High‑Precision Waveform Generation with a DAC‑Based Custom PCB

High‑Precision Waveform Generation with a DAC‑Based Custom PCB

In this guide we walk through firmware and hardware design that delivers fast, accurate arbitrary waveforms using a C8051F360 microcontroller and an Analog Devices TxDAC.

Project Overview

The board is centered around the C8051F360 MCU and a TxDAC DAC. The two ICs communicate over a parallel bus, allowing us to push the DAC update rate to the limits of the MCU’s 100 MHz core clock.

Prior posts have detailed the schematic, component selection, and PCB layout. This article focuses on the firmware framework that moves data from memory to the DAC, ensuring a stable, high‑precision interface.

Design Objectives

• High‑Precision Timing – The AWG must produce signals that match mathematical waveforms with minimal jitter.
• Processor Efficiency – Firmware must run concurrently with other tasks, keeping the MCU responsive.
• Maximum Speed – Target at least 10 MHz sampling, yielding usable signals in the 1–2 MHz band.

Sampling Strategy

The board uses a MEMS oscillator with ±50 ppm stability. Firmware generates a 500 kHz reference clock for the DAC via the Programmable Counter Array (PCA).

Configuration is performed with the Silicon Labs Config2 utility, as the C8051F360 is not supported by Simplicity Studio’s built‑in tool.

Key PCA settings:

/* PCA enabled; time base set to system clock (10 MHz) */
PCA0MD = 0x01;          // Enable PCA
PCA0CPM0 = 0x40;       // Timer mode, edge‑triggered
PCA0CPH0 = 0x00;       // High byte of compare value
PCA0CPL0 = 0x01;       // Low byte of compare value (500 kHz)

Measurement with a Tektronix MDO3104 scope confirms a 500 kHz clock with 50 % duty cycle. Standard deviation of frequency is 13.54 Hz (0.0027 %) and duty cycle is 2.117 × 10⁻³ (0.0042 %). Histogram analysis shows negligible jitter.

Synchronizing Data Updates

To align DAC updates with the clock, the falling edge of the 500 kHz square wave triggers /INT0. The DAC latches data on the rising edge, so updating on the falling edge guarantees stable data.

External interrupt 0 is configured on pin P0.7, falling‑edge sensitive, and highest priority.

Interrupt Service Routine

All data transfer occurs inside the /INT0 ISR. A minimal routine keeps the processor free for other tasks:

SI_INTERRUPT(INT0_ISR, INT0_IRQn)
{
    DAC_WORD--;
}

Here, DAC_WORD is a #define alias for port P2. The single decrement instruction yields a linear ramp that wraps from 0 to 255, producing a sawtooth waveform. The DAC’s inverting output amplifier inverts the voltage, so the analog ramp appears to increase.

Verification

Running the firmware produces a clean ramp waveform. A scope trace shows a stable 1 kHz ramp with a 5 V peak‑to‑peak amplitude, confirming correct timing and data handling.

Further tests with higher sample rates (up to 10 MHz) demonstrate the board’s ability to generate multi‑MHz waveforms while keeping the MCU busy with other tasks.

For deeper dives into the schematic, component selection, and PCB layout, see the companion posts in this series.

Ready to build your own high‑precision AWG? Download the source, start coding, and let the firmware drive the next level of signal fidelity.