Timer 0 Modes of Operation. Normal Mode Clear Timer on Compare Match (CTC) Fast PWM Mode Phase Corrected PWM Mode

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Cordelia Conley
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1 Timer 0 Modes of Operation Normal Mode Clear Timer on Compare Match (CTC) Fast PWM Mode Phase Corrected PWM Mode

2 PWM - Introduction Recall: PWM = Pulse Width Modulation We will mostly use it for controlling motors PWM is used in MANY other applications! Standard PWM signal Frequency can vary!

3 PWM Servo control Standard servos require a 50Hz (20ms) signal Typically, a 1ms 2ms high pulse 1ms = CCW, 2ms = CW, 1.5ms = neutral

4 Generating a PWM signal One way to generate a PWM signal on a uc is to use interrupts and a timer Set up the timer to interrupt in deterministic intervals Write a digital high to an output port in the ISR Wait... Write a digital low to the same port in the ISR Wait... Repeat (possibly updating the timer count value in the ISR to change the duty cycle)

5 Generating a PWM signal What is wrong with doing this???

6 Generating a PWM signal What is wrong with doing this??? You have to interrupt several times to generate the signal You have to interrupt to change the signal duty cycle VERY inefficient!!!

7 Generating a PWM signal Is there any other way to generate these signals???

8 Generating a PWM signal Is there any other way to generate these signals??? Of course! Use the output compare modes within the timer's functionality We will discuss both methods used on Timer 0 Fast PWM Phase corrected PWM

9 Fast PWM High frequency operation Hence the name FAST PWM Uses a single-slope operation!

10 Fast PWM Functionality Counter is incremented until it reaches the TOP and then it's cleared When a match occurs on EITHER OCR0A/B the corresponding output compare pin is changed Changed according to the COM0A/B bits in TCCR0A

11 Fast PWM vs. CTC How is this different than CTC mode??? Recall: In CTC mode the TCNT value never reaches the TOP unless the value in OCR0A is 255! The counter is cleared on a match In Fast PWM mode, the counter is only cleared when the TOP is reached So what is TOP (depends on the mode) Determined by the WGM bits

12 Fast PWM vs. CTC How is this different than CTC mode??? Recall: In CTC mode the match value is only compared to OCR0A OCR0B is disabled! In Fast PWM mode, both OCR0A and B are used for corresponding comparisons (depending on the mode) This allows two different PWM signals to be generated from a single timer This allows three different PWM signals to be generated if using 16-bit timers

13 Fast PWM vs. CTC How is this different than CTC mode??? Recall: In CTC mode the match value can only be toggled when a match occurs Unless we play interrupt tricks with the value in OCR0A we can only generate a 50% duty cycle signal In Fast PWM mode, we can toggle, set, or clear the pin on match (as we will see) Since we don't clear the timer on a match, we can also generate PWM signals with different duty cycles Further, this allows us to generate inverted PWM signals

14 So, how do we use it? Depends on which mode we want to use! WGM = 3 or WGM = 7

15 What's the difference? WGM = 3 Standard operation Timer counts from 0x00 to 0xFF When a match occurs on OCR0A/B the OC0A/B pins are changed accordingly WGM = 7 The TOP (0xFF) value is set to the value in OCR0A As a result, OC0A is ALWAYS either high or low (depending on the COM bits) We can generate a desired frequency for OC0B by adjusting the value in OCR0A

16 Lets have a look at WGM = 3 We have to make a choice for the behavior of OC0A/B Set the COM0A/B 1:0 bits

17 What frequency does it operate at? Depends on the prescale value! Clock select bits in TCCR0B (1,8,64,256,1024) Recall the single slope operation: if operating in WGM=3 mode F_PWM = F_clk/(N*256) Prescale Frequency Period khz 16 us khz 128 us Hz ms Hz ms Hz ms

18 Servo control Looking at the frequencies and period of the signal, is it possible to control our servos using Timer 0 in Fast PWM mode??? Recall: We need a 50 Hz signal (20 ms period) Prescale Frequency Period khz 16 us khz 128 us Hz ms Hz ms Hz ms

19 DC Motor control Looking at the frequencies and period of the signal, is it possible to control standard DC Motors using Timer 0 in Fast PWM mode??? Recall: We need a duty cycle that ranges from 0%- 100% Prescale Frequency Period khz 16 us khz 128 us Hz ms Hz ms Hz ms

20 DC Motor control Answer: Yes! The frequency doesn't matter (sort-of) we just need to see how we control the duty cycle! Recall, we can set OCR0A/B to toggle, set, or clear on a match! We can change the values of OCR0A/B to change the duty cycle 64 = 25% duty cycle 128 = 50% duty cycle 192 = 75% duty cycle

21 Example!

22 Example!

23 Notes on the Example Two things to notice about our example We generate TWO different PWM signals on OC0A and OC0B OC0A changes from 0% - 100% -0% OC0B is a 75% duty cycle square wave We never have to interrupt to change the duty cycle We can write to OCR0A anytime This may cause glitches but shouldn't have any effect on your motor We can eliminate the glitches using one of the 16-bit timers or an RTOS

24 Fast PWM mode cont. So what about WGM = 7??? We will look at this option in phase corrected PWM mode The basic principles are the same

