Reducing code size for embedded ARM processors

There are a few options that make the code size smaller when using GCC for embedded ARM processors.

The newlib has a nano version which is optimized for small embedded processor. To activate newlib-nano, add the option -specs=nano.specs to the linker.  The newlib-nano printf does not print floating number format.

Another linker option --gc-sections removes unused sections.  To facilitate gc-section, compile each function and data item into its own section by passing `-ffunction-sections' and -fdata-sections. 

The optimizer can be specified to generate smaller code with the -Os option.

Keil ARM compiler consistently produces much smaller code, often 50% smaller.   Especially the math library is not only much smaller but also much high performance.

Some considerations in I2C interface

The first thing is the value of the pull-up resistor.  Consider the fast mode 400KHz, or 2.5us period.  The I2C standard calls for a rise time (30%-70%) of 0.3us max.  The RC circuit V(t) = Vdd*(1 - exp(t/τ)), where τ is the RC constant; the rise time is τ*(ln(0.7) - ln(0.3)) = 0.85*a, so τ = 0.35us.  (Note that the scope measures 10%-90% rise time, which is 2.2*τ.)  If the line capacitance is 100pF, the resistance should be less than 3.5KOhms.  I find the usual 10K only good for very short wires, and does not work when I attach a scope probe.  The capacitance between the adjacent wires of a 50mil pitch ribbon cable is ~15pF/ft; twisted pair ~25pF/ft; an IC pin 8pF. The bus driver's minimum current sink capability is 3mA and VOL = 0.4V, so for Vdd = 3.3V, the minimum allowable resistor is 1.0KOhms.  Then the maximum capacitance is 350pF.

If the slave fails to acknowledge within a reasonable amount of time, the master generates stop to release the bus.  Another more serious problem is bus stuck, especially SDA stuck low.  One situation this could arise is that noise in the bus or master reset leaves the device in the middle of transferring data.  To recover, the master has to either reset the device if possible or clock for 9 cycles.  On a microcontroller, the clocking may be done manually in the GPIO mode.