DAC

The built-in DAC (Digital-to-Analog converter) can turn a digital value into a voltage. This can be useful to produce a given voltage at a pin dynamically by code rather than fixed by discrete components (i.e. via a voltage divider using resistors). There are many use cases. One is generating wave forms so you can create signal generators or produce audio output.

Support

A built-in DAC is supported by these ESPxxxx modules:

Benefit of Built-In DAC

You can get separate breakout boards with external DACs like the popular MCP4725, and add simple-to-use DAC capability to any microcontroller.

However, external DACs are not the same as having an internal built-in DAC:

The built-in DAC tightly integrates with the rest of the microcontroller infrastructure. Most importantly, it can directly communicate with its memory (Direct Memory Access, DMA).

Since the CPU is not involved when DMA is used, the DAC can change the voltage much more often per second than external DACs.

External DACs like the MCP4725 do support very high speeds: its I2C interface is ready for 100kbps, 400kbps, and even 3.4Mbps. However, it cannot directly access memory, and always needs the CPU to manage I2C and do the memory transfer, thus placing a high burden on the CPU.

Overview

A Digital-to-Analog Converter (DAC) converts digital data into an analog signal (voltage), enabling digital audio playback, video, and other data that can be used by analog devices such as speakers, monitors, and sensors.

A DAC is not a buck or boost converter: it can only output voltages within the range of the supply voltage.

Voltage Range

The analog reference voltage for ESP32 is 3.3V. An ideal DAC would be able to produce voltages in the range of 0.0-3.3V in a linear way.

In practice, DACs are not perfectly ideal and show a zero drift (cannot deliver exactly 0.0V, range ends at around 0.08V) and early saturation (cannot reach the upper voltage limit and deliver around 3.165V at most).

Resolution

The DAC resolution defines the voltage steps that are creatable. The built-in DAC has a 8bit resolution, providing 256 voltage steps.

Speed

The internal DAC can be operated in different modes, yielding different update speeds:

When looking at speeds, keep in mind that for API methods, this is the speed for a single voltage change. A waveform like a sine wave consists of many different voltage points, often hundreds. DMA is much faster and can for example create a sine wave at 100kHz. The same sine wave created by individual API calls in a loop would reach a maximum frequency of around 150Hz (and keep the CPU at 100% load).

DAC Output Current

The DAC outputs a control signal that can be used to drive other circuitry, i.e. an amplifier, an oscilloscope, a voltmeter, or an audio amplifier.

You cannot and should never directly drive any component that requires significant currents. DAC output can typically source up to about 12mA and sink up to about 10mA.

For reliable operation and to avoid damaging the DAC, it is advisable to limit the current draw to much lower values and not exceed 1-2mA.

If you need to output higher currents, you should use an external buffer, such as an operational amplifier (op-amp) configured as a voltage follower.

Operational Modes

The ESP32 DAC supports three modes of operation:

Producing Fixed Voltages

Often, your project just needs a fixed voltage that can be controlled by your code.

The easiest way for this is to use the default Arduino API (not requiring any additional libraries).

While the Espressif API is about 4x faster than the built-in Arduino API, this does not really matter: when switching an output pin to a fixed voltage, it makes no difference whether this takes 20uS or just 5uS.

Setting Pin Voltage

To set one of the DAC output pins to a given voltage, use this call:

dacWrite(uint8_t dac_pin, uint8_t value);

Turning Off Voltage

To turn off the voltage completely on a pin, use this call:

void dacDisable(uint8_t dac_pin);

[NOTE] In a simple sketch you can test the DAC output voltage with a multimeter. Only use voltmeters with internal power supply. Do not connect voltmeters that draw their energy from the DAC output, i.e. to power its display LEDs. This would likely exceed the recommended maximum output current of 1-2mA.

Stepping Up Voltage Slowly

To see the DAC in action, you can use a simple multimeter provided you ask the DAC to perform voltage changes slowly enough for the multimeter to pick up the voltage.

The following sketch asks the DAC to change the voltage every second. It increments the digital input by 25 each time, so the DAC is producing the analog voltage for the digital inputs 0, 25, 50, 75, 100, 125, 150, 175, 200, 225, and 250.

The sketch uses DAC Channel 1 which corresponds to GPIO25 on a classic ESP32, and GPIO17 on a S2.

Make sure you adjust the sketch to the type of ESPxxxx you are using by uncommenting the appropriate line defining the DAC GPIO to use.

#include <Arduino.h>

#define DAC1 25       // ESP32:    DAC Ch1 
//#define DAC1 17     // ESP32-S2: DAC Ch1 
                 

void setup(){
}

void loop(){
 for(int i=0;i<256;i+=25)
 {
   dacWrite(DAC1, i);
   delay(1000);
 }
}

To see the effect, connect the red plus cable of a multimeter to the DAC pin and the black ground cable to GND.