25 Phase corrected PWM mode Similar operation to Fast PWM mode Difference: Dual slope operation

26 WGM for Phase Corrected PWM Similar to Fast PWM we have two options to choose from WGM = 1 or WGM = 5

27 What's the difference??? In phase corrected PWM the timer counts from 0x00 to TOP then back down to 0x00 TOP is defined as 0xFF when WGM = 1 and the value in OCR0A when WGM = 5 Some important things to note: The timer does not overflow at the TOP but rather at the BOTTOM Depending on the COM0A/B bits and the WGM bits, OC0A/B is cleared when a match happens while up-counting and set when down-counting

28 COM bits Essentially the same as with Fast PWM In BOTH PWM modes, the DDR must be set to be an output on OC0A/B

29 So what frequencies do we have available? Again we need to look at the prescale value Since we have a dual-slope, we can compute the frequency of the PWM as: f_pwm = f_clk/(n*510) Prescale Frequency Period khz us khz 255 us Hz 2.04 ms Hz 8.16 ms Hz ms

30 Servo control Looking at the frequencies and period of the signal, is it possible to control our servos using Timer 0 in Phase Correct PWM mode??? Recall: We need a 50 Hz signal (20 ms period) Prescale Frequency Period khz us khz 255 us Hz 2.04 ms Hz 8.16 ms Hz ms

31 Yes!!! How??? Servo control

32 Servo control Yes!!! How??? We need to choose the correct prescale value Prescale Frequency Period khz us khz 255 us Hz 2.04 ms Hz 8.16 ms Hz ms

33 Servo control Yes!!! How??? We need to choose the correct prescale value Prescale Frequency Period khz us khz 255 us Hz 2.04 ms Hz 8.16 ms Hz ms Only one choice will work!!! (divide by 1024)

34 Servo control Using WGM = 1 and a prescale value of 1024 we get a 32 ms period signal Will this work for servo control???

35 Servo control Using WGM = 1 and a prescale value of 1024 we get a 32 ms period signal Will this work for servo control??? NO!!! We need a 20 ms period signal so standard operation is too long! Can we slow it down a bit??

36 Servo control Using WGM = 1 and a prescale value of 1024 we get a 32 ms period signal Will this work for servo control??? NO!!! We need a 20 ms period signal so standard operation is too long! Can we slow it down a bit?? YES!!! Use WGM = 5 and set OCR0A to a smaller value than 0xFF!!! Recall: It takes 32 ms to count from 0x00 to 0xFF back to 0x00

37 Servo control So how do we determine what value to use for OCR0A??? Recall: We have f_pwm = f_clk/(n*510) = f_clk/(2*n*255) This assumes we count from 0x00 to 0xFF but we want to stop at some OCR0A < 0xFF f_pwm = f_clk/(2*n*ocr0a) OR OCR0A = f_clk/(2*n*f_pwm)

38 Servo control For a 50 Hz PWM signal and prescale of 1024 OCR0A = 16 Mhz/(2*1024*50) = We can't have a.25 (8-bit integer value), so what is our true frequency??? Round down: OCR0A = 156 f_pwm = 16 Mhz/(1*1024*156) = Hz Which has a ms period (close enough!)

39 Servo control Recall: to control the servo we need a 20 ms period signal We have just calculated how to generate this! We also need the signal to be high from 1ms 2ms and low the rest! Hmmm. How do we go about doing this??? Think OCR0B!!!

40 Servo control Recall that we can use the OCR0B register to control the behavior of OC0B OCR0A is in use and therefore OC0A is always a constant!!!!! How do we compute what OCR0B needs to be??? The derivation is left as an exercise to YOU!!! OCR0B = f_clk * desired high pulse (in ms)/(2*n) This is for COMB set to clear on match up-counting (10)! Think about how you might change it for COMB = 11

41 Servo control Assuming we have COMB = 10 OCR0B = f_clk * desired high pulse (in ms)/(2*n) 1ms = 8 1.5ms = 12 2ms = 16

42 Example (servo control)

43 Example (servo control)

44 Some notes on the example Again, we can control the servos without the need to interrupt program flow!!! What's the issue with Timer 0 though??

45 Some notes on the example Again, we can control the servos without the need to interrupt program flow!!! What's the issue with Timer 0 though?? Too low of resolution!!! We only have four values to get us from full CCW to neutral Our servo control will be very course! How do we fix this???

46 Some notes on the example Again, we can control the servos without the need to interrupt program flow!!! What's the issue with Timer 0 though?? Too low of resolution!!! We only have four values to get us from full CCW to neutral Our servo control will be very course! How do we fix this??? Use the 16-bit timer Setting one up is left to you (LAB)

47 DC Motor control Looking at the frequencies and period of the signal, is it possible to control standard DC Motors using Timer 0 in Phase Correct PWM mode??? Recall: We need a duty cycle that ranges from 0%- 100% Prescale Frequency Period khz us khz 255 us Hz 2.04 ms Hz 8.16 ms Hz ms

48 DC Motor control Answer: Yes! The frequency doesn't matter (sort-of) we just need to see how we control the duty cycle! Recall, we can set OCR0A/B to toggle, set, or clear on a match! We can change the values of OCR0A/B to change the duty cycle similar to Fast PWM!!! This method is preferred to control motors since the phase is correct in this mode What do I mean by phase correct??? (draw figure)!!

ATmega16A Microcontroller

ATmega16A Microcontroller Timers 1 Timers Timer 0,1,2 8 bits or 16 bits Clock sources: Internal clock, Internal clock with prescaler, External clock (timer 2), Special input pin 2 Features The choice of