Merge pull request #129 from mattheww/2024-02_gpio-order

Non-blocking display: change the order of GPIO writes

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Fix: non-blocking display on micro:bit V2 could spuriously light LEDs briefly
 - Fix the `blocking::Display::set_refresh_rate` calculation for the micro:bit V2
 - Double the non-blocking display refresh frequency for the micro:bit V2
 - Fix faulty doc test in `blocking.rs`

microbit-common/src/display/nonblocking/control.rs

@@ -128,19 +128,28 @@ impl DisplayControl for MicrobitGpio {
  let rows_to_set = 1 << pins::P0_ROWS[row];
  let rows_to_clear = P0_ROW_BITS ^ rows_to_set;
 
- let cols_to_clear = column_pins(p0cols, &pins::P0_COLS);
- let cols_to_set = P0_COL_BITS ^ cols_to_clear;
-
- p0.outset.write(|w| w.bits(rows_to_set | cols_to_set));
- p0.outclr.write(|w| w.bits(rows_to_clear | cols_to_clear));
+ #[cfg(feature = "v1")]
+ {
+ let cols_to_clear = column_pins(p0cols, &pins::P0_COLS);
+ let cols_to_set = P0_COL_BITS ^ cols_to_clear;
+ p0.outset.write(|w| w.bits(rows_to_set | cols_to_set));
+ p0.outclr.write(|w| w.bits(rows_to_clear | cols_to_clear));
+ }
 
  #[cfg(feature = "v2")]
  {
  let p1 = &*P1::ptr();
- let cols_to_clear = column_pins(p1cols, &pins::P1_COLS);
- let cols_to_set = P1_COL_BITS ^ cols_to_clear;
- p1.outset.write(|w| w.bits(cols_to_set));
- p1.outclr.write(|w| w.bits(cols_to_clear));
+ let p0_cols_to_clear = column_pins(p0cols, &pins::P0_COLS);
+ let p0_cols_to_set = P0_COL_BITS ^ p0_cols_to_clear;
+ let p1_cols_to_clear = column_pins(p1cols, &pins::P1_COLS);
+ let p1_cols_to_set = P1_COL_BITS ^ p1_cols_to_clear;
+ // We do the row-clearing write first and the row-setting write last, so that
+ // intermediate states never light LEDs which aren't lit in either the old or new state.
+ p0.outclr
+ .write(|w| w.bits(rows_to_clear | p0_cols_to_clear));
+ p1.outset.write(|w| w.bits(p1_cols_to_set));
+ p1.outclr.write(|w| w.bits(p1_cols_to_clear));
+ p0.outset.write(|w| w.bits(rows_to_set | p0_cols_to_set));
  }
  }
  }