ST7789 Add offset constructor arg.

DRIVERS.md

@@ -400,14 +400,23 @@ below. An example file for the Pi Pico is in `color_setup/ssd7789.py`.
  * `width=240`
  * `disp_mode=0` By default the display chip operates in landscape mode. This
  arg enables portrait mode and other configurations. See below.
- * `init_spi=False` This optional arg enables flexible options in configuring
- the SPI bus. The default assumes exclusive access to the bus. In this normal
- case, `color_setup.py` initialises it and the settings will be left in place.
- If the bus is shared with devices which require different settings, a callback
- function should be passed. It will be called prior to each SPI bus write. The
- callback will receive a single arg being the SPI bus instance. It will
- typically be a one-liner or lambda initialising the bus. A minimal example is
- this function:
+ * `init_spi=False` For shared SPI bus applications. See note below.
+ * `offset=(0, 0)` This is intended for display hardware where the display
+ hardware's coordinate system is offset relative to the chip's RAM origin. An
+ `(x, y)` tuple (where `x` and `y` are positive integers) can offset this. In
+ practice, when other display hardware is supported, this doc will be amended
+ to specify the values to be used.
+
+### init_spi
+
+This optional arg enables flexible options in configuring the SPI bus. The
+default assumes exclusive access to the bus. In this normal case,
+`color_setup.py` initialises it and the settings will be left in place. If the
+bus is shared with devices which require different settings, a callback
+function should be passed. It will be called prior to each SPI bus write. The
+callback will receive a single arg being the SPI bus instance. It will
+typically be a one-liner or lambda initialising the bus. A minimal example is
+this function:
 ```python
 def spi_init(spi):
     spi.init(baudrate=30_000_000)

drivers/st7789/st7789_4bit.py

@@ -53,13 +53,15 @@ class ST7789(framebuf.FrameBuffer):
         return ((b & 0xf8) << 5 | (g & 0x1c) << 11 | (g & 0xe0) >> 5 | (r & 0xf8)) ^ 0xffff
 
     # rst and cs are active low, SPI is mode 0
-    def __init__(self, spi, cs, dc, rst, height=240, width=240, disp_mode=0, init_spi=False):
+    def __init__(self, spi, cs, dc, rst, height=240, width=240,
+                 disp_mode=0, init_spi=False, offset=(0, 0):
         self._spi = spi  # Clock cycle time for write 16ns 62.5MHz max (read is 150ns)
         self._rst = rst  # Pins
         self._dc = dc
         self._cs = cs
         self.height = height  # Required by Writer class
         self.width = width
+        self._offset = offset
         self._spi_init = init_spi  # Possible user callback
         self._lock = asyncio.Lock()
         mode = framebuf.GS4_HMSB  # Use 4bit greyscale.
@@ -138,8 +140,10 @@ class ST7789(framebuf.FrameBuffer):
         # Determine x and y start and end. Defaults for LANDSCAPE and PORTRAIT
         ys = 0  # y start
         ye = wht - 1  # y end
+        yoff = self._offset[1]
         xs = 0
         xe = wwd - 1
+        xoff = self._offset[0]
         if mode & PORTRAIT:
             if mode & REFLECT:
                 ys = rwd - wht
@@ -154,9 +158,13 @@ class ST7789(framebuf.FrameBuffer):
             if mode & USD:
                 ys = rht - wwd
                 ye = rht - 1
-        # Col address set
+        # Col address set. Add in any offset.
+        xs += xoff
+        xe += xoff
         self._wcd(b'\x2a', int.to_bytes(xs, 2, 'big') + int.to_bytes(xe, 2, 'big'))
         # Row address set
+        ys += yoff
+        ye += yoff
         self._wcd(b'\x2b', int.to_bytes(ys, 2, 'big') + int.to_bytes(ye, 2, 'big'))
 
     #@micropython.native # Made virtually no difference to timing.