ULP Coprocessor programming


The ULP (Ultra Low Power) coprocessor is a simple FSM (Finite State Machine) which is designed to perform measurements using the ADC, temperature sensor, and external I2C sensors, while the main processors are in deep sleep mode. The ULP coprocessor can access the RTC_SLOW_MEM memory region, and registers in RTC_CNTL, RTC_IO, and SARADC peripherals. The ULP coprocessor uses fixed-width 32-bit instructions, 32-bit memory addressing, and has 4 general-purpose 16-bit registers.

Installing the Toolchain

The ULP coprocessor code is written in assembly and compiled using the binutils-esp32ulp toolchain.

If you have already set up ESP-IDF with CMake build system according to the Getting Started Guide, then the ULP toolchain will already be installed.

Compiling the ULP Code

To compile the ULP code as part of the component, the following steps must be taken:

  1. The ULP code, written in assembly, must be added to one or more files with .S extension. These files must be placed into a separate directory inside the component directory, for instance ulp/.

  1. Call ulp_embed_binary from the component CMakeLists.txt after registration. For example:

    set(ulp_app_name ulp_${COMPONENT_NAME})
    set(ulp_s_sources ulp/ulp_assembly_source_file.S)
    set(ulp_exp_dep_srcs "ulp_c_source_file.c")
    ulp_embed_binary(${ulp_app_name} "${ulp_s_sources}" "${ulp_exp_dep_srcs}")

The first argument to ulp_embed_binary specifies the ULP binary name. The name specified here will also be used by other generated artifacts such as the ELF file, map file, header file and linker export file. The second argument specifies the ULP assembly source files. Finally, the third argument specifies the list of component source files which include the header file to be generated. This list is needed to build the dependencies correctly and ensure that the generated header file will be created before any of these files are compiled. See section below for the concept of generated header files for ULP applications.

  1. Build the application as usual (e.g. idf.py app)

    Inside, the build system will take the following steps to build ULP program:

    1. Run each assembly file (foo.S) through the C preprocessor. This step generates the preprocessed assembly files (foo.ulp.S) in the component build directory. This step also generates dependency files (foo.ulp.d).

    2. Run preprocessed assembly sources through the assembler. This produces object (foo.ulp.o) and listing (foo.ulp.lst) files. Listing files are generated for debugging purposes and are not used at later stages of the build process.

    3. Run the linker script template through the C preprocessor. The template is located in components/ulp/ld directory.

    4. Link the object files into an output ELF file (ulp_app_name.elf). The Map file (ulp_app_name.map) generated at this stage may be useful for debugging purposes.

    5. Dump the contents of the ELF file into a binary (ulp_app_name.bin) which can then be embedded into the application.

    6. Generate a list of global symbols (ulp_app_name.sym) in the ELF file using esp32ulp-elf-nm.

    7. Create an LD export script and header file (ulp_app_name.ld and ulp_app_name.h) containing the symbols from ulp_app_name.sym. This is done using the esp32ulp_mapgen.py utility.

    8. Add the generated binary to the list of binary files to be embedded into the application.

Accessing the ULP Program Variables

Global symbols defined in the ULP program may be used inside the main program.

For example, the ULP program may define a variable measurement_count which will define the number of ADC measurements the program needs to make before waking up the chip from deep sleep:

                        .global measurement_count
measurement_count:      .long 0

                        /* later, use measurement_count */
                        move r3, measurement_count
                        ld r3, r3, 0

The main program needs to initialize this variable before the ULP program is started. The build system makes this possible by generating the ${ULP_APP_NAME}.h and ${ULP_APP_NAME}.ld files which define the global symbols present in the ULP program. Each global symbol defined in the ULP program is included in these files and are prefixed with ulp_.

The header file contains the declaration of the symbol:

extern uint32_t ulp_measurement_count;

Note that all symbols (variables, arrays, functions) are declared as uint32_t. For functions and arrays, take the address of the symbol and cast it to the appropriate type.

The generated linker script file defines the locations of symbols in RTC_SLOW_MEM:

PROVIDE ( ulp_measurement_count = 0x50000060 );

To access the ULP program variables from the main program, the generated header file should be included using an include statement. This will allow the ULP program variables to be accessed as regular variables:

#include "ulp_app_name.h"

// later
void init_ulp_vars() {
    ulp_measurement_count = 64;

Note that the ULP program can only use lower 16 bits of each 32-bit word in RTC memory, because the registers are 16-bit, and there is no instruction to load from the high part of the word.

Likewise, the ULP store instruction writes register value into the lower 16 bits part of the 32-bit word. The upper 16 bits are written with a value which depends on the address of the store instruction, thus when reading variables written by the ULP, the main application needs to mask the upper 16 bits, e.g.:

printf("Last measurement value: %d\n", ulp_last_measurement & UINT16_MAX);

Starting the ULP Program

To run a ULP program, the main application needs to load the ULP program into RTC memory using the ulp_load_binary function, and then start it using the ulp_run function.

