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type: investigation status: closed summary: The 2026-06 SWD bitbang optimization of the F042 bridge on tag-breakout-base-l432-v1: about 23% faster downloads, and why ll_swd.c stays at the normal optimization level.

SWD bitbang optimization on tag-breakout-base-l432-v1 (2026-06)

Closed: the optimization shipped, and the -O3 experiment was rejected. Extracted verbatim from base design notes; ideas for a future base remain there.

The tag-breakout-base-l432-v1 base uses an STM32F042 as the SWD bridge MCU and is already running at the F042 maximum 48 MHz SYSCLK/HCLK. The 2026-06 SWD bitbang optimization pass replaced hot-path PAL line calls with lower-level GPIO helpers, cached direction changes, and added fixed-width unrolled shift helpers. Measured download speed improved by about 23%.

An experiment to compile only tag-breakout-base-l432-v1/src/ll_swd.c at -O3 was a net performance loss and increased flash size, so keep that file at the normal target optimization level.