As per discussion in #2449, using write requests instead of read requests
for I2C.scan() seems to support a larger number of devices, especially
ones that are write-only. Even a read-only I2C device has to implement
writes in order to be able to receive the address of the register to read.
The memory read/write I2C functions now take an optional keyword-only
parameter that specifies the number of bits in the memory address.
Only mem-addrs that are a multiple of 8-bits are supported (otherwise
the behaviour is undefined).
Due to the integer type used for the address, for values larger than 32
bits, only 32 bits of address will be sent, and the rest will be padded
with 0s. Right now no exception is raised when that happens. For values
smaller than 8, no address is sent. Also no exception then.
Tested with a VL6180 sensor, which has 16-bit register addresses.
Due to code refactoring, this patch reduces stmhal and esp8266 builds
by about 50 bytes.
When the clock is too fast for the i2c slave, it can temporarily hold
down the scl line to signal to the master that it needs to wait. The
master should check the scl line when it is releasing it after
transmitting data, and wait for it to be released.
This change has been tested with a logic analyzer and an i2c slace
implemented on an atmega328p using its twi peripheral, clocked at 8Mhz.
Without the change, the i2c communication works up to aboy 150kHz
frequency, and above that results in the slave stuck in an unresponsive
state. With this change, communication has been tested to work up to
400kHz.
