Files needed to program the esp32 ULP coprocessor in Arduino
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Arduino ULP v2.0.0

This guide explains how to setup Arduino to use ULP coprocessor assembly files for your esp32 projects using the perfered method of the board manager to install the esp32 core. The old method of manually installing the esp32 core is no longer supported. Currently the how-to is only geared for MacOS and has not been tested with Windows or Linux at this time. Python 2.7 is required and python3 will not currently work. Being beta many things could go wrong so let me know if you encounter any issues.

Typically in Arduino you can compile assembly files using the '.S' extension. Using the ESP32 Arduino core framework these files would correspond to the Xtensa processors whose toolchain is incompatible with the ULP coprocessor. Luckily, Arduino provides a fairly easy albeit not that flexible build framework using series of recipes. This guide extends those esp32 recipes for building the ULP assembly files. We will use the '.s' extensions for ULP assembly files which Arduino will let you create. Remeber thats a lower case 's'. I tried to keep the ulp build process the same as the esp-if framework with only a few small modifications the user needs to do in order to compile in Arduino.

Setup Steps:

  1. Download the latest release of this repository and unpack-> 'arduino_ulp'.

  2. Download the latest pre-compiled binutils-esp32ulp toolchain for Mac/Linux/Windows:

  3. Find your Arduino-esp32 core directory which Arduino IDE uses:

         Typically (Mac OS) -> ~/Library/Arduino15/packages/esp32
         Typically (Windows) ->?
         Typically (Linux) ->?
  4. In the 'arduino_ulp' release you downloaded and unpacked, copy the folder 'ulp' to .../esp32/hardware/esp32/1.0.0/tools/sdk/include/ replacing the existing folder named 'ulp', "1.0.0" is the version number of the core you installed, change version number accordingly.

  5. In the 'arduino_ulp' repository folder you downloaded and unpacked, copy the file 'platform.txt' to .../esp32/hardware/esp32/1.0.0/ replacing the one you have. If you want, just remain the old "platform.txt" so you can revert back. Remeber "1.0.0" has to match your version.

  6. In the 'arduino_ulp' release you downloaded, copy the 'ulp_example' folder to where Arduino saves your sketches.

  7. Create a new folder named exactly 'binutils' in directory .../esp32/tools/

  8. Unpack and copy the folder of the pre-compiled binutils-esp32ulp toolchain you downloaded to .../esp32/tools/binutils/

Thats it, you now have all the files in place, lets look at very simple example to get you compiling ulp assembly code!


Open a blank Arduino sketch and copy and paste the code below into the that sketch.

#include "esp32/ulp.h"
// include ulp header you will create
#include "ulp_main.h"

// Unlike the esp-idf always use these binary blob names
extern const uint8_t ulp_main_bin_start[] asm("_binary_ulp_main_bin_start");
extern const uint8_t ulp_main_bin_end[]   asm("_binary_ulp_main_bin_end");

static void init_run_ulp(uint32_t usec);

void setup() {
    init_run_ulp(100 * 1000); // 100 msec

void loop() {
    // ulp variables data is the lower 16 bits
    Serial.printf("ulp count: %u\n", ulp_count & 0xFFFF);

static void init_run_ulp(uint32_t usec) {
    // initialize ulp variable
    ulp_count = 0;
    ulp_set_wakeup_period(0, usec);
    esp_err_t err = ulp_load_binary(0, ulp_main_bin_start, (ulp_main_bin_end - ulp_main_bin_start) / sizeof(uint32_t));
    // ulp coprocessor will run on its own now
    err = ulp_run((&ulp_entry - RTC_SLOW_MEM) / sizeof(uint32_t));

Create a new tab named ulp.s, take notice that the extension is a lower case 's'. Copy the code below into that ulp assembly file tab.

/* Define variables, which go into .bss section (zero-initialized data) */
/* Store count value */
    .global count
    .long 0

/* Code goes into .text section */
    .global entry
    move    r3, count
    ld      r0, r3, 0 
    add     r0, r0, 1
    st      r0, r3, 0

Create a new tab named ulp_main.h. This header allows your sketch to see global variables whose memory is allocated your ulp assembly file. This memory is in the SLOW RTC section. Copy the code below into that header file. As with the esp-idf you have to add 'ulp_' to the front of the variable name. Unlike esp-idf the name of this header is always this name.

    Put your ULP globals here you want visibility
    for your sketch. Add "ulp_" to the beginning
    of the variable name and must be size 'uint32_t'
#include "Arduino.h"

extern uint32_t ulp_entry;
extern uint32_t ulp_count;

Compile and run and you should see the variable 'ulp_count' increment every 100 msecs.

Under the Hood:

All the magic happens in the python script called This along with espressif's in which both are located in the ulp directory from this repository.


While almost a complete solution to programing the ULP coprocessor in assembly, there are currently a few limitations. Once I fix these, I'll remove them from this list.

  1. Not tested with Windows or Linux yet.
  2. Errors can be non-informative.
  3. Probably more that I can't think of now...