In SX Microcontrollers, SX/B Compiler and SX-Key Tool, davehein wrote:

If your interrupt service routine is short you can probably leave interrupts enabled.  With a system clock of 4 MHz, the 9600 buad bit time is about 417 cycles.  This allows for a tolerance of about +/- 200 cycles.  If your ISR is less than 100 cycles you should not have a problem, assuming that you don't get more than one interrupt per transmitted byte.  At 19.2 or 38.4 things become a bit tighter.
 

I'm working on a project where I get 100 interrupts per second.  My ISR increments a memory location and then returns, so it only uses a few cycles.  I've never seen a problem with serial input or output with interrupts enabled.

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