This page is best engaged after completing both,| AVR Optimization #3: ATmega328P Timer/Counter Interrupts Using Register-Level Coding |
This page is best engaged after completing both,
It is difficult to overstate the importance of the onboard Timer/Counter units embedded within MCUs. So important is an understanding and competency in their Interrupt usage I have committed this separate page to exploring their configuration and application to digital signal generation.
Your Grade 11 course introduced you to the DS1307 RTC, in particular, to its dependency on a 32.768 KHz crystal oscillator to keep accurate track of time by internal counter registers. As you are aware, the default source of your ATmega328P MCU clock is a 16 MHz crystal. It should be noted that alternate system clock sources are always an option. One possible motivation for doing so is the relatively awkward frequency calculations associated with the Arduino's standard 16 MHz crystal. This is, indeed, 16 000 000 oscillations per second. Since Timer/Counters are either 8 or 16-bit, and prescalers are also powers of 2, a crystal frequency that is also a power of 2 (such as the 215 or the 32768 Hz crystal used with the DS1307 would make for improved timekeeping. For this reason I have ordered a small number of the 8.192 MHz crystals from Digikey for a clock project I (or one of you for an ISP?) may like to take on.
Here's some possible inspiration for your Long ISP:
How a quartz watch works - its heart beats at 32.768 kHz
| Crystal Oscillators | ATmega328P Timer/Counters |
|---|---|
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The ATmega328P has three builtin Timer/Counter units onboard (by way of contrast, the ATtiny84 has two, the ATtiny85 has one, and the ATmega2560 has SIX!) that tap into the 16 MHz crystal oscillator for sources of secondary timekeeping. These Timer/Counter units are referred to simply as Timer 0, Timer 1, and Timer 2. Timer 0 and Timer 2 are built on 8-bit counting and Timer 1 on 16-bit. Many of the basic functions Arduino users rely on invoke Timer/Counter performance in the background. To this point, the Arduino's UNO's ATmega328P leaves the factory preflashed with the Blink sketch's dependency on Timer 0, ready to rely on system clock counts to support the delay() function.
(Informative) Sparkfun Video Series: Level-Up Your Arduino Code: Timer Interrupts
Associated with each Timer unit is either an 8- or 16-bit register called a counter. Under default operation, each cycle of the MCU's clock source is accumulated within the counter. When the accumulation reaches the maximum value (255 or 65535) the counter starts over again at 0. We call this an overflow event.
Understandably, with a 16 MHz crystal oscillator as the clock source, the default counter overflow frequencies are 62.5 kHz (16 MHz/256) for 8-bit counters and 244.14 Hz (16 MHz/65536) for 16-bit counters (interestingly, if you attached this signal to a speaker with sufficient amplification, humans would not hear anything with these frequencies). As mentioned, these are the default behaviours. A review of the Timer/Counter Block Diagram to the right suggests there are many options for tailoring counter behvaiour and the resulting digital signal forms that can be generated from strategic interrupt designs.
One significant option for tailoring the behaviour of the input clock signal the Timer sees is the prescaler. As previously encountered with the 128 kHz Watchdog timer oscillator, a prescaler divides the clock signal into multiples to effectively slow the input signal down. This and numerous other options, are explored below.
Interrupts #8-#17. Timer/Counter Interrupts
First off, another look at the ATmega328P's Vector Table below left reveals that the jump addresses #8-#17 are reserved for a number of Timer/Counter events. ACES are also encouraged to review the graphic below right to ensure they are are aware of the integration of the ATmega328P's timer in support of commonly used functionality. Overlapping uses of Timers will, understandably, result in ineffective performance. For various reasons we'll begin our Timer/Counter interrupt investigations with Timer 2, Normal Mode. Chapter 18 of the ATmega328P datasheet discusses the use of Timer/Counter 2 with details of its Register usage on page 153 of the datasheet.
| ATmega328P Interrupt Vector Table | Timer/Counter-Related Uses and Applications |
|---|---|
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1. Timer2: Normal Mode (Interrupt #10: TIMER2_OVF_vect) (OC2A and OC2B Disconnected)
Each timer offers a slightly different set of Modes that can be configured for various purposes. The graphic below left summarizes the modes offered by Timer 2. In the simplest mode (Normal) an interrupt can be periodically generated when its 8-bit counter (TCNT2) overflows (one count after the TOP:255). The diagram below right depicts the counting as blue segments that, at their peak, trigger an interrupt as the counter simply continues from BOTTOM:0). It should be noted that all three timers offer this Normal Mode 0.
| ATmega328P Timer 2 Modes | Normal Mode (0) |
|---|---|
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Task A. (No Prescaler)
The goal of this task is to create the simplest configuration of Timer2's Normal Mode resulting in an interrupt generated each time its TCNT2 register overflows. The associated ISR will inform our code that the state of the development board's onboard LED could to be toggled.Task B. With a Prescaler
For many (most?) applications the overflow interrupt frequency of the previous task is too high. This is where the concept of a system clock prescaler can be introduced.
