Configuring timers and GPIO for interrupt latency testing

The key to measuring interrupt latency is utilizing the ARM Cortex chip’s timers and GPIO pins. By configuring a timer to trigger a GPIO interrupt at precise intervals, we can measure the time elapsed between the timer interrupt request and the execution of the GPIO interrupt handler. This provides an accurate measurement of the interrupt latency experienced by the system.

1. Choosing the Right Timer

Most ARM Cortex chips have multiple timer peripherals available. For interrupt latency testing, we want a timer that offers high precision and flexibility in setting interrupt intervals. The SysTick timer found in Cortex-M cores is generally not suitable, since it has limited configurability. A better choice is one of the general purpose timers available on the chip.

Factors to consider when selecting a timer peripheral include:

• Bit width – wider timers allow more precise intervals.
• Number of channels – more channels provide flexibility.
• Interrupt capabilities – the ability to trigger interrupts is required.
• Clock source – typically the core clock or a dedicated timer clock.

For example, the STM32F103 chips have several 32-bit general purpose timers with 4 channels each and extensive interrupt capabilities. These would be an excellent choice for interrupt latency testing.

2. Configuring the Timer for Periodic Interrupts

Once an appropriate timer peripheral has been selected, we can configure it to trigger interrupts at regular intervals. The steps involved are:

1. Enable the timer clock in RCC registers.
2. Configure the timer’s prescaler to reach the desired interrupt frequency.
3. Set the auto-reload register (ARR) to define the timer interval.
4. Enable interrupts on the timer channel we will use.
5. Write the timer’s interrupt handler function.
6. Enable the NVIC interrupt for the timer in the ARM core.
7. Start the timer running in the desired mode (free-running).

Choosing the right prescaler and auto-reload values is key for generating the interrupts at precise intervals. Smaller intervals provide better latency resolution. Aim for interrupts at least 10x faster than the expected interrupt latency.

3. GPIO Interrupt Configuration

Next we need to configure a GPIO pin to generate interrupts on rising/falling edges that will be triggered by the timer interrupts. The steps are:

1. Enable GPIO clock in RCC registers.
2. Configure the pin as a digital input.
3. Enable interrupts on the GPIO pin for rising or falling edges.
4. Write the pin interrupt handler function.
5. Enable the GPIO pin interrupt in the NVIC.

It’s important to connect the GPIO pin to something that will cause rapid rising and falling edges at the timer’s interrupt frequency. A simple circuit with an oscillator or PWM signal driving the pin works well.

4. Measuring Interrupt Latency

With the timer and GPIO interrupts configured, we can now measure the interrupt latency in the GPIO handler function. The basic steps are:

1. Record the timer counter value on entry to the handler.
2. Perform any GPIO interrupt handling tasks.
3. Record the timer counter value before exiting the handler.
4. Compute the elapsed time between the two readings.
5. This value provides the interrupt latency for that occurrence.

Averaging many measurements provides an accurate overall latency figure. Optimally measuring the minimum and maximum latencies gives insight into the worst-case determinism.

The timer’s counter can be used directly if the prescaler and auto-reload values yield sufficient resolution. Otherwise, additional timers or hardware timers can measure the elapsed time.

5. Optimizing for Low Interrupt Latency

If interrupt latency is higher than desired, there are a number of techniques that can help reduce it:

• Assign higher priority to the timer and GPIO interrupts.
• Use processor’s lowest latency interrupt handling mode.
• Avoid lengthy computations in the interrupt handlers.
• Minimize higher priority interrupts that increase blocking time.
• Choose a GPIO pin that maps to a low-latency interrupt.
• Use hardware timers to improve time measurement resolution.
• Disable unnecessary peripherals that may be consuming CPU cycles.

Measuring the impact of each change is key to zeroing in on an optimal configuration. Getting interrupt latency as low and deterministic as possible improves real-time responsiveness in embedded systems.

Example Code

Here is example code for an ARM Cortex-M4 chip that configures a timer and GPIO pin to measure interrupt latency. It can serve as a starting point for your own testing. // Timer configuration RCC->APB1ENR |= RCC_APB1ENR_TIM2EN; // Enable timer 2 clock TIM2->PSC = 15; // Set prescaler to reach 10 MHz TIM2->ARR = 999; // Interrupts every 1 ms TIM2->DIER |= TIM_DIER_UIE; // Enable interrupt on update event // Timer2_Handler called on timer interrupt // GPIO configuration RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN; // Enable GPIOA clock GPIOA->MODER &= ~GPIO_MODER_MODER5; // Set PA5 to input GPIOA->PUPDR &= ~GPIO_PUPDR_PUPDR5; // No pull-up/down EXTI->IMR |= EXTI_IMR_MR5; // Unmask interrupt on line 5 EXTI->RTSR |= EXTI_RTSR_TR5; // Trigger on rising edge // GPIOA5_Handler called on GPIO interrupt // NVIC configuration NVIC_EnableIRQ(TIM2_IRQn); NVIC_EnableIRQ(EXTI4_15_IRQn); // Start timer TIM2->CR1 |= TIM_CR1_CEN; // GPIOA5_Handler uint32_t t_start = TIM2->CNT; // Record timer value // Handle GPIO interrupt uint32_t t_end = TIM2->CNT; // Record timer value uint32_t latency = t_end – t_start; // Compute interrupt latency

Conclusion

Measuring interrupt latency by utilizing timers and GPIO provides valuable insights into real-time system performance. Careful configuration allows latency to be quantified and optimizations to be tested. The techniques described here leverage the ARM Cortex peripherals to build an effective interrupt latency test system with high resolution and precision.

With the ability to accurately characterize latency, developers can construct responsive embedded designs and guarantee their real-time behavior. These testing capabilities are invaluable for verifying that stringent timing requirements are met, ensuring robust system performance in interrupting timing-critical tasks.

While interrupt handling overhead is usually minimal, when microseconds matter it becomes critical. By thoroughly testing and optimizing interrupt latency, robust real-time performance can be achieved. The methods outlined provide a pathway to maximizing responsiveness for peripheral interaction in interrupt-driven devices.

We have covered the key steps involved in utilizing the on-chip timers and GPIO of ARM Cortex chips to accurately measure interrupt latency. With the insight this provides, you can analyze your system determinism and make optimizations to reduce latency. Configuring the timers and GPIO correctly is crucial for high resolution measurements.

There are additional techniques that can be applied to further enhance testing and optimization. But the foundation is now in place to leverage your ARM Cortex chip’s capabilities for quantifying and improving interrupt responsiveness. With the methodology and example code provided, you can configure your own latency test setup and ensure your system’s real-time performance is as robust as possible.

In summary, we discussed:

• Choosing the right timer peripheral for precision and flexibility.
• Configuring the timer for periodic interrupts at a high frequency.
• Setting up a GPIO pin to generate interrupts from the timer.
• Measuring the latency between timer and GPIO interrupts.
• Techniques for optimizing latency – priorities, lower overhead modes, reducing blocking, etc.
• Example code to illustrate configuration steps.

With this foundation, you can now leverage your ARM Cortex chip’s capabilities to thoroughly test and optimize interrupt latency in your system’s real-time applications.