In this lesson, you will learn how to bypass the CPU for sample timing by using hardware timers to trigger DAC updates, ensuring your analog signals are free from jitter.
The Problem with CPU-Driven Updates
Previously, we used software loops or interrupt-driven routines to update the DAC. However, relying on the CPU to "poke" the DAC register is dangerous. If an interrupt occurs or the processor is busy executing other code, your update timing will jitter. This leads to distortion in the output waveform because the time intervals between samples become inconsistent.
The Hardware Solution
Most modern microcontrollers allow a hardware timer to act as a "trigger source" for the DAC peripheral. Instead of the CPU writing the value, the DAC hardware monitors a specific signal from a timer (like an Output Compare event or a Timer Update event).
When the timer reaches a pre-configured period, it sends an electrical trigger signal directly to the DAC. The DAC then immediately latches the data from its input register to the conversion circuitry. Because this happens at the hardware level, the update occurs with near-zero latency and perfect timing relative to the timer clock.
Practical Implementation
To set this up, you must configure two systems to work in tandem:
- Timer Configuration: Set the timer frequency to match your desired sampling rate (e.g., 44.1 kHz).
- DAC Configuration: Instead of enabling the standard software-trigger mode, you enable the "External Trigger" mode and select the specific timer as the source.
// Conceptually, configuring the DAC to listen for a timer
DAC_Control_Register |= DAC_TRIGGER_ENABLE;
DAC_Trigger_Selection = TIMER_2_TRGO_EVENT;
// The timer then manages the cadence
Timer_Period = (System_Clock / Desired_Sample_Rate);
By offloading the timing to the hardware, your CPU is freed from the burden of precise timing, and your analog reconstruction gains the stability required for high-fidelity signal generation. When combined with the Double Buffering and DMA techniques we discussed earlier, you can stream complex waveforms perfectly without ever manually triggering a DAC update.
Key Takeaway: Linking a hardware timer to a DAC trigger ensures that analog sample updates occur at mathematically precise intervals, completely eliminating CPU-induced jitter.