SMBus Read Byte protocol w/ Synapse Wireless Snappy

smbus_read_byte_protocol_with_synapse_wireless_snappy.py

"""
Synapse Wireless RF200 devices use some terribly hobbled version of Python2 coupled with a terribly limited library.

They are in general terrible devices.

Anyway- to read SMBus (a protocol layer on top of i2c- see Page 41 of 85 of http://smbus.org/specs/SMBus_3_0_20141220.pdf) the process is as follows:

- write 2 bytes to i2c
    - byte 0 = slave address shifted left by 1 bit, write bit of 0
    - byte 1 = command
- send restart bit
- write 1 byte to i2c
    - byte 0 = slave address shifted left by 1 bit, read bit of 1
- read n bytes from i2c (in our case, 2)
"""

#
# mocked out functions so you can test this on a desktop Python implementation
#

def i2cWrite(byteStr, retries, ignoreFirstAck, endWithRestart=False):
    return len(byteStr)


def i2cRead(byteStr, numToRead, retries, ignoreFirstAck):
    return chr(0x00) * numToRead

#
# stuff to actually paste into your Portal sketch
#

def get_address_byte(address, read):
    return chr((address << 1) + (0x01 if read else 0x00))

def smbus_read_word(address, command, length):
    write_address = get_address_byte(address, False) + chr(command)
    read_address = get_address_byte(address, True)

    # critical that last argument (endWithRestart) is True
    _ = i2cWrite(write_address, 1, True, True)

    return i2cRead(read_address, length, 1, True)

#
# example usage
#

data = smbus_read_word(0x68, 0x3B, 2)