ULP RISC-V Coprocessor programming

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The ULP RISC-V coprocessor is a variant of the ULP present in ESP32-S2. Similar to ULP FSM, the ULP RISC-V coprocessor can perform tasks such as sensor readings while the main CPU stays in low power modes. The main difference between ULP FSM and ULP RISC-V is that the latter can be programmed in C using standard GNU tools. The ULP RISC-V coprocessor can access the RTC_SLOW_MEM memory region, and registers in RTC_CNTL, RTC_IO, and SARADC peripherals. The RISC-V processor is a 32-bit fixed point machine. Its instruction set is based on RV32IMC which includes hardware multiplication and division, and compressed code.

Installing the ULP RISC-V Toolchain

The ULP RISC-V coprocessor code is written in C (assembly is also possible) and compiled using the RISC-V toolchain based on GCC.

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

Note

In earlier versions of ESP-IDF, RISC-V toolchain had a different prefix: riscv-none-embed-gcc.

Compiling the ULP RISC-V Code

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

  1. The ULP RISC-V code, written in C or assembly (must use the .S extension), must be placed in a separate directory inside the component directory, for instance, ulp/.

Note

When registering the component (via idf_component_register), this directory should not be added to the SRC_DIRS argument as it is currently done for the ULP FSM. See the step below for how to properly add ULP source files.

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

    ...
idf_component_register()

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

ulp_embed_binary(${ulp_app_name} "${ulp_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 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 the 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 source file through the C compiler and assembler. This step generates the object files (.obj.c or .obj.S depending of source file processed) in the component build directory.

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

    3. 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.

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

    5. Generate a list of global symbols (ulp_app_name.sym) in the ELF file using riscv32-esp-elf-nm.

    6. Create an LD export script and a 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.

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

Accessing the ULP RISC-V Program Variables

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

For example, the ULP RISC-V 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.

volatile int measurement_count;

int some_function()
{
    //read the measurement count for use it later.
    int temp = measurement_count;

    ...do something.
}

The main program can access the global ULP RISC-V program variables as 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 RISC-V program. Each global symbol defined in the ULP RISC-V 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 RISC-V program variables from the main program, the generated header file should be included using an include statement. This will allow the ULP RISC-V program variables to be accessed as regular variables.

#include "ulp_app_name.h"

void init_ulp_vars() {
    ulp_measurement_count = 64;
}

Mutual Exclusion

If mutual exclusion is needed when accessing a variable shared between the main program and ULP, then this can be achieved by using the ULP RISC-V lock API:

The ULP does not have any hardware instructions to facilitate mutual exclusion, so the lock API achieves this through a software algorithm (Peterson’s algorithm).

The locks are intended to only be called from a single thread in the main program, and will not provide mutual exclusion if used simultaneously from multiple threads.

Starting the ULP RISC-V Program

To run a ULP RISC-V program, the main application needs to load the ULP program into RTC memory using the ulp_riscv_load_binary() function, and then start it using the ulp_riscv_run() function.

Note that CONFIG_ULP_COPROC_ENABLED and CONFIG_ULP_COPROC_TYPE_RISCV options must be enabled in menuconfig to work with ULP RISC-V. To reserve memory for the ULP, the RTC slow memory reserved for coprocessor option must be set to a value big enough to store ULP RISC-V 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 RISC-V 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_riscv_load_binary( bin_start,
        (bin_end - bin_start)) );
}

Once the program is loaded into RTC memory, the application can start it by calling the ulp_riscv_run() function:

ESP_ERROR_CHECK( ulp_riscv_run() );

ULP RISC-V Program Flow

The ULP RISC-V coprocessor is started by a timer. The timer is started once ulp_riscv_run() is called. The timer counts the number of RTC_SLOW_CLK ticks (by default, produced by an internal 90kHz RC oscillator). The number of ticks is set using RTC_CNTL_ULP_CP_TIMER_1_REG register. When starting the ULP, RTC_CNTL_ULP_CP_TIMER_1_REG will be used to set the number of timer ticks.

The application can set ULP timer period values (RTC_CNTL_ULP_CP_TIMER_1_REG) using the ulp_set_wakeup_period() function.

Once the timer counts the number of ticks set in the RTC_CNTL_ULP_CP_TIMER_1_REG register, the ULP RISC-V coprocessor will power up and start running the program from the entry point set in the call to ulp_riscv_run().

