Echo Tail

This board echoes serial traffic back to the console, and firmware is already flashed and running. A field report came in: when the console sends a quick run of characters back-to-back, only the first part of the run echoes. The tail of the run never comes back. Typing one key at a time echoes fine every time.

The hardware runs at 16 MHz. USART1 is on the APB2 bus at 9600 baud, 8N1.

The firmware that is on the board is in the editor. Find why a fast run loses its tail and fix it so every byte of the run echoes back, in order, with nothing missing. Single-byte echo must keep working.