Note that “Enable Ultra Low Power (ULP) Coprocessor” option must be enabled in menuconfig to reserve memory for the ULP. “RTC slow memory reserved for coprocessor” option must be set to a value sufficient to store ULP code and data. If the application components contain multiple ULP programs, then the size of the RTC memory must be sufficient to hold the largest one.

Each ULP program is embedded into the ESP-IDF application as a binary blob. The application can reference this blob and load it in the following way (suppose ULP_APP_NAME was defined to ulp_app_name):

extern const uint8_t bin_start[] asm("_binary_ulp_app_name_bin_start");
extern const uint8_t bin_end[]   asm("_binary_ulp_app_name_bin_end");

void start_ulp_program() {
    ESP_ERROR_CHECK( ulp_load_binary(
        0 /* load address, set to 0 when using default linker scripts */,
        (bin_end - bin_start) / sizeof(uint32_t)) );
esp_err_t ulp_load_binary(uint32_t load_addr, const uint8_t *program_binary, size_t program_size)

Load ULP program binary into RTC memory.

ULP program binary should have the following format (all values little-endian):

  1. MAGIC, (value 0x00706c75, 4 bytes)

  2. TEXT_OFFSET, offset of .text section from binary start (2 bytes)

  3. TEXT_SIZE, size of .text section (2 bytes)

  4. DATA_SIZE, size of .data section (2 bytes)

  5. BSS_SIZE, size of .bss section (2 bytes)

  6. (TEXT_OFFSET - 12) bytes of arbitrary data (will not be loaded into RTC memory)

  7. .text section

  8. .data section

Linker script in components/ulp/ld/esp32.ulp.ld produces ELF files which correspond to this format. This linker script produces binaries with load_addr == 0.


  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if load_addr is out of range

  • ESP_ERR_INVALID_SIZE if program_size doesn’t match (TEXT_OFFSET + TEXT_SIZE + DATA_SIZE)

  • ESP_ERR_NOT_SUPPORTED if the magic number is incorrect

  • load_addr: address where the program should be loaded, expressed in 32-bit words

  • program_binary: pointer to program binary

  • program_size: size of the program binary

Once the program is loaded into RTC memory, the application can start it, passing the address of the entry point to the ulp_run function:

ESP_ERROR_CHECK( ulp_run(&ulp_entry - RTC_SLOW_MEM) );
esp_err_t ulp_run(uint32_t entry_point)

Run the program loaded into RTC memory.


ESP_OK on success

  • entry_point: entry point, expressed in 32-bit words

Declaration of the entry point symbol comes from the generated header file mentioned above, ${ULP_APP_NAME}.h. In the assembly source of the ULP application, this symbol must be marked as .global:

        .global entry
        /* code starts here */

ULP Program Flow

The ULP coprocessor is started by a timer. The timer is started once ulp_run is called. The timer counts the number of RTC_SLOW_CLK ticks (by default, produced by an internal 150kHz RC oscillator). The number of ticks is set using SENS_ULP_CP_SLEEP_CYCx_REG registers (x = 0..4). When starting the ULP for the first time, SENS_ULP_CP_SLEEP_CYC0_REG will be used to set the number of timer ticks. Later the ULP program can select another SENS_ULP_CP_SLEEP_CYCx_REG register using the sleep instruction.

The application can set ULP timer period values (SENS_ULP_CP_SLEEP_CYCx_REG, x = 0..4) using the ulp_set_wakeup_period function.

esp_err_t ulp_set_wakeup_period(size_t period_index, uint32_t period_us)

Set one of ULP wakeup period values.

ULP coprocessor starts running the program when the wakeup timer counts up to a given value (called period). There are 5 period values which can be programmed into SENS_ULP_CP_SLEEP_CYCx_REG registers, x = 0..4. By default, wakeup timer will use the period set into SENS_ULP_CP_SLEEP_CYC0_REG, i.e. period number 0. ULP program code can use SLEEP instruction to select which of the SENS_ULP_CP_SLEEP_CYCx_REG should be used for subsequent wakeups.

However, please note that SLEEP instruction issued (from ULP program) while the system is in deep sleep mode does not have effect, and sleep cycle count 0 is used.


The ULP FSM requires two clock cycles to wakeup before being able to run the program. Then additional 16 cycles are reserved after wakeup waiting until the 8M clock is stable. The FSM also requires two more clock cycles to go to sleep after the program execution is halted. The minimum wakeup period that may be set up for the ULP is equal to the total number of cycles spent on the above internal tasks. For a default configuration of the ULP running at 150kHz it makes about 133us.


  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if period_index is out of range

  • period_index: wakeup period setting number (0 - 4)

  • period_us: wakeup period, us

Once the timer counts the number of ticks set in the selected SENS_ULP_CP_SLEEP_CYCx_REG register, the ULP coprocessor will power up and start running the program from the entry point set in the call to ulp_run.

The program runs until it encounters a halt instruction or an illegal instruction. Once the program halts, the ULP coprocessor will power down, and the timer will be started again.

To disable the timer (effectively preventing the ULP program from running again), please clear the RTC_CNTL_ULP_CP_SLP_TIMER_EN bit in the RTC_CNTL_STATE0_REG register. This can be done both from the ULP code and from the main program.