The program runs until the field RTC_CNTL_COCPU_DONE in register RTC_CNTL_COCPU_CTRL_REG gets written or when a trap occurs due to illegal processor state. Once the program halts, the ULP RISC-V 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_ULP_CP_TIMER_REG register. This can be done both from the ULP code and from the main program.

ULP RISC-V Peripheral Support

To enhance the capabilities of the ULP RISC-V coprocessor, it has access to peripherals which operate in the low-power (RTC) domain. The ULP RISC-V coprocessor can interact with these peripherals when the main CPU is in sleep mode, and can wake up the main CPU once a wakeup condition is reached. The following peripherals are supported.

RTC I2C

The RTC I2C controller provides I2C master functionality in the RTC domain. The ULP RISC-V coprocessor can read from or write to I2C slave devices using this controller. To use the RTC I2C peripheral, call the ulp_riscv_i2c_master_init() from your application running on the main core before initializing the ULP RISC-V core and going to sleep.

Once the RTC I2C controller is initialized, the I2C slave device address must be programmed via the ulp_riscv_i2c_master_set_slave_addr() API before any read or write operation is performed.

Note

The RTC I2C peripheral always expects a slave sub-register address to be programmed via the ulp_riscv_i2c_master_set_slave_reg_addr() API. If it is not, the I2C peripheral uses the SENS_SAR_I2C_CTRL_REG[18:11] as the sub register address for the subsequent read or write operations. This could make the RTC I2C peripheral incompatible with certain I2C devices or sensors which do not need any sub-register to be programmed.

Note

There is no hardware atomicity protection in accessing the RTC I2C peripheral between the main CPU and the ULP RISC-V core. Therefore, care must be taken that both cores are not accessing the peripheral simultaneously.

In case your RTC I2C based ULP RISC-V program is not working as expected, the following sanity checks can help in debugging the issue:

  • Incorrect SDA/SCL pin selection: The SDA pin can only be set up as GPIO1 or GPIO3 and SCL pin can only be set up as GPIO0 or GPIO2. Make sure that the pin configuration is correct.

  • Incorrect I2C timing parameters: The RTC I2C bus timing configuration is limited by the I2C standard bus specification. Any timing parameters which violate the standard I2C bus specifications would result in an error. For details on the timing parameters, please read the standard I2C bus specifications.

  • If the I2C slave device or sensor does not require a sub-register address to be programmed, it may not be compatible with the RTC I2C peripheral. Please refer the notes above.

  • If the RTC driver reports a Write Failed! or Read Failed! error log when running on the main CPU, then make sure:

    • The I2C slave device or sensor works correctly with the standard I2C master on Espressif SoCs. This would rule out any problems with the I2C slave device itself.

    • If the RTC I2C interrupt status log reports a TIMEOUT error or ACK error, it could typically mean that the I2C device did not respond to a START condition sent out by the RTC I2C controller. This could happen if the I2C slave device is not connected properly to the controller pins or if the I2C slave device is in a bad state. Make sure that the I2C slave device is in a good state and connected properly before continuing.

    • If the RTC I2C interrupt log does not report any error status, it could mean that the driver is not fast enough in receiving data from the I2C slave device. This could happen as the RTC I2C controller does not have a TX/RX FIFO to store multiple bytes of data but rather, it depends on single byte transmissions using an interrupt status polling mechanism. This could be mitigated to some extent by making sure that the SCL clock of the peripheral is running as fast as possible. This can be tweaked by configuring the SCL low period and SCL high period values in the initialization config parameters for the peripheral.

  • Other methods of debugging problems would be to ensure that the RTC I2C controller is operational only on the main CPU without any ULP RISC-V code interfering and without any sleep mode being activated. This is the basic configuration under which the RTC I2C peripheral must work. This way you can rule out any potential issues due to the ULP or sleep modes.

Debugging Your ULP RISC-V Program

When programming the ULP RISC-V, it can sometimes be challenging to figure out why the program is not behaving as expected. Due to the simplicity of the core, many of the standard methods of debugging, e.g., JTAG or printf, are simply not available.

Keeping this in mind, here are some ways that may help you debug you ULP RISC-V program:

  • Share program state through shared variables: as described in Accessing the ULP RISC-V Program Variables, both the main CPU and the ULP core can easily access global variables in RTC memory. Writing state information to such a variable from the ULP and reading it from the main CPU can help you discern what is happening on the ULP core. The downside of this approach is that it requires the main CPU to be awake, which will not always be the case. Keeping the main CPU awake might even, in some cases, mask problems, as some issues may only occur when certain power domains are powered down.

