ulptool v2.4.2

Now Arduino can program the ULP coprocessor for your esp32 projects. The guide below assumes you installed the esp32 core with the preferred method using the board manager.

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 series of recipes for building the ULP assembly files by using the '.s' extension which Arduino will let you create. Take note, the extension is a lower case s. In tring to keep the ulp build process the same as the esp-idf framework only a few small modifications are needed to the esp32 Arduino installation.

A new experimental c compiler (lcc) for the ulp implemented by Jason Fuller is included now. Currently only Mac and Linux have been built but hopefully I'll have Windows soon:) There are many limitations and those can found at his github page here: https://github.com/jasonful/lcc Examples can be found in the ulp_examples/ulpcc folder.

Note - platform.local.txt version does not follow ulptool version.

Manual Setup Steps:

Download the latest release of this repository and unpack-> https://github.com/duff2013/ulptool/releases/latest delete the release version number so the folder is just called 'ulptool'
Download and unpack the latest pre-compiled binutils-esp32ulp toolchain for Mac/Linux/Windows: https://github.com/espressif/binutils-esp32ulp/releases/latest

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

    Typically (Mac OS) -> ~/Library/Arduino15/packages/esp32

    Typically (Windows) -> C:\Users\<USERNAME>\AppData\Local\Arduino15\packages\esp32

    Typically (Linux) -> ~/.arduino15/packages/esp32

Move the ulptool folder you downloaded and unpacked to the tools folder here -> ... /esp32/tools/ulptool/.
Copy the 'platform.local.txt' file to ... /esp32/hardware/esp32/1.0.0/. Remember 1.0.0 has to match your esp32 core version.
In the ulptool folder, move or copy the ... /ulptool/src/ulp_examples folder to where Arduino saves your sketches.
Move esp32ulp-elf-binutils folder you downloaded and unpacked to -> ... /esp32/tools/ulptool/src/esp32ulp-elf-binutils/.

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

Assembly Example:

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

#include "esp32/ulp.h"// Must have this!!!

// include ulp header you will create
#include "ulp_main.h"// Must have this!!!

// Custom binary loader
#include "ulptool.h"// Must have this!!!

// 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() {
    Serial.begin(115200);
    delay(1000);
    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);
    delay(100);
}

static void init_run_ulp(uint32_t usec) {
    // initialize ulp variable
    ulp_count = 0;
    ulp_set_wakeup_period(0, usec);
    // use this binary loader instead
    esp_err_t err = ulptool_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 the ulp assembly file you just created.

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

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

Create a new tab named ulpmain.h. This header allows your sketch to see global variables whose memory is allocated in your ulp assembly file. This memory is from the SLOW RTC section. Copy the code below into the 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 ulp_main.h.

/*
    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;

Upload the code then open your serial monitor, you should see the variable 'ulp_count' increment every 100 msecs.

ULPCC Example:

Open a blank Arduino sketch and copy and paste the code below into the that sketch. This is basically the same as the example above but written in c:)

#include "esp32/ulp.h"
// include ulp header you will create
#include "ulp_main.h"
// must include ulptool helper functions also
#include "ulptool.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() {
  Serial.begin(115200);
  delay(1000);
  init_run_ulp(100 * 1000); // 100 msec
}

void loop() {
  // ulp variables are 32-bit but only the bottom 16-bits hold data
  Serial.printf("Count: %i\n", ulp_counter & 0xFFFF);
  delay(100);
}

static void init_run_ulp(uint32_t usec) {
  ulp_set_wakeup_period(0, usec);
  esp_err_t err = ulptool_load_binary(0, ulp_main_bin_start, (ulp_main_bin_end - ulp_main_bin_start) / sizeof(uint32_t));
  err = ulp_run((&ulp_entry - RTC_SLOW_MEM) / sizeof(uint32_t));

  if (err) Serial.println("Error Starting ULP Coprocessor");
}

Create a new tab named ulp_counter.c, take notice of the #ifdef _ULPCC_, code between these will be compiled by the ulp c compiler. Do not place any code outside of the #ifdef _ULPCC_ or your sketch will fail to build. For further information about the limitations see Jason Fullers github site which is developed for the esp-idf not Arduino.

#ifdef _ULPCC_ // Do not add anything above this def
// must include ulpcc helper functions
#include <ulp_c.h>

// global variable that the main processor can see
unsigned counter = 0;

// all ulpcc programs have have this function
void entry() {
  // increment counter
  counter++;
}
#endif // do not add anything after here

/*
    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"
// points to the entry function in counter.c.
extern uint32_t ulp_entry;
// pointer to counter in counter.c
extern uint32_t ulp_counter;

Upload the code then open your serial monitor, you should see the variable 'ulp_counter' increment every 100 msecs.

Under the Hood:

All the magic happens in the python script called esp32ulp_build_recipe.py along with espressif's esp32ulp_mapgen.py script.

Limitations:

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.

ulpcc is still experimental, Mac and Linux only so far.
Errors can be non-informative.
Have to use the custom binary loader function now. (ulptool_load_binary)