Only use battery-powered multimeters. Do not hook up a voltage display that is powered solely by the input voltage. Keep in mind that a DAC output should never be exceeding 1-2mA and can be destroyed by currents above 12mA.

Sawtooth Wave

Next, let’s generate a wave form by changing the voltage in high(er) frequency. The next sketch increases and then decreases the voltage in a linear way, effectively producing a sawtooth wave.

The next sketch is not using delay(1000); so the voltage changes occur at the maximum possible frequency. You can no longer use a normal multimeter to check the effect: it would take much too long for its measurements and just display an average voltage. To see the effect, you need an oscilloscope. Since the generated waveform will have a very low frequency of around 100Hz, you can use cheap pocket-size oscilloscopes or even multimeters with oscilloscope functionality.

#include <Arduino.h>

#define DAC1 25       // ESP32:    DAC Ch1 
//#define DAC1 17     // ESP32-S2: DAC Ch1 


void setup(){
}

void loop() {
 for (int i=0; i<255; i++){
   dacWrite(DAC1, i);
 }

 for (int i=255; i>=0; i--){
   dacWrite(DAC1, i);
 }
}

This is the generated waveform displayed on a pocket oscilloscope:

And this would be an output from a real oscilloscope:

Sine Wave

Now that you know how to instruct the DAC to produce a voltage, you can generate basically any waveform.

To create more complex waveforms such as a sine wave, use an array with the voltages required for the waveform, then index into the array to let the DAC produce the required voltages.

#include <Arduino.h>

#define DAC1 25       // ESP32:    DAC Ch1 
//#define DAC1 17     // ESP32-S2: DAC Ch1 

int Sin_Array[256]; 
float Period = (2*PI)/256;
float Rad_Angle;                   

void setup(){
    
   for(int Angle=0; Angle<256; Angle++) {
       Rad_Angle = Angle*Period;
       Sin_Array[Angle] = (sin(Rad_Angle)*127)+128;
   }
}

void loop(){
 for(int i=0;i<256;i++)
   dacWrite(DAC1, Sin_Array[i]);
}

This is the generated sine wave displayed by an oscilloscope:

Speed Check

Let’s finally come back to the initial example where the sketch incremented the voltage in steps of 25. The sketch used a delay of 1000ms between each voltage change so that a regular multimeter would have sufficient time to measure the signal.

Take the same sketch but comment out the line with delay(1000); to see what the maximum speed is for regular API calls to change voltages:

#include <Arduino.h>

#define DAC1 25       // ESP32:    DAC Ch1 
//#define DAC1 17     // ESP32-S2: DAC Ch1 
                 

void setup(){
}

void loop(){
 for(int i=0;i<256;i+=25)
 {
   dacWrite(DAC1, i);
   //delay(1000);
 }
}

This is the generated waveform displayed on a pocket oscilloscope:

And this would be an output from a real oscilloscope:

Higher Frequencies

Setting one discrete voltage on one of the DAC pins is simple and straight-forward as you have seen: the Arduino method dacWrite(); sets the desired voltage - done.

Once you need to set voltages more often, for example if you want to generate complex wave forms at higher frequency, things become more complex. As you have already seen above, generating sawtooth or sine waves or entirely different wave forms is absolutely possible using the simple dacWrite();. It just takes comparably long: each call requires approximately 20uS, severely limiting the frequencies you can generate. At best, you will be able to create a sine wave at around 200Hz.

In the remaining part, I look into alternative approaches allowing higher frequencies:

• Espressif API: First, I look at a much faster way of setting DAC output voltage. It still requires the CPU to do the calls but allows for sine waves at up to 1kHz.
• I2S and DMA: Then, I’ll dive into I2S and how this can use DMA (Direct Memory Access) so that the DAC can communicate directly with memory, effectively bypassing the CPU bottleneck and allowing sine waves at frequencies of up to 100kHz.

Espressif Libraries

Using the Espressif Libraries to control the DAC (instead of the generic Arduino libraries) improves speed quite a lot. You can compare the difference in maximum achievable frequency using this sketch:

#include <Arduino.h>
#include <driver/dac.h>

#define DAC1 25       // ESP32:    DAC Ch1 
//#define DAC1 17     // ESP32-S2: DAC Ch1 


const uint8_t sineLookupTable[] = {
128, 136, 143, 151, 159, 167, 174, 182,
189, 196, 202, 209, 215, 220, 226, 231,
235, 239, 243, 246, 249, 251, 253, 254,
255, 255, 255, 254, 253, 251, 249, 246,
243, 239, 235, 231, 226, 220, 215, 209,
202, 196, 189, 182, 174, 167, 159, 151,
143, 136, 128, 119, 112, 104, 96, 88,
81, 73, 66, 59, 53, 46, 40, 35,
29, 24, 20, 16, 12, 9, 6, 4,
2, 1, 0, 0, 0, 1, 2, 4,
6, 9, 12, 16, 20, 24, 29, 35,
40, 46, 53, 59, 66, 73, 81, 88,
96, 104, 112, 119};

void setup(){
   dac_output_enable(DAC_CHANNEL_1);
}

void loop(){
 for(int i=0;i<100;i++) {
   dacWrite(DAC1, sineLookupTable[i]);
   //dac_output_voltage(DAC_CHANNEL_1, sineLookupTable[i]);
 }
}