  • Use the bit-banged UART driver to print: the ULP RISC-V component comes with a low-speed bit-banged UART TX driver that can be used for printing information independently of the main CPU state. See system/ulp_riscv/uart_print for an example of how to use this driver.

  • Trap signal: the ULP RISC-V has a hardware trap that will trigger under certain conditions, e.g., illegal instruction. This will cause the main CPU to be woken up with the wake-up cause ESP_SLEEP_WAKEUP_COCPU_TRAP_TRIG.

Application Examples

API Reference

Header File

Functions

esp_err_t ulp_riscv_isr_register(intr_handler_t fn, void *arg, uint32_t mask)

Register ULP signal ISR.

Note

The ISR routine will only be active if the main CPU is not in deepsleep

Parameters

  • fn – ISR callback function

  • arg – ISR callback function arguments

  • mask – Bit mask to enable the required ULP RISC-V interrupt signals

Returns

  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if callback function is NULL or if the interrupt bits are invalid

  • ESP_ERR_NO_MEM if heap memory cannot be allocated for the interrupt

  • other errors returned by esp_intr_alloc

esp_err_t ulp_riscv_isr_deregister(intr_handler_t fn, void *arg, uint32_t mask)

Deregister ULP signal ISR.

Parameters

  • fn – ISR callback function

  • arg – ISR callback function arguments

  • mask – Bit mask to enable the required ULP RISC-V interrupt signals

Returns

  • ESP_OK on success

  • ESP_ERR_INVALID_ARG if callback function is NULL or if the interrupt bits are invalid

  • ESP_ERR_INVALID_STATE if a handler matching both callback function and its arguments isn’t registered

esp_err_t ulp_riscv_config_and_run(ulp_riscv_cfg_t *cfg)

Configure the ULP and run the program loaded into RTC memory.

Parameters

cfg – pointer to the config struct

Returns

ESP_OK on success

esp_err_t ulp_riscv_run(void)

Configure the ULP with default settings and run the program loaded into RTC memory.

Returns

ESP_OK on success

esp_err_t ulp_riscv_load_binary(const uint8_t *program_binary, size_t program_size_bytes)

Load ULP-RISC-V program binary into RTC memory.

Different than ULP FSM, the binary program has no special format, it is the ELF file generated by RISC-V toolchain converted to binary format using objcopy.

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

Parameters

  • program_binary – pointer to program binary

  • program_size_bytes – size of the program binary

Returns

  • ESP_OK on success

  • ESP_ERR_INVALID_SIZE if program_size_bytes is more than 8KiB

void ulp_riscv_timer_stop(void)

Stop the ULP timer.

Note

This will stop the ULP from waking up if halted, but will not abort any program currently executing on the ULP.

void ulp_riscv_timer_resume(void)

Resumes the ULP timer.

Note

This will resume an already configured timer, but does no other configuration

void ulp_riscv_halt(void)

Halts the program currently running on the ULP-RISC-V.

Note

Program will restart at the next ULP timer trigger if timer is still running. If you want to stop the ULP from waking up then call ulp_riscv_timer_stop() first.

void ulp_riscv_reset(void)

Resets the ULP-RISC-V core from the main CPU.

Note

This will reset the ULP core from the main CPU. It is intended to be used when the ULP is in a bad state and cannot run as intended due to a corrupt firmware or any other reason. The main core can reset the ULP core with this API and then re-initilialize the ULP.

Structures

struct ulp_riscv_cfg_t

ULP riscv init parameters.

Public Members

ulp_riscv_wakeup_source_t wakeup_source

ULP wakeup source

Macros

ULP_RISCV_DEFAULT_CONFIG()
ULP_RISCV_SW_INT
ULP_RISCV_TRAP_INT

Enumerations

enum ulp_riscv_wakeup_source_t

Values:

enumerator ULP_RISCV_WAKEUP_SOURCE_TIMER
enumerator ULP_RISCV_WAKEUP_SOURCE_GPIO

Header File

Structures

struct ulp_riscv_lock_t

Structure representing a lock shared between ULP and main CPU.