Timers.xlsx workbook.
Tab to the worksheet entitled Frequency Formula and review the calculations required to yield specific frequencies.2. Timer2: Frequency Confirmation with WDT
The previous investigation began our journey into Timer-generated waveforms. Before we get too far along, it is instructive to confirm the frequency of the interrupt generation. The formula to the right is a good starting point for determining the expected overflow frequency, fovf of Normal mode where the count starts at 0 and through the intervention of a prescaler value (PVal) until it reaches the capacity of the respective Counter (255 or 65535) before triggering an interrupt.
OK, time to roll our skills forward. To assist us in monitoring the overflow frequency for a particular Timer configuration, we can employ our recently mastered Watchdog Timer interrupt to schedule and trigger the display of fovf to the Serial Monitor at periodic intervals.
Task.
In your setup() function configure Timer2 for Normal Mode under a prescaler of 1024 and a ISR(TIMER2_OVF_vect) to simply count overflows.| ATmega328P Timer 2 Normal Mode with Preload ? | (PS1240P02BT) Piezo Buzzer Drive Circuit |
|---|---|
3. Timer2: Normal Mode (Interrupt #10: TIMER2_OVF_vect) (OC2A and OC2B Connected)
In the two previous tasks, we exploited Timer 2 hardware to signal our software to maintain the square wave driving an LED. As I stated early in AVR Optimization #2,
"The most efficient software for the embedded developer is typically none at all".
In this next task, we'll use hardware alone to produce the square wave driving the frequency of the LED.
| ATmega328P Timer/Counter Output Compare Pins | Timer 2 Normal Mode (OC2A/B connected) |
|---|---|
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A closer look at the pin mapping of the Arduino below left reveals 6 digital pins with alternate functionality labeled OCnX (pins 3,5,6,9,10,& 11). These pins can be configured to change their state (cleared, set, or toggled) when a Timer/Counter register such as TCNT2, matches the value preloaded into either of it Output Compare Registers (OCR2A or OCR2B). Since this functionality is achieved strictly at the hardware level (software ISR not required) it offers improved efficiency.
Task.
Configure PB3 to present a ground for the time being.| Timer 2 Normal Mode (TCNT2) | Timer 2 Normal Mode (OC2A, OC2B, and OVF Signals) |
|---|---|
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4. Timer2: Normal Mode, Interrupt #8 (TIMER2_COMPA_vect) and Interrupt #9 (TIMER2_COMPB_vect)
The previous task bypassed software altogether in presenting matches between TCNT2 and OCR2A and OCR2B as 50% duty cycle square waves, with a limited range of frequencies, to digital IO pins 11 (OC2A) and 3 (OC2B), respectively. Although independent hardware performance is often desirable, there are application where synchronous software intervention would be desirable.
Another look at the 328P's IVT below left, reveals a pair of additional interrupt vector addresses, conveniently, for all three timers. In this next exercise you are asked to piggyback on the previous task to perform an audio frequency task.
| ATmega328P Interrupt Vector Table | Timer/Counter-Related Uses and Applications |
|---|---|
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Task.
Online Pitch Detector. I would like to predict/experiment/confirm that we can develop a crude register-level simulation of what the Tone library achieves.Timers.xlsx workbook as a guide, see how close you code can be developed to produce a desired frequency and have the Online Pitch Detector confirm your calculations.| Timer2 REgsiter Summary | Piezo Buzzers |
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TBD...
More to Come...2022 01 15
A good blog post: https://wolles-elektronikkiste.de/en/timer-and-pwm-part-1-8-bit-timer0-2
Source code for analogWrite():
https://github.com/arduino/ArduinoCore-avr/blob/master/cores/arduino/wiring_analog.c
Audio Tones
pitches.h: http://darcy.rsgc.on.ca/ACES/TEI4M/Assembly/code/pitches.h
40. [2017/2018] THE 4-WIRE DC FAN. Researching and developing this project has been extremely rewarding and it is a superb context for assembling and reviewing much of the engineering content we have explored over the past two and a half years. Furthermore, this project requires the use of all three timers available on the Arduino!
