stmhal: Add preliminary driver for ST32F4DISC accelerometer.

Written in Python, not currently working.  See issue #725.

stmhal/boards/STM32F4DISC/staccel.py

+"""
+Driver for accelerometer on STM32F4 Discover board.
+
+Assumes it's a LIS302DL MEMS device.
+
+Not currently working.
+
+See:
+    STM32Cube_FW_F4_V1.1.0/Drivers/BSP/Components/lis302dl/lis302dl.h
+    STM32Cube_FW_F4_V1.1.0/Drivers/BSP/Components/lis302dl/lis302dl.c
+    STM32Cube_FW_F4_V1.1.0/Drivers/BSP/STM32F4-Discovery/stm32f4_discovery.c
+    STM32Cube_FW_F4_V1.1.0/Drivers/BSP/STM32F4-Discovery/stm32f4_discovery.h
+    STM32Cube_FW_F4_V1.1.0/Drivers/BSP/STM32F4-Discovery/stm32f4_discovery_accelerometer.c
+    STM32Cube_FW_F4_V1.1.0/Drivers/BSP/STM32F4-Discovery/stm32f4_discovery_accelerometer.h
+    STM32Cube_FW_F4_V1.1.0/Projects/STM32F4-Discovery/Demonstrations/Src/main.c
+"""
+
+from pyb import Pin
+from pyb import SPI
+
+READWRITE_CMD = const(0x80) 
+MULTIPLEBYTE_CMD = const(0x40)
+LIS302DL_WHO_AM_I_ADDR = const(0x0f)
+LIS302DL_CTRL_REG1_ADDR = const(0x20)
+
+class STAccel:
+    def __init__(self):
+        self.cs_pin = Pin('PE3', Pin.OUT_PP, Pin.PULL_NONE)
+        self.cs_pin.high()
+        self.spi = SPI(1, SPI.MASTER, baudrate=328125, polarity=0, phase=1, bits=8)
+
+    def rd(self, addr, nbytes):
+        if nbytes > 1:
+            addr |= READWRITE_CMD | MULTIPLEBYTE_CMD
+        else:
+            addr |= READWRITE_CMD
+        self.cs_pin.low()
+        self.spi.send(addr)
+        buf = self.spi.send_recv(bytearray(nbytes * [0])) # read data, MSB first
+        self.cs_pin.high()
+        return buf
+
+    def wr(self, addr, buf):
+        if len(buf) > 1:
+            addr |= MULTIPLEBYTE_CMD
+        self.cs_pin.low()
+        self.spi.send(addr)
+        for b in buf:
+            self.spi.send(b)
+        self.cs_pin.high()
+
+    def read_id(self):
+        return self.rd(LIS302DL_WHO_AM_I_ADDR, 1)
+
+    def init(self, init_param):
+        self.wr(LIS302DL_CTRL_REG1_ADDR, bytearray([init_param]))