Fix partial update coordinates

The addresses are little endian. CircuitPython 8.1.0+ is needed for this to work.

Author: tannewt

adafruit_ssd1681.py

@@ -69,6 +69,8 @@ def __init__(self, bus: displayio.FourWire, **kwargs) -> None:
         start_sequence[21] = (width - 1) & 0xFF
         start_sequence[22] = ((width >> 8) - 1) & 0xFF
 
+        # RAM is actually only 200 bits high but we use 296 to match the 9 bits
+        # (and therefore two bytes) used to address height.
         super().__init__(
             bus,
             start_sequence,
@@ -86,4 +88,5 @@ def __init__(self, bus: displayio.FourWire, **kwargs) -> None:
             set_current_row_command=0x4F,
             refresh_display_command=0x20,
             always_toggle_chip_select=True,
+            address_little_endian=True
         )

examples/ssd1681_four_corners.py

@@ -0,0 +1,85 @@
+# SPDX-FileCopyrightText: Copyright (c) 2023 Scott Shawcroft for Adafruit Industries
+# SPDX-FileCopyrightText: Copyright (c) 2021 Melissa LeBlanc-Williams for Adafruit Industries
+#
+# SPDX-License-Identifier: Unlicense
+
+"""Test partial updates by moving a simple label around each of the four corners."""
+
+# The top left is 0 or 4, top right is 1 or 5, bottom left is 2 or 6 and bottom
+# right is 3 or 7. (It does % 8 for the label and % 4 for position.)
+# pylint: disable=no-member
+
+import time
+import board
+import busio
+import displayio
+import digitalio
+import terminalio
+import adafruit_ssd1681
+
+displayio.release_displays()
+
+# This pinout works on a Feather RP2040 EPD and may need to be altered for other
+# boards. The 200x200px SSD1681 display with "SYX 2024" on the ribbon cable (tricolor) or
+# "SYX 2118" (bicolor) is connected directly via the ribbon cable.
+spi = busio.SPI(board.EPD_SCK, board.EPD_MOSI)  # Uses SCK and MOSI
+epd_cs = board.EPD_CS
+epd_dc = board.EPD_DC
+epd_reset = board.EPD_RESET
+epd_busy = board.EPD_BUSY
+
+display_bus = displayio.FourWire(
+  spi, 
+  command=epd_dc, 
+  chip_select=epd_cs, 
+  reset=epd_reset, 
+  baudrate=1000000
+)
+display = adafruit_ssd1681.SSD1681(
+    display_bus,
+    width=200,
+    height=200,
+    busy_pin=epd_busy,
+    highlight_color=0xFF0000,
+    rotation=180,
+    seconds_per_frame=15
+)
+
+# Make the display context
+main_group = displayio.Group()
+display.show(main_group)
+
+palette = displayio.Palette(2)
+palette[0] = 0x000000
+palette[1] = 0xffffff
+
+zero_glyph = terminalio.FONT.get_glyph(ord('0'))
+
+padding = max(zero_glyph.height, zero_glyph.width) + 1
+label = displayio.TileGrid(terminalio.FONT.bitmap, pixel_shader=palette, tile_width=zero_glyph.width, tile_height=zero_glyph.height)
+main_group.append(label)
+
+# Number each of the 4 corners
+i = 0
+while True:
+    if i % 2 == 0:
+      label.x = padding
+    else:
+      label.x = display.width - padding - zero_glyph.width
+    if (i % 4) // 2 == 0:
+      label.y = padding
+    else:
+      label.y = display.height - padding - zero_glyph.height
+
+    label[0] = zero_glyph.tile_index + i
+
+    # update text property to change the text showing on the display
+    sleep_time = display.time_to_refresh
+    print(f"Sleeping {sleep_time} seconds")
+    time.sleep(sleep_time + 0.1)
+
+    print(f"{i % 8} @ ({label.x}, {label.y})")
+    display.refresh()
+
+    i += 1
+    i %= 8