Few components are available in a greater variety than the humble DC Cooling Fan. The 4-wire Sunon 12VDC ball bearing fan was chosen for this project because it offers numerous options that can be explored under elementary and advanced microcontroller operation.
PART A. GETTING ACQUAINTED WITH THIS FAN
PART B. HARDWARE DESIGN. The web has a great deal of instructional content for interfacing 2- and 3-wire fans. For 4-wire fans there is a far less and some if it is inefficiently designed or simply wrong.
PART C. PWM DESIGN. PWM signal design (modulation) requires consideration of two characteristics: frequency and duty cycle. Our (fun) challenge is to configure an AVR timer to provide a PWM signal to match the optimal requirements outlined in fan's datasheet. Whereas the frequency of the signal remains constant, the duty cycle (speed) should be variable (in this stage, we'll hard code the changes to the duty cycle. In the next stage we'll consider external control (manual or sensor) over the duty cycle). Finally, as you develop your solution, remember, the best (most efficient) strategy places as much, if not all, the burden on the hardware that can perform in the background.
T. Morland (RSGC '18) developed a working solution after class Wednesday and offers us this scope trace of his assembly implementation ...
PART C Task.
PART D. Fan Speed (ADC). We've reached the stage in this project where the speed of the fan (duty cycle) is turned over to the operator. For this, we need to explore low-level register manipulation of the AVR's ADC unit.
PART D Task.
PART E. SENSE (FEEDBACK, TACHOMETRY, YELLOW) WIRE (Demodulation of PART C's PWM).
OPEN loop systems (no feedback) are undesirable as the user has no idea if the process is functioning as designed.
CLOSED loop devices such as our 4-wire fan offer a means of monitoring and adjusting our input PWM by sensing the speed of rotation and placing this signal on the fourth (yellow) wire. This variable PWM signal can be captured and demodulated by our microcontroller, analyzed and the data used to adjust the input PWM as necessary to ensure the fan is maintaining a desired performance. Note. All sorts of information can be carried by a PWM signal. An image from the datasheet you are about to read suggests how the duty cycle of a PWM signal could be used to carry ASCII data. Inspiring stuff.
The 328P's Timer1 facilitates the capture of the yellow wire's PWM signal through it's ICP1 function (mapped to Arduino's digital pin 8). Demodulation of this signal will expose the fan's actual RPM. Furthermore, through Atmel Studio's Data Visualizer feature we can explore AVR's Serial Communication feature and plot both the theoretical and actual RPM. This sets us up well for the final stage of this project, PART F: PID.
PART E. Task (TACH Wire Demodulation)
PART F. Serial Communication (USART: Universal Synchronous/Asynchronous serial Receive and Transmit) After all the groundwork previously laid we're now in the enviable position of acquiring data from our system and placing it into serial data stream for external retrieval.
In this stage we're going to confirm and display as text, the value of OC2B and number of falling edges the tach wire is yielding per second.
Just as the Serial Monitor and Serial Plotter are available tools within the Arduino IDE, Atmel Studio offer a far more comprehensive utlity, the Data Visualizer, that does a great deal more. To fully exploit its capabilities we must learn to navigate the AVR's USART subsystem.
Reference 1: Serial Communication is discussed in Chapter 20 of the ATmega328P datasheet
Reference 2: Microchip's Data Visualizer Home Page
Reference 3: Microchip's Data Visualizer Users' Guide
PART F. Task.
PART G. Graphic Data Visualization. In this stage we wish to generate a graphic plot comparing the theoretical RPM to the empirical RPM obtained through feedback, setting the stage for the final leg of ths [epic 8-part] marathon: PID in which we expect to the exteced and actual plots to merge. A plot of the Duty Cycle, Theoretical RPM and Empirical RPM are displayed over a FULL range of input duty cycles provided by a potentiometer was undertaken weeks ago in the high-level Arduino IDE and visualized by it's Serial Plotter utility over 1 s intervals. in this stage we'll see to what extent we can produce a similar visualization through AVR assembly exploiting Atmel Studio's Data Visualizer.
| Arduino Serial Plotter | Atmel Studio 6 Data Visualizer |
|---|---|
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PART G. Task.
, will access context-sensitive support. Under External Connections, double-click Serial Port to launch the Serial Port Control Panel. Click Connect to see the Terminal window. Confirm the data stream.
to the plot area. Confirm.PART H. PID. You are well-aware that simply supplying a device with a required signal is not sufficient to GUARANTEE required performance. To achieve this you must create a closed loop that continually measures the process output (actual device performance) and dynamically adjusts the setpoint signal by reducing or eliminating the variance (error-e).
