Our previous program was not very efficient and burned CPU clock cycles and power just to create a delay , wouldn't it be better if the CPU could be interrupted at the correct time and flip the LED only when it needed to ? If so then we would have all the time in the world to do other things or just let the CPU sleep and save on our battery.

Well we can by using the stm8s105's timers. The timers available are very sophisticated an we will only touch on some of the capabilities here. The stm8s105C6T6 has a total of 4 Timers at it disposal.

	Resolution	Capture/Compare  Channels	Up/Down  Counting	Complementary  Outputs	External  Trigger	External  Break	Max Prescale  Divide
Timer 1	16	4	Up / Down	3	Yes	Yes	65536
Timer 2	16	3	Up	0	No	No	32768
Timer 3	16	2	Up	0	No	No	32768
Timer 4	8	-	Up	0	No	No	128

Timers can be used for a variety of purposes, including:

● Time base generation
● Measuring the pulse lengths of input signals (input capture)
● Generating output waveforms (output compare, PWM and One Pulse Mode)
● Interrupt capability on various events (capture, compare, overflow,update)

We are going to use Timer 2 and its capability to send an interrupt when it is updated.

Essentially timers are counters and Timer 2 can be used in it's most basic way to count upwards from zero until a specific value has been reached , it is then reset back to zero and starts again triggering an interrupt at that time.

The timers take their clock source from the Master Clock but can be prescaled in order to lower the clock frequency that is counted. This is done for Timer2  by setting the lower 4 bits in the TIM2_PSCR register.
The prescale divide is calculated as 2 to the power of TIM2_PSCR as listed below:

TIM2_PSCR	TIM2_PSCR (decimal)	Prescale   Divide	Timer Clock Freq  @ fMaster=16mhz
0x00	0	1	16 mhz
0x01	1	2	8 mhz
0x02	2	4	4 mhz
0x03	3	8	2 mhz
0x04	4	16	1 mhz
0x05	5	32	500 khz
0x06	6	64	250 khz
0x07	7	128	125 kz
0x08	8	256	62.5 khz
0x09	9	512	31.25 khz
0x0A	10	1024	15,625 khz
0x0B	11	2048	7.8125 khz
0x0C	12	4096	3.90625 khz
0x0D	13	8192	1.953125 khz
0x0E	14	16384	976.5625 hz
0x0F	15	32768	488.28125 hz

Using the Maximum prescale divide we have to count to 488 clock pulses to get an interrupt about every second.

The Procedure is as follows ..

1. Prescale the timers clock by 32768 by placing 0x0F into the TIM2_PSCR prescale register , this must be done by loading both the High and Low bytes separately with the Highest first as per technical reference.
   
2. Place 0x01E8 (488 in decimal) into the Auto Refresh Register TIM2_ARR this is the value we count to before returning to zero.
3. Enable interrupts on timer update so we get an interupt when the counter is updated ( reset to zero ) after 1 second.
4. Set the CEN bit on the Timer Control Register 1 to enable the timer
5. Reset the CPU interrupt mask so interrupts are enabled with the rim instruction , sim would disable them again by the way
   

But first we must set up an interrupt handler to catch the interrupt when it happens. In this interrupt hander we toggle the LED state as we have done so previously.



Then place its address in the interrupt table.


The directive interrupt Timer2UpdateInterupt is to tell the debugger that this is an interrupt routine , otherwise interrupt routines are like subroutines but return with the iret instruction, when an interrupt routine is triggered all of the CPU's registers are preserved on the stack so you don't have to worry about modifying them , the iret instruction restores the registers to there previous values before the interrupt routine was called , the execution of the program then continues for the point is was interrupted

One final point we need to reset the timers interrupt flag in the timer2's Status Register 1 (TIM2_SR1) to signal that it has be handled, otherwise the interrupt will keep on triggering.

Now the complete code ..


   
This will blink the LED every second , you may have noticed the wfi instruction commented out, This is "wait for interrupt" and it does just that puts the CPU in a low power state while it waits for an interrupt to happen the code the executes after it, which will just call wfi again in this case. However from some reason it interferes with the debugger and so it is left commented out. Just uncomment it for a release version.


We can use multiple timers together , try adding the code below that uses the low resolution 8 bit timer 4 in order to slow down the LED over time.


    



This code showed us how to use interrupts to blink the LED , however there is an even better way to blink the LED without using any CPU cycles , find out more in always on the pulse.