Why Reception Must Use Interrupt Flag

Transmission can use time delay because transmitter controls timing; SBUF loaded after >character time ensures previous byte finished.

Reception is asynchronous – sender timing unpredictable, start bit arrives anytime. Polling by delay may miss bytes because:

1. If polling period > character time, bytes are overwritten (overrun). 8051 double-buffering means second byte overwrites before RI flags.
2. Flag RI is set at middle of stop bit, accurate timing; polling delay not synchronized to start bit, so sampling may miss.
3. Continuous back-to-back characters require detection of each stop→start transition; interrupt on RI ensures every byte captured and software can read SBUF before next. Hence reception must check RI flag (poll or interrupt) not blind delay.