avr/ds3231
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build: the libavr pin advances, and the tiny85 image grows 78 B

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43fc479 -> aec9955. The mega328P does not move at 1480 B - it drives the RTC
from the TWI block, which neither change touches. The tiny85 has no TWI, so
dev::i2c resolves to the bit-banged master there and takes both:

  cb40a13, the bit period                      1238 -> 1262  (+24)
  943ffbe, one call shape over every master    1262 -> 1316  (+54)

The first is a bug being paid for. software_master delayed a fixed quarter
period and let its own instructions make up the rest of the bit, so the rate
on the wire followed the inliner; each bit now delays a low and a high phase
net of what that phase already spends. The DS3231 is a 400 kHz part and this
example asks for 100, so nothing here was out of spec - the bytes buy a rate
that no longer moves when the image does.

The second is a trade, and it is worth writing down which way it went. The
shared ladder is smaller than the two it replaced - run_transfer is 232 B
against read_regs 162 + write_regs 102 + stop 30 - but the call-site thunks
that marshal spans into it cost 116 B where the specialised ladders were
inlined, so a driver making both calls nets +54. Filed upstream with these
numbers rather than worked around here: which way the trade falls is a
property of the consumer's mix, and that is the library owner's call.

Size re-recorded in the two places that state it. Four presets green,
cross-mode identity held on both chips.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-08-23 02:17:44 +02:00
parent 176f78c2e9
commit bd604ec288
3 changed files with 3 additions and 3 deletions
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2
README.md
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@@ -27,7 +27,7 @@ cmake --build --preset attiny85-generated
Presets cover attiny85/atmega328p in both libavr modes (generated and Presets cover attiny85/atmega328p in both libavr modes (generated and
reflect); `ctest` runs the consteval battery over the BCD, the hours reflect); `ctest` runs the consteval battery over the BCD, the hours
register's two formats, the weekday and the alarm rate encodings, and holds register's two formats, the weekday and the alarm rate encodings, and holds
the example to its stated size — **1480 B** on the mega328P and **1238 B** the example to its stated size — **1480 B** on the mega328P and **1316 B**
on the tiny85, byte-identical across the two modes. The legacy on the tiny85, byte-identical across the two modes. The legacy
yazoalfa-based driver lives on the `master` branch. yazoalfa-based driver lives on the `master` branch.

2
libavr

Submodule libavr updated: 43fc4796c2...aec9955ad3

2
test/CMakeLists.txt
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@@ -5,7 +5,7 @@ libavr_format_test()
if(LIBAVR_MCU STREQUAL "atmega328p") if(LIBAVR_MCU STREQUAL "atmega328p")
libavr_size_claim_test(clock 1480) libavr_size_claim_test(clock 1480)
elseif(LIBAVR_MCU STREQUAL "attiny85") elseif(LIBAVR_MCU STREQUAL "attiny85")
libavr_size_claim_test(clock 1238) libavr_size_claim_test(clock 1316)
endif() endif()
# The README says the two modes emit the same image, and only a tree with # The README says the two modes emit the same image, and only a tree with