Make BCDDigits chainable

adafruit_max7219/bcddigits.py

@@ -26,14 +26,15 @@
 _DISPLAYTEST = const(15)
 
 
-class BCDDigits(max7219.MAX7219):
+class BCDDigits(max7219.ChainableMAX7219):
     """
     Basic support for display on a 7-Segment BCD display controlled
     by a Max7219 chip using SPI.
 
     :param ~busio.SPI spi: an spi busio or spi bitbangio object
     :param ~digitalio.DigitalInOut cs: digital in/out to use as chip select signal
-    :param int nDigits: number of led 7-segment digits; default 1; max 8
+    :param int nDigits: number of led 7-segment digits; default 1; max 8 if one
+        MAX7219; max 16 if two MAX7219 are chained together
     """
 
     def __init__(self, spi: busio.SPI, cs: digitalio.DigitalInOut, nDigits: int = 1):
@@ -46,10 +47,10 @@ def init_display(self) -> None:
             (_SHUTDOWN, 0),
             (_DISPLAYTEST, 0),
             (_SCANLIMIT, 7),
-            (_DECODEMODE, (2**self._ndigits) - 1),
-            (_SHUTDOWN, 1),
         ):
             self.write_cmd(cmd, data)
+        self.write_cmd(_DECODEMODE, (2**self._ndigits) - 1, data_overflow=True)
+        self.write_cmd(_SHUTDOWN, 1)
 
         self.clear_all()
         self.show()
@@ -61,6 +62,15 @@ def set_digit(self, dpos: int, value: int) -> None:
         :param int dpos: the digit position; zero-based
         :param int value: integer ranging from 0->15
         """
+
+        if not 0 <= dpos < self.chain_length * 8:
+            raise ValueError(
+                "Digit with index {0} cannot be set with only {1} MAX7219(s)".format(
+                    dpos,
+                    self.chain_length,
+                )
+            )
+
         dpos = self._ndigits - dpos - 1
         for i in range(4):
             # print('digit {} pixel {} value {}'.format(dpos,i+4,v & 0x01))

adafruit_max7219/max7219.py

@@ -193,18 +193,25 @@ def __init__(
         self._buffer = bytearray(self.chain_length * 8)
         self.framebuf = framebuf.FrameBuffer1(self._buffer, self.chain_length * 8, 8)
 
-    def write_cmd(self, cmd: int, data: int) -> None:
+    # pylint: disable=arguments-differ
+    def write_cmd(self, cmd: int, data: int, *, data_overflow: bool = False) -> None:
         """
         Writes a command to spi device.
 
         :param int cmd: register address to write data to
         :param int data: data to be written to commanded register
+        :param bool data_overflow: if data is larger than byte, write byte
+            by byte in big-endian order
         """
         # print('cmd {} data {}'.format(cmd,data))
         self._chip_select.value = False
         with self._spi_device as my_spi_device:
-            for _ in range(self.chain_length):
-                my_spi_device.write(bytearray([cmd, data]))
+            for byte_num in range(1, self.chain_length + 1):
+                byte_data = data
+                if data_overflow:
+                    byte_data = data >> 8 * (self.chain_length - byte_num)
+                    byte_data &= 0xFF
+                my_spi_device.write(bytearray([cmd, byte_data]))
 
     def show(self) -> None:
         """