"                                                                          
|STRELOAD|| 0xE000 E014  || System Timer Reload value register
|-                                                                                        
|STCURR|| 0xE000 E018 || System Timer Current value register
|-class="active"                                                                           
|STCALIB|| 0xE000 E01C || System Timer Calibration value register
|}<br><br><br><br>


{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|STCTRL
|-
|31-17 || 16 || 15-3 || 2 || 1 || 0 
|-
| RESERVED || COUNTFLAG || RESERVED || CLKSOURCE || TICKINT || ENABLE 
|}

'''Bit 0 - ENABLE '''<br>
This bit is used to enable/disable the systick counter.<br>
0-Disable the systick timer.
1-Enables the systick timer.


'''Bit 1 - TICKINT'''<br>
This bit is used to enable/disable the systick timer interrupt. When enabled the SysTick_Handler ISR will be called.<br>
0-Disable the systick timer Interrupt.
1-Enables the systick timer Interrupt.


'''Bit 2 - CLKSOURCE'''<br>
This bit is used to select clock source for System Tick timer.<br>
0-The external clock pin (STCLK) is selected.
1-CPU clock is selected.


'''Bit 16- COUNTFLAG'''<br>
System Tick counter flag. This flag is set when the System Tick counter counts down to 0, and is cleared by reading this register.
<br><br>




{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|STRELOAD
|-
|31-24 || 23 - 0 
|-
| RESERVED ||  RELOAD
|}

'''Bit 23:0 - RELOAD'''<br>
This is the 24-bit timer value that is loaded into the System Tick counter when it counts down to 0.
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{| class="table table-striped table-hover table-condensed table-bordered"
|-class="info"
|STCURR
|-
|31-24 || 23 - 0 
|-
| RESERVED ||  CURRENT
|}

'''Bit 23:0 - CURRENT'''<br>
Reading this register returns the current value of the System Tick counter. Writing any value clears the System Tick counter and the COUNTFLAG bit in STCTRL.
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=Timer Calculation=
SysTick timer calculation for 1ms tick with 100 Mhz CPU clock (cclk).<br>
cclk = 100 Mhz <br>
ticks = 1000/sec <br>

$$RELOAD = (cclk / ticks) - 1 = (100 Mhz / 1000) - 1 = 100,000 - 1 = 99,999$$
<br><br><br>

=Steps To configure Systick =
#Set the Reload value for required tick in STRELOAD.
#Enable the Systick Module by setting Enable bit in STCTRL.
#Select the CPU Clock Source by setting CLKSOURCE bit in STCTRL.
#Finally enable the SysTick interrupt by setting TICKINT bit in STCTRL.
<br><br><br>

=Code=
<html>
<script src="https://gist.github.com/SaheblalBagwan/4093e06a1139507f95c7adc0611e926b.js"></script>
</html>
<br><br>


=Using ExploreEmbedded Libraries=
<html>
<script src="https://gist.github.com/SaheblalBagwan/204ae5d68ea7c2db911cc2c5398ab540.js"></script>
</html>
<br><br>

=Using ExploreEmbedded Libraries=

[[File:00_Lpc1768_Timer.jpg]]