Question

Using Timer1, no prescaler, and CTC mode, write a program that generates a square wave with a frequency of \(1 \mathrm{kHz}\). Assume \(\mathrm{XTAL}=8 \mathrm{MHz}\).

Short answer

Set OCR1A to 3999, configure the TCCR1A and TCCR1B registers for CTC mode (set WGM12, clear WGM13, WGM11, and WGM10, and set CS10 for no prescaler), set the output pin to output mode, and enable pin toggle on compare match by setting COM1A0 or COM1B0.

Step-by-step solution

Understanding Timer1 and CTC Mode

Timer1 in CTC (Clear Timer on Compare Match) mode will count up to a value specified in the Output Compare Register (OCR1A), then reset to 0, toggling the output pin, and continue counting. This mode is used to generate precise timing events like a square wave.

Calculating the Compare Match Value

To create a 1 kHz square wave, Timer1 needs to toggle the output pin every 0.5 ms (half-period of a 1 kHz signal). The formula to calculate the output compare register value (OCR1A) is: OCR1A = (XTAL / (2 * frequency * prescaler)) - 1. No prescaler means the prescaler value is 1. Thus, OCR1A = (8,000,000 / (2 * 1,000 * 1)) - 1 = 3999.

Setting up Timer1 in CTC Mode

Configure Timer1 for CTC mode by setting the appropriate bits in the TCCR1A and TCCR1B registers. Clear the WGM13:0 bits to 0 except for WGM12 which should be set to 1 to enable CTC mode.

Configuring the Output Pin

Set the data direction of the output pin (e.g., OC1A or OC1B) to output. This is done by writing a 1 to the corresponding bit in the DDR (Data Direction Register).

Writing the Compare Value to OCR1A

Write the calculated value (3999) to the OCR1A register to set the compare match value.

Starting the Timer

Start Timer1 by writing to the TCCR1B register to select CTC mode (with WGM12 set) and no prescaler (CS10 set to 1).

Toggling the Pin on Compare Match

Ensure that the Timer1 output compare match will toggle the output pin OC1A or OC1B by setting the COM1A0 or COM1B0 bit in the TCCR1A register.

Key concepts

Square Wave Generation

Generating a square wave with a microcontroller such as those found in AVR embedded systems involves toggling a digital pin on and off at a specific frequency, resulting in a signal that alternates between a high and a low state in equal duration. This is useful in a variety of applications from digital clocks to pulse width modulation for motor control.

To do this, timers within the microcontroller can be configured in CTC (Clear Timer on Compare Match) mode. In the AVR microcontroller, when Timer1 is set to CTC mode, it counts up and compares its value with a predetermined one. Once it reaches this 'compare match value', it resets back to zero, and if configured correctly, can toggle an output pin, thus generating a predictable square wave. It's a simple yet elegant way to achieve this using embedded systems programming without constant CPU intervention.

Compare Match Value Calculation

The calculation of the compare match value is critical to ensure that the square wave generated has the desired frequency. As seen in the step-by-step solution, the formula involves the clock frequency of the system \( XTAL \) and the desired square wave frequency.

For a 1 kHz square wave, the output pin needs to toggle every 0.5 ms (the half-period of 1 kHz frequency), considering the wave consists of a high and a low state of equal duration. The formula \( OCR1A = (XTAL / (2 \times frequency \times prescaler)) - 1 \) utilizes no prescaler, directly leveraging the clock frequency.

Calculating OCR1A for 1 kHz:

Inserting the values, \( OCR1A = (8,000,000 / (2 \times 1,000 \times 1)) - 1 = 3999 \). This calculated value is then set in the OCR1A register, so every time the Timer1 matches this value, the output pin toggles, forming the desired square wave.

Embedded Systems Programming

Embedded systems programming involves writing software to control machines or devices that are not typically considered computers, like microcontrollers. In programming AVR microcontrollers for tasks such as generating a square wave, we use direct register manipulation to configure the device's hardware peripherals.

One sets up the timer, controls the output pins, and configures the clock system using C-language instructions that interact with hardware registers such as TCCR1A/B for timer settings or DDR (Data Direction Register) to specify pin direction. The combination of these settings and manipulations, as seen in the programming steps provided, constitutes the crux of embedded systems programming. Each change directly affects the microcontroller's behavior at a very low level, allowing for precise control over the hardware's functionality.