Public Members

bool critical_section_flag_ulp

ULP wants to enter the critical sections

bool critical_section_flag_main_cpu

Main CPU wants to enter the critical sections

ulp_riscv_lock_turn_t turn

Which CPU is allowed to enter the critical section

Enumerations

enum ulp_riscv_lock_turn_t

Enum representing which processor is allowed to enter the critical section.

Values:

enumerator ULP_RISCV_LOCK_TURN_ULP

ULP’s turn to enter the critical section

enumerator ULP_RISCV_LOCK_TURN_MAIN_CPU

Main CPU’s turn to enter the critical section

Header File

Functions

void ulp_riscv_lock_acquire(ulp_riscv_lock_t *lock)

Locks are based on the Peterson’s algorithm, https://en.wikipedia.org/wiki/Peterson%27s_algorithm.

Acquire the lock, preventing the ULP from taking until released. Spins until lock is acquired.

Note

The lock is only designed for being used by a single thread on the main CPU, it is not safe to try to acquire it from multiple threads.

Parameters

lock – Pointer to lock struct, shared with ULP

void ulp_riscv_lock_release(ulp_riscv_lock_t *lock)

Release the lock.

Parameters

lock – Pointer to lock struct, shared with ULP

Header File

Functions

void ulp_riscv_i2c_master_set_slave_addr(uint8_t slave_addr)

Set the I2C slave device address.

Parameters

slave_addr – I2C slave address (7 bit)

void ulp_riscv_i2c_master_set_slave_reg_addr(uint8_t slave_reg_addr)

Set the I2C slave device sub register address.

Note

The RTC I2C peripheral always expects a slave sub register address to be programmed. If it is not, the I2C peripheral uses the SENS_SAR_I2C_CTRL_REG[18:11] as the sub register address for the subsequent read or write operation.

Parameters

slave_reg_addr – I2C slave sub register address

void ulp_riscv_i2c_master_read_from_device(uint8_t *data_rd, size_t size)

Read from I2C slave device.

Note

The I2C slave device address must be configured at least once before invoking this API.

Parameters

  • data_rd – Buffer to hold data to be read

  • size – Size of data to be read in bytes

void ulp_riscv_i2c_master_write_to_device(uint8_t *data_wr, size_t size)

Write to I2C slave device.

Note

The I2C slave device address must be configured at least once before invoking this API.

Parameters

  • data_wr – Buffer which holds the data to be written

  • size – Size of data to be written in bytes

esp_err_t ulp_riscv_i2c_master_init(const ulp_riscv_i2c_cfg_t *cfg)

Initialize and configure the RTC I2C for use by ULP RISC-V Currently RTC I2C can only be used in master mode.

Parameters

cfg – Configuration parameters

Returns

esp_err_t ESP_OK when successful

Structures

struct ulp_riscv_i2c_pin_cfg_t

ULP RISC-V RTC I2C pin config.

Public Members

uint32_t sda_io_num

GPIO pin for SDA signal. Only GPIO#1 or GPIO#3 can be used as the SDA pin.

uint32_t scl_io_num

GPIO pin for SCL signal. Only GPIO#0 or GPIO#2 can be used as the SCL pin.

bool sda_pullup_en

SDA line enable internal pullup. Can be configured if external pullup is not used.

bool scl_pullup_en

SCL line enable internal pullup. Can be configured if external pullup is not used.

struct ulp_riscv_i2c_timing_cfg_t

ULP RISC-V RTC I2C timing config.

Public Members

float scl_low_period

SCL low period in micro seconds

float scl_high_period

SCL high period in micro seconds

float sda_duty_period

Period between the SDA switch and the falling edge of SCL in micro seconds

float scl_start_period

Waiting time after the START condition in micro seconds

float scl_stop_period

Waiting time before the END condition in micro seconds

float i2c_trans_timeout

I2C transaction timeout in micro seconds

struct ulp_riscv_i2c_cfg_t

ULP RISC-V RTC I2C init parameters.

Public Members

ulp_riscv_i2c_pin_cfg_t i2c_pin_cfg

RTC I2C pin configuration

ulp_riscv_i2c_timing_cfg_t i2c_timing_cfg

RTC I2C timing configuration

Macros

ULP_RISCV_I2C_DEFAULT_GPIO_CONFIG()
ULP_RISCV_I2C_FAST_MODE_CONFIG()
ULP_RISCV_I2C_STANDARD_MODE_CONFIG()
ULP_RISCV_I2C_DEFAULT_CONFIG()

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