OVMS3-idf/components/ulp

Programming ULP coprocessor using C macros
==========================================

In addition to the existing binutils port for the ESP32 ULP coprocessor, it is possible to generate programs for the ULP by embedding assembly-like macros into an ESP32 application. Here is an example how this can be done::

    const ulp_insn_t program[] = {
        I_MOVI(R3, 16),         // R3 <- 16
        I_LD(R0, R3, 0),        // R0 <- RTC_SLOW_MEM[R3 + 0]
        I_LD(R1, R3, 1),        // R1 <- RTC_SLOW_MEM[R3 + 1]
        I_ADDR(R2, R0, R1),     // R2 <- R0 + R1
        I_ST(R2, R3, 2),        // R2 -> RTC_SLOW_MEM[R2 + 2]
        I_HALT()
    };
    size_t load_addr = 0;
    size_t size = sizeof(program)/sizeof(ulp_insn_t);
    ulp_process_macros_and_load(load_addr, program, &size);
    ulp_run(load_addr);

The ``program`` array is an array of ``ulp_insn_t``, i.e. ULP coprocessor instructions. Each ``I_XXX`` preprocessor define translates into a single 32-bit instruction. Arguments of these preprocessor defines can be register numbers (``R0 — R3``) and literal constants. See `ULP coprocessor instruction defines`_ section for descriptions of instructions and arguments they take.

.. note::

    Because some of the instruction macros expand to inline function calls, defining such array in global scope will cause the compiler to produce an "initializer element is not constant" error. To fix this error, move the definition of instructions array into local scope.

Load and store instructions use addresses expressed in 32-bit words. Address 0 corresponds to the first word of ``RTC_SLOW_MEM`` (which is address 0x50000000 as seen by the main CPUs).

To generate branch instructions, special ``M_`` preprocessor defines are used. ``M_LABEL`` define can be used to define a branch target. Label identifier is a 16-bit integer. ``M_Bxxx`` defines can be used to generate branch instructions with target set to a particular label. 

Implementation note: these ``M_`` preprocessor defines will be translated into two ``ulp_insn_t`` values: one is a token value which contains label number, and the other is the actual instruction. ``ulp_process_macros_and_load`` function resolves the label number to the address, modifies the branch instruction to use the correct address, and removes the the extra ``ulp_insn_t`` token which contains the label numer.

Here is an example of using labels and branches::

    const ulp_insn_t program[] = {
        I_MOVI(R0, 34),         // R0 <- 34
        M_LABEL(1),             // label_1
        I_MOVI(R1, 32),         // R1 <- 32
        I_LD(R1, R1, 0),        // R1 <- RTC_SLOW_MEM[R1]
        I_MOVI(R2, 33),         // R2 <- 33
        I_LD(R2, R2, 0),        // R2 <- RTC_SLOW_MEM[R2]
        I_SUBR(R3, R1, R2),     // R3 <- R1 - R2
        I_ST(R3, R0, 0),        // R3 -> RTC_SLOW_MEM[R0 + 0]
        I_ADDI(R0, R0, 1),      // R0++
        M_BL(1, 64),            // if (R0 < 64) goto label_1
        I_HALT(),
    };
    RTC_SLOW_MEM[32] = 42;
    RTC_SLOW_MEM[33] = 18;
    size_t load_addr = 0;
    size_t size = sizeof(program)/sizeof(ulp_insn_t);
    ulp_process_macros_and_load(load_addr, program, &size);
    ulp_run(load_addr);


Functions
^^^^^^^^^

.. doxygenfunction:: ulp_process_macros_and_load
.. doxygenfunction:: ulp_run

Error codes
^^^^^^^^^^^

.. doxygendefine:: ESP_ERR_ULP_BASE
.. doxygendefine:: ESP_ERR_ULP_SIZE_TOO_BIG
.. doxygendefine:: ESP_ERR_ULP_INVALID_LOAD_ADDR
.. doxygendefine:: ESP_ERR_ULP_DUPLICATE_LABEL
.. doxygendefine:: ESP_ERR_ULP_UNDEFINED_LABEL
.. doxygendefine:: ESP_ERR_ULP_BRANCH_OUT_OF_RANGE

ULP coprocessor registers
^^^^^^^^^^^^^^^^^^^^^^^^^

ULP co-processor has 4 16-bit general purpose registers. All registers have same functionality, with one exception. R0 register is used by some of the compare-and-branch instructions as a source register.
 
These definitions can be used for all instructions which require a register.

.. doxygengroup:: ulp_registers
    :content-only:
    
ULP coprocessor instruction defines
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. doxygendefine:: I_DELAY
.. doxygendefine:: I_HALT
.. doxygendefine:: I_END
.. doxygendefine:: I_ST
.. doxygendefine:: I_LD
.. doxygendefine:: I_WR_REG
.. doxygendefine:: I_RD_REG
.. doxygendefine:: I_BL
.. doxygendefine:: I_BGE
.. doxygendefine:: I_BXR
.. doxygendefine:: I_BXI
.. doxygendefine:: I_BXZR
.. doxygendefine:: I_BXZI
.. doxygendefine:: I_BXFR
.. doxygendefine:: I_BXFI
.. doxygendefine:: I_ADDR
.. doxygendefine:: I_SUBR
.. doxygendefine:: I_ANDR
.. doxygendefine:: I_ORR
.. doxygendefine:: I_MOVR
.. doxygendefine:: I_LSHR
.. doxygendefine:: I_RSHR
.. doxygendefine:: I_ADDI
.. doxygendefine:: I_SUBI
.. doxygendefine:: I_ANDI
.. doxygendefine:: I_ORI
.. doxygendefine:: I_MOVI
.. doxygendefine:: I_LSHI
.. doxygendefine:: I_RSHI
.. doxygendefine:: M_LABEL
.. doxygendefine:: M_BL
.. doxygendefine:: M_BGE
.. doxygendefine:: M_BX
.. doxygendefine:: M_BXZ
.. doxygendefine:: M_BXF

Defines
^^^^^^^

.. doxygendefine:: RTC_SLOW_MEM