improve README

Make it friendlier and easier to get started:
* Generally shorten sentences/paragraphs, where possible
* Add Installation and Getting Started instructions
* Mention counter.py example as the simplest way to get started
* Shorten supported/not-supported features into a bulleted list
* Add Advanced Usage section showing how to assemble to and load from file
* Add example of how to assemble on a PC
* Add reference to GitHub Actions workflow for how to run tests
* Add mention of ESP32-S2 not being supported
* Add 2 more links to ULP examples
* Add section mentioning the license
* Remove reference to missing stuff, bugs and other beta quality symptoms
* Remove mention of the issue tracker, as this is common on GitHub
* Remove old README from examples dir

Author: wnienhaus

Diff for: README.rst

@@ -1,63 +1,177 @@
-What is py-esp32-ulp?
----------------------
+=====================
+py-esp32-ulp
+=====================
 
-It is an assembler toolchain for the ESP32 ULP (Ultra Low-Power) Co-Processor,
-written in MicroPython.
+py-esp32-ulp is an assembler toolchain for the ESP32 ULP (Ultra Low-Power)
+Co-Processor, written in MicroPython.
 
-It is able to translate small, simple assembler language programs to a
-loadable/executable machine code binary, at runtime, on the ESP32
-microcontroller, from projects implemented in MicroPython.
+It can translate small assembly language programs to a loadable/executable
+ULP machine code binary, directly on the ESP32 microcontroller.
 
-This is intended as an alternative approach to assembling such programs on a
-development machine using the binutils-esp32ulp toolchain from Espressif.
+This is intended as an alternative approach to assembling such programs using
+the binutils-esp32ulp toolchain from Espressif on a development machine.
 
 
-Status
-------
+Installation
+------------
 
-The most commonly used stuff should work. Many ULP code examples found on
-the web will work unmodified. Notably, assembler macros and #include processing
-are not supported.
+On the ESP32, install using upip:
 
-Expressions in assembly source code are supported and get evaluated during
-assembling. Only expressions evaluating to a single integer are supported.
-Constants defined with ``.set`` are supported in expressions.
+.. code-block:: python
 
-We have some unit tests and also compatibility tests that compare the output
-whether it is identical with binutils-esp32ulp output.
+   # ensure the ESP32 is connected to a network with internet connectivity
+   import upip
+   upip.install('micropython-py-esp32-ulp')
+
+On a PC, simply ``git clone`` this repo.
+
+
+Getting Started
+---------------
+
+The quickest way to get started is to try one of the `examples <examples/>`_.
+
+The simplest example is `counter.py <examples/counter.py>`_. It shows how to
+assemble code, load and run the resulting binary and exchange data between the
+ULP and the main CPU.
+
+Run the ``counter.py`` example:
+
+1. Install py-esp32-ulp onto the ESP32 as shown above
+2. Upload the `counter.py <examples/counter.py>`_ file to the ESP32
+3. Run with ``import counter``
+
+You can also try the `blink.py <examples/blink.py>`_ example, which shows how to
+let the ULP blink an LED.
+
+Look inside each example for a more detailed description.
+
+
+Support
+-------
+
+The following features are supported:
+
+* The entire `ESP32 ULP instruction set <https://esp-idf.readthedocs.io/en/latest/api-guides/ulp_instruction_set.html>`_
+* Constants defined with ``.set``
+* Constants defined with ``#define``
+* Expressions in assembly code and constant definitions
+* RTC convenience macros (e.g. WRITE_RTC_REG)
+* Many ESP32 ULP code examples found on the web will work unmodified
+
+Not currently supported:
+
+* Assembler macros using ``.macro``
+* Preprocessor macros using ``#define A(x,y) ...``
+* Including files using ``#include``
+* ESP32-S2 (not binary compatible with the ESP32)
+
+
+Requirements
+------------
+
+The minimum supported version of MicroPython is v1.12.
+
+py-esp32-ulp has been tested on the Unix port of MicroPython and on real ESP32
+devices with the chip type ESP32D0WDQ6 (revision 1) without SPIRAM.
+
+
+Advanced usage
+--------------
+
+In real world applications, you might want to separate the assembly stage from
+the loading/running stage, to avoid having to assemble the code on every startup.
+
+The ``esp32_ulp.assemble_file`` function stores the assembled and linked binary
+into a file with a ``.ulp`` extension, which can later be loaded directly without
+assembling the source again.
+
+1. Create/upload an assembly source file and run the following to get a loadable
+   ULP binary as a ``.ulp`` file:
+
+   .. code-block:: python
+
+      import esp32_ulp
+      esp32_ulp.assemble_file('code.S')  # this results in code.ulp
+
+   Alternatively you can assemble the source on a PC with MicroPython, and transfer
+   the resulting ULP binary to the ESP32.
+
+   .. code-block:: python
+
+      git clone https://github.com/ThomasWaldmann/py-esp32-ulp
+      cd py-esp32-ulp
+      micropython -m esp32_ulp path/to/code.S  # this results in path/to/code.ulp
+      # now upload path/to/code.ulp to the ESP32
+
+2. The above prints out the offsets of all global symbols/labels. For the next step,
+   you will need to note down the offset of the label, which represents the entry
+   point to your code.
+
+3. Now load and run the resulting binary as follows:
+
+   .. code-block:: python
+
+      from esp32 import ULP
+
+      ulp = ULP()
+      with open('test.ulp', 'r') as f:
+          # load the binary into RTC memory
+          ulp.load_binary(0, f.read())
+
+          # configure how often the ULP should wake up
+          ulp.set_wakeup_period(0, 500000)  # 500k usec == 0.5 sec
+
+          # start the ULP
+          # assemble_file printed offsets in number of 32-bit words.
+          # ulp.run() expects an offset in number of bytes.
+          # Thus, multiply the offset to our entry point by 4.
+          # e.g. for an offset of 2:
+          #   2 words * 4 = 8 bytes
+          ulp.run(2*4)  # specify the offset of the entry point label
+
+
+Preprocessor
+------------
 