When you compile and run this sketch as-is, it uses the default Arduino API and produces a sine wave at 537Hz:

You probably noted that this example already increased the frequency from around 200Hz to almost 600Hz without changing the API calls yet.

Note that this example uses only 100 samples (100 voltage values) that define the curve. The previous examples had used 256 samples (2.56x more). 537Hz/2.56 equals to 209Hz, the frequency we saw earlier.

So from a performance perspective, both examples ran the DAC at 53.7ksps (53.700 samples per second).

Testing Espressif API

Now comment in the line dac_output_voltage(DAC_CHANNEL_1, sineLookupTable[i]); and comment out the line dacWrite(DAC1, sineLookupTable[i]);, effectively just changing the method that tells the DAC the voltage it should deliver.

Once you compile and upload the changed sketch, this is the result:

We now see a frequency of 3.25kHz which is roughly 6x faster, or from a performance perspective: the DAC now runs at 100 samples x 3250Hz = *325ksps**.

This example is pushing it to the extreme showing the maximum archievable frequency going this route. One CPU core is most likely working at close to 100% load.

More Control Through Interrupts

The code above is hard to integrate with other tasks as it blocks the CPU.

You can slightly modify the code though and i.e. use hardware interrupts to take care of updating the DAC voltages.

This has two benefits:

• Non-Blocking: loop() is not doing anything and could be used to execute other code, i.e. a user interface to change signal parameters on the fly.
• Frequency Adjustments: Adjusting the signal frequency is now trivial as it just requires to change the timer interrupt interval.

Here is the modified code that generates a sine wave at exactly 1kHz:

#include <Arduino.h>
#include <driver/dac.h>
 
hw_timer_t *Timer0_Cfg = NULL;
 
uint8_t SampleIdx = 0;

const uint8_t sineLookupTable[] = {
128, 136, 143, 151, 159, 167, 174, 182,
189, 196, 202, 209, 215, 220, 226, 231,
235, 239, 243, 246, 249, 251, 253, 254,
255, 255, 255, 254, 253, 251, 249, 246,
243, 239, 235, 231, 226, 220, 215, 209,
202, 196, 189, 182, 174, 167, 159, 151,
143, 136, 128, 119, 112, 104, 96, 88,
81, 73, 66, 59, 53, 46, 40, 35,
29, 24, 20, 16, 12, 9, 6, 4,
2, 1, 0, 0, 0, 1, 2, 4,
6, 9, 12, 16, 20, 24, 29, 35,
40, 46, 53, 59, 66, 73, 81, 88,
96, 104, 112, 119};
 
// this is the timer interrupt procedure updating the DAC:
void IRAM_ATTR Timer0_ISR()
{
  dac_output_voltage(DAC_CHANNEL_1, sineLookupTable[SampleIdx++]);
  if(SampleIdx == 100) 
    SampleIdx = 0;

}
 
void setup()
{
  // setup timer interrupt
  Timer0_Cfg = timerBegin(0, 80, true);
  timerAttachInterrupt(Timer0_Cfg, &Timer0_ISR, true);
  timerAlarmWrite(Timer0_Cfg, 10, true);
  timerAlarmEnable(Timer0_Cfg);
  dac_output_enable(DAC_CHANNEL_1);
}
 
void loop()
{
  // all DAC operations are handled by interrupts
  // loop() can be used for anything else you need to do, i.e. manage a user interface
}

This is the result:

I2S And DMA

By using DMA, the built-in DAC can produce wave forms at much higher sampling rates and frequencies.

