add first example: a simple counter running on the ULP coprocessor

Author: ThomasWaldmann

examples/README

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+To run the micropython examples which load and run binaries on the ULP,
+you need a ESP32 MicroPython build that was made after 2018-05-01.
+

examples/counter.py

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+"""
+Very basic example showing data exchange main CPU <--> ULP coprocessor.
+
+To show that the ULP is doing something, it just increments the value <data>.
+It does that once per ulp timer wakeup (and then the ULP halts until it gets
+waked up via the timer again).
+
+The timer is set to a rather long period, so you can watch the data value
+incrementing (see loop at the end).
+"""
+
+from esp32 import ULP
+from machine import mem32
+
+from esp32_ulp.__main__ import src_to_binary
+
+source = """\
+data:       .long 0
+
+entry:      move r3, data    # load address of data into r3
+            ld r2, r3, 0     # load data contents ([r3+0]) into r2
+            add r2, r2, 1    # increment r2
+            st r2, r3, 0     # store r2 contents into data ([r3+0])
+
+            halt             # halt ULP co-prozessor (until it gets waked up again)
+"""
+
+binary = src_to_binary(source)
+
+load_addr, entry_addr = 0, 4
+
+ULP_MEM_BASE = 0x50000000
+ULP_DATA_MASK = 0xffff  # ULP data is only in lower 16 bits
+
+ulp = ULP()
+ulp.set_wakeup_period(0, 50000)  # use timer0, wakeup after 50.000 cycles
+ulp.load_binary(load_addr, binary)
+
+mem32[ULP_MEM_BASE + load_addr] = 0x1000
+ulp.run(entry_addr)
+
+while True:
+    print(hex(mem32[ULP_MEM_BASE + load_addr] & ULP_DATA_MASK))
+