RTL Design #32 · 2026-10-25 · Serial transmitters · Medium · about 15 min

Serial transmitter that forgets its byte

ser_tx sends one byte at a time on txd, one bit per clock: a start bit 0, then data[0] to data[7] (least significant first), then a stop bit 1. txd idles at 1.

• When idle, start makes the transmitter take data on that rising edge. From the next cycle busy is 1 and txd shows the start bit; busy is 1 for exactly the ten bit-cycles.
• The caller holds data for the start cycle only, and may change it, or pulse start again, at any time after that. A start while busy is ignored.
• rst_n (synchronous) abandons any frame in progress.

Run sends two bytes and prints the bits that came out. Fix tx.sv.

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