PID Principle 1: Introduction: PID Controller Design
PID Principle 2: PID Controller on AVR Devices
PID Principle 3: Detailed PID Library Discussion
PID Principle 4: The PID Controller (Nults&Bolts Magazine)
Application 1: Arduino: DC Motor Control Position Control using PID
Part H. Task.
AVR TIMERS
38. 
Timer1. Fast PWM Mode 15. In this exercise you are asked to develop standalone assembly code that will result in the the horn of your HXT500 standard servo motor sweeping through its 180ˆ of rotation, continuously.
Task.
RSGC ACES: HXT50037. 
USART Debugging Utility. In this exercise you are asked to develop a utility that can be used to debug your assembly code by dumping the contents of SRAM the Serial Monitor.
Task.
36. Timer1. Clear Timer on Compare Match. Modes 4 (Output Compare) and 12 (Input Capture) result in the TCNTn clearing (0) when it matches the preset OCRnA/B register. The attached pins can be cleared, set or toggled. This is somewhat easier to use than setting the start count less than the TCNTn resolution an climbing all the way to the overflow. The
OCRnA/B register can be manipulated in the COMPA/B ISR to modify the resulting PWM Signal.
Task.
decreases.
The mono waveform, recorded in stereo, is shown to the right as it appears in Adobe's SoundBooth.
35. Timer1. Normal Mode with Output Compare (Software and Hardware Interrupts). In addition to a strictly hardware output response to the occurrence of a Compare event, on A and/or B, one can add a software response as well.
Task.
.global TIMER1_COMPA_vect
TIMER1_COMPA_vect:
...
reti
33b. Light It Up! (This should be fun:) Each of you has been given a Piezo Buzzer with which we'll attempt to crank up the audio in the DES confirming each of the 7 frequency bands of the MSGEQ7 used in Ethan's Equalizer at the front of the room.
33a. Sound: Timer2 Normal Mode with PreLoad. In the previous exercise you simply let the Timer's counter run freely, restarting at 0x00 on the overflow. In this exercise you are asked to undertake an audio exploration of Timer2's Overflow Interrupt (in Normal Mode) while manipulating the TCNT2 register.
In this exercise you are asked to toggle Pin 13 with Timer2 Interrupt Overflows so as to recreate the provided audio file.
Task.
the volume remains relatively stable while the frequency (pitch) increases. The mono waveform, recorded in stereo, is shown below right as it appear in Adobe's SoundBooth.
32. Timers as Audio Frequency Generators. AnalogWrite PWM and the tone() Function.
AnalogWrite PWM
We have put the 6 Arduino PWM pins to good use in the past to affect the duty cycle of our signals but not the frequency. The frequency remained beyond the scope of the analogWrite(pin,dutyCycle) function.
uint8_t pwmPin = 11;
void setup(){
pinMode(pwmPin,OUTPUT);
}
void loop() {
for (int i=0; i<255; i++) {
analogWrite(pwmPin,i);
delay(30);
}
}
You will recall that the parameter to the analogWrite() function affects the duty cycle (mark/space ratio). Frequency and volume are the two primary audio characteristics you'll experience. Which of the two are affectd by the changing parameter? Does this make sense?tone(pin, frequency, duration) Function
31. Timer1. Normal Mode. The simplest of Timer1 modes results in the system clock incrementing its 16-bit register pair TCNT1H:TCNT1L with every tick. At 16MHz (`2^24`), the count will reach the top (`2^16`), 256 times per second. The use of one of the prescalers can slow the accumulation down.
When the count overflows, an interrupt overflow (TIMER1_OVF_vect) is generated and the count continues from 0x0000 whether you service the request or not. Setting the TOIE1 bit in the TIMSK1 register allows your code to respond to the request. Review the document, Timer1InterruptNormalMode.docx. I have provided both a mid-level implementation of a Timer1 Normal Mode Overflow Interrupt Routine to ease you into the register configurations, followed by the equivalent low-level standalone assembly version in which the ISR toggles an LED on PB5 (digital pin 13 on the Arduino) at rate of 60Hz (120 ovf/s). (Note: although 24Hz is the understood threshold for Persistence of Vision I found this far too low. Even at 40Hz, flicker is detectable. 50-60Hz is more like it)
Task.
Create an Arduino project entitled Timer1InterruptNormalMode and add the file, Timer1Interrupt1NormalMode.S. Be sure to leave the high-level file Timer1InterruptNormalMode.ino blank. Note: For future reference, you may wish to copy the mid-level version from the document and paste it in as the .ino version, as long as it is completely commented out.