The sketch below produces a sine wave at a frequency of 65kHz:

#include <Arduino.h>

#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "freertos/queue.h"

#include "soc/rtc_io_reg.h"
#include "soc/rtc_cntl_reg.h"
#include "soc/sens_reg.h"
#include "soc/rtc.h"

#include "driver/dac.h"

// signal frequency is determined by frequency_step like this:
// frequency_step = desiredFrequencyHz x 65536 / 8500000
// -or-
// frequency = 8500000 x frequency_step / 65536
// 8: 1.04kHz
// 16: 2.08kHz
// 50: 6.5kHz
// 100: 12.97kHz
// 200: 25.94kHz
// 500: 64.85kHz
int frequency_step = 500;  // Frequency step for CW generator

int clk_8m_div = 0;      // RTC 8M clock divider (0=8MHz)
int scale = 1;           // 1/2 scale
int offset;              // no offset
int invert = 2;          // invert MSB (most significant bit) for sine wave

float frequency = RTC_FAST_CLK_FREQ_APPROX / (1 + clk_8m_div) * (float) frequency_step / 65536;
        

// all manipulations are direct writes to a particular register
// DAC is configured using register SENS_SAR_DAC_CTRL1_REG and SENS_SAR_DAC_CTRL2_REG
// SENS_SAR_DAC_CTRL1_REG enables the cosine generator
// SENS_SAR_DAC_CTRL2_REG connects it to a DAC channel

// Register bits can be changed using SET_PERI_REG_MASK();


// enables the cosine generator for a DAC channel:
void dac_cosine_enable(dac_channel_t channel)
{
    // Step 1: ENABLE the cosine generator:
    SET_PERI_REG_MASK(SENS_SAR_DAC_CTRL1_REG, SENS_SW_TONE_EN);
    
    // Step 2: CONNECT the cosine generator to a DAC channel:
    switch(channel) {
        // cosine generator is enabled per channel using SENS_DAC_CW_EN1_M and SENS_DAC_CW_EN2_M
        // MSB must be inverted by SENS_DAC_INV1 and SENS_DAC_INV2 
        case DAC_CHANNEL_1:
            SET_PERI_REG_MASK(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_CW_EN1_M);
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_INV1, 2, SENS_DAC_INV1_S);
            break;
        case DAC_CHANNEL_2:
            SET_PERI_REG_MASK(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_CW_EN2_M);
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_INV2, 2, SENS_DAC_INV2_S);
            break;
    }
}


// frequency (for both channels)is determined by two parameters:
// clk_8m_div (RTC 8M clock divider): 0=8Mhz clock, range is 0-273
// frequency_step: range is 1-65535
void dac_frequency_set(int clk_8m_div, int frequency_step)
{
    REG_SET_FIELD(RTC_CNTL_CLK_CONF_REG, RTC_CNTL_CK8M_DIV_SEL, clk_8m_div);
    SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL1_REG, SENS_SW_FSTEP, frequency_step, SENS_SW_FSTEP_S);
}

// scaling, range is 0-3:
// 0: no scale
// 1: scale to 1/2
// 2: scale to 1/4
// 3: scale to 1/8
void dac_scale_set(dac_channel_t channel, int scale)
{
    switch(channel) {
        case DAC_CHANNEL_1:
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_SCALE1, scale, SENS_DAC_SCALE1_S);
            break;
        case DAC_CHANNEL_2:
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_SCALE2, scale, SENS_DAC_SCALE2_S);
            break;
    }
}

// Offset output for a particular channel: range is 0-255
void dac_offset_set(dac_channel_t channel, int offset)
{
    switch(channel) {
        case DAC_CHANNEL_1:
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_DC1, offset, SENS_DAC_DC1_S);
            break;
        case DAC_CHANNEL_2:
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_DC2, offset, SENS_DAC_DC2_S);
            break;
    }
}

// Invert output for a particular channel, range is 0-3:
// 0: no inversion
// 1: completely inverted
// 2: invert MSB (most significant bit)
// 3: invert all but MSB
void dac_invert_set(dac_channel_t channel, int invert)
{
    switch(channel) {
        case DAC_CHANNEL_1:
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_INV1, invert, SENS_DAC_INV1_S);
            break;
        case DAC_CHANNEL_2:
            SET_PERI_REG_BITS(SENS_SAR_DAC_CTRL2_REG, SENS_DAC_INV2, invert, SENS_DAC_INV2_S);
            break;
    }
}

// task that handles the DAC
void dactask(void* arg) {
    while(1){
        // set frequency
        dac_frequency_set(clk_8m_div, frequency_step);
        vTaskDelay(2000/portTICK_PERIOD_MS);
    }
}

void setup() {
    dac_cosine_enable(DAC_CHANNEL_1);
    dac_output_enable(DAC_CHANNEL_1);
    xTaskCreate(dactask, "dactask", 1024*3, NULL, 10, NULL);
}

void loop() {}

This is the result on an oscilloscope:

The desired frequency is set via the variable frequency_step which is an integer. You can calculate other values either by submitting a desired frequency:

frequency_step = desiredFrequencyHz x 65536 / 8500000

Or you can fill in integer values for frequency_step and then calculate the effective frequency like this:

frequency = 8500000 x frequency_step / 65536

For example, when you change the value for frequency_step in the code from 500 to 2000, according to the formula above, the resulting frequency now would be:

frequency = 8500000 x 2000 / 65536 = 259.4kHz

Here is the result on an oscilloscope:

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