Core code causes timing issues at some alternative clock speeds

[trthanhquang]

I recently tried to run my Atmega328p with a 1.8432Mhz crystal (to use a faster baud rate). I managed to recompile the bootloader by changing AVR_FREQ and FUSES in Optiboot Makefile, boards.txt

However I found out some timing issue with Arduino's implementation of delay, millis, micros....
As defined
in Arduino.h (line 96)
#define clockCyclesPerMicrosecond() ( F_CPU / 1000000L )
in wiring.c (line 103)

unsigned long micros() {
 ...
 return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

This integer division really limit the choices of clock!

The baud rate is correct and avr version of delay (_delay_ms) work correctly for me. I am not sure if any other libraries are affected.