add reference to battery-powered applications to advanced usage in README

wnienhaus · wnienhaus · commit 26048c85e7af · 2021-10-24T18:12:15.000+03:00
While py-esp32-ulp could be used on a PC to generate the ULP binary,
which in turn could be uploaded to an ESP32, this workflow can also
be realised using binutils-esp32ulp on the PC.

py-esp32-ulp makes it easy to work directly on the ESP32 to assemble
code for the ULP. However, the ESP32 is rather slow at doing this,
which becomes relevant in battery-powered applications, where every
second of sleep time is valuable.

The documentation therefore no longer refers to the use-case of
assembling ULP code on a PC, and adds a short reference to battery-
powered applications as one reason, why one might want to split the
assembly and load phase and store the ULP binary into a file.
diff --git a/README.rst b/README.rst
@@ -86,6 +86,12 @@ 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.
+This can be useful for battery-powered applications, where every second of sleep
+time matters.
+
+Splitting the assembly and load stage can be combined with other techniques to
+for example implement a caching mechansim for the ULP binary, which automatically
+updates the binary every time the assembly source code changes.
 
 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
@@ -99,16 +105,6 @@ assembling the source again.
       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.
@@ -135,6 +131,10 @@ assembling the source again.
           #   2 words * 4 = 8 bytes
           ulp.run(2*4)  # specify the offset of the entry point label
 
+To update the binary every time the source code changes, you would need a mechanism
+to detect that the source code changed. Whenever needed, manually re-running the
+``assemble_file`` function as shown above, would also work.
+
 
 Preprocessor
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