fix(nrf): move the panel SPI off SPIM3 onto SPIM2 (anomalies 195/198)

platformio.ini

@@ -51,6 +51,39 @@ build_flags =
   ; -DOPENDISPLAY_ZLIB_WINDOW_BITS=15
   ; nRF keeps the zlib history window in static storage for deterministic allocation.
   -DOPENDISPLAY_ZLIB_USE_HEAP_WINDOW=0
+  ; Move the panel SPI off SPIM3 and onto SPIM2.
+  ;
+  ; The Adafruit core binds the global `SPI` object to an instance chosen by this
+  ; macro (framework SPI.cpp): 0 (its default) gives SPIM3, 1 gives SPIM2. Only
+  ; `SPI` matters here -- bb_epaper drives the panel through that global
+  ; (arduino_io.inl `SPI.begin()`), and nothing in this firmware ever calls
+  ; SPI1.begin(), so the instance this hands to SPI1 is left uninitialised.
+  ;
+  ; Why: SPIM3 is the only nRF52840 SPIM instance carrying anomalies 195 (keeps
+  ; drawing current after disable; the nrfx workaround pokes 0x4002F004 on every
+  ; uninit and never undoes it on re-init) and 198 (transmit data may be corrupted
+  ; when another bus master touches the TX buffer's RAM block mid-transfer -- and
+  ; the Arduino per-byte path makes that buffer a one-byte STACK local, contended
+  ; by SoftDevice interrupts on every byte). Both workarounds in nrfx_spim.c are
+  ; gated on `p_spim == NRF_SPIM3`, and 198's is compiled out entirely here because
+  ; NRFX_SPIM3_NRF52840_ANOMALY_198_WORKAROUND_ENABLED is never defined. Moving to
+  ; SPIM2 makes both inapplicable rather than worked around.
+  ;
+  ; Free of cost at our clock: SPIM2 tops out at 8 MHz and bbepInitIO already asks
+  ; for exactly 8000000 (display_service.cpp), which setClockDivider maps to
+  ; NRF_SPIM_FREQ_8M on either instance. It does forfeit the 16/32 MHz that only
+  ; SPIM3 offers -- deliberately, since raising the panel clock increases the DMA
+  ; bus pressure this flag exists to avoid.
+  ;
+  ; No pin change: the instance is swapped, PIN_SPI_* are not.
+  ; No I2C conflict: Wire is TWIM0 and Wire1 is TWIM1 (Wire_nRF52.cpp), while
+  ; SPIM2's block is shared only with SPIS2/SPI2, neither of which is used.
+  ;
+  ; VERIFY AFTER ANY PLATFORM UPDATE. This macro is consumed by the framework's
+  ; SPI.cpp, not by our sources, so it only works if -D reaches the framework
+  ; build. If it silently stops doing so we are back on SPIM3 with no error. Check
+  ; with: pio run -e nrf52840custom -v 2>&1 | grep -c 'SPI_32MHZ_INTERFACE=1.*SPI\.cpp'
+  -DSPI_32MHZ_INTERFACE=1
 platform = https://github.com/maxgerhardt/platform-nordicnrf52
 framework = arduino
 board_build.variants_dir = variants