This is the story of how I hacked my beautiful baud rate divider down to a measly 74 bytes. It wasn't without compromise, but as with any good task of engineering, it's all about trade-offs. So lets begin.

Start: 144 bytes

To start off, the PIC18F USART supports different divider ranges selected using the BRGH bit in the TXSTA register. Having BRGH clear has almost no redeeming value, except for supporting baud rates below 183 when running at 48MHz. Setting BRGH however, increases the resolution of the divider by exactly 4. What this implies, is that any baud rate producible with BRGH clear, can also be done with BRGH set with the exception of rates below 183. So, removing support for low baudrates has no other drawback. This shaved off 42 bytes.

Remaining: 102 bytes

Next up, after the division is completed, the PIC specifies the divider to be one less than the result of the division. But, of course, the division doesn't always result in a nice round number, so the code I had to refactor down would use the remainder to determine whether the answer would round up or down, and include the implied subtraction in that rounding operation. It was tight. It felt like an elegant solution. Alas, fudging the numerator, and doing a subtraction still results in the best divider for all of the standard rates above 300 baud which I've checked. I bumped up the numerator from 12,000,000 to 12,001,024, and subtract 1 from the result. Net savings for this compromise came in at another 28 bytes.

Final size: 74 bytes

On to the next one...