What should stay out of a small 8051 interrupt handler?

Keep the handler focused on acknowledging the source and capturing the minimum information needed. Formatting, blocking transmission and long calculations extend interrupt latency and may use non-reentrant library code. Check the compiler's calling and register-bank conventions rather than assuming desktop-library behavior applies.

Pass events through a bounded buffer or a carefully synchronized shared structure, then format and transmit outside the handler. Define the overflow policy and count lost events. If events arrive at 1,000 per second but output supports only 200 records per second, buffering delays the overflow; it cannot solve the sustained throughput mismatch.

Measure worst-case handler duration and inspect nesting behavior if priorities are enabled. Use a short diagnostic pin pulse or a hardware capture method when possible, since logging can alter the timing being measured. Review stack usage and multi-byte shared accesses as part of the same investigation.