 There is a simple preprocessor that understands just enough to allow assembling
-ULP source files containing convenience macros such as WRITE_RTC_REG. The
-preprocessor and how to use it is documented here:
-`Preprocessor support <docs/preprocess.rst>`_.
+ULP source files containing convenience macros such as WRITE_RTC_REG. This is
+especially useful for assembling ULP examples from Espressif or other ULP code
+found as part of Arduino/ESP-IDF projects.
+
+The preprocessor and how to use it is documented here: `Preprocessor support <docs/preprocess.rst>`_.
 
-The minimum supported version of MicroPython is v1.12. py-esp32-ulp has been
-tested with MicroPython v1.12 and v1.17. It has been tested on real ESP32
-devices with the chip type ESP32D0WDQ6 (revision 1) without SPIRAM. It has
-also been tested on the Unix port.
 
-There might be some stuff missing, some bugs and other symptoms of beta
-software. Also, error and exception handling is rather rough yet.
+Testing
+-------
 
-Please be patient or contribute missing parts or fixes.
+There are unit tests and also compatibility tests that check whether the binary
+output is identical with what binutils-esp32ulp produces.
 
-See the issue tracker for known bugs and todo items.
+Consult the Github Actions `workflow definition file <.github/workflows/run_tests.yaml>`_
+for how to run the different tests.
 
 
 Links
 -----
 
-We are NOT (fully) compatible with "as", but we try to be close for the stuff
-that is actually implemented:
+Espressif documentation:
+
+* `ESP32 ULP coprocessor instruction set <https://esp-idf.readthedocs.io/en/latest/api-guides/ulp_instruction_set.html>`_
+* `ESP32 Technical Reference Manual <https://www.espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf>`_
+
+GNU Assembler "as" documentation (we try to be compatible for all features that are implemented)
 
-https://sourceware.org/binutils/docs/as/index.html
+* `GNU Assembler manual <https://sourceware.org/binutils/docs/as/index.html>`_
 
-Espressif docs:
+More ULP examples:
 
-https://esp-idf.readthedocs.io/en/latest/api-guides/ulp_instruction_set.html
+* https://github.com/espressif/esp-iot-solution/tree/master/examples/ulp_examples
+* https://github.com/duff2013/ulptool
+* https://github.com/joba-1/Blink-ULP/blob/master/main/ulp/
 
-https://www.espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf
 
-Espressif ULP examples:
+License
+-------
 
-https://github.com/espressif/esp-iot-solution/tree/master/examples/ulp_examples
+This project is released under the `MIT License <LICENSE>`_.