mcp4728: Update to better match printrbot revf requirements

Reported by @Grumps49.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -890,27 +890,28 @@
 #i2c_mcu: mcu
 #   The name of the micro-controller that the MCP4451 chip is
 #   connected to. The default is "mcu".
-#i2c_address:
+#i2c_address: 96
 #   The i2c address that the chip is using on the i2c bus. The default
-#   is zero.
+#   is 96.
 #channel_a:
 #channel_b:
 #channel_c:
 #channel_d:
 #   The value to statically set the given MCP4728 channel to. This is
 #   typically set to a number between 0.0 and 1.0 with 1.0 being the
-#   highest voltage and 0.0 being the lowest voltage. However, the
-#   range may be changed with the 'scale' parameter (see below). If a
-#   channel is not specified then it is left unconfigured.
+#   highest voltage (2.048V) and 0.0 being the lowest voltage. However,
+#   the range may be changed with the 'scale' parameter (see
+#   below). If a channel is not specified then it is left
+#   unconfigured.
 #scale:
 #   This parameter can be used to alter how the 'channel_x' parameters
 #   are interpreted. If provided, then the 'channel_x' parameters
 #   should be between 0.0 and 'scale'. This may be useful when the
 #   MCP4728 is used to set stepper voltage references. The 'scale' can
 #   be set to the equivalent stepper amperage if the MCP4728 were at
-#   its highest voltage, and then the 'channel_x' parameters can be
-#   specified using the desired amperage value for the stepper. The
-#   default is to not scale the 'channel_x' parameters.
+#   its highest voltage (2.048V), and then the 'channel_x' parameters
+#   can be specified using the desired amperage value for the
+#   stepper. The default is to not scale the 'channel_x' parameters.
 
 
 # Statically configured MCP4018 digipot connected via two gpio "bit

docs/Config_Changes.md

@@ -6,6 +6,10 @@ All dates in this document are approximate.
 
 # Changes
 
+20190726: The mcp4728 digital-to-analog code has changed. The default
+i2c_address is now 0x60 and the voltage reference is now relative to
+the mcp4728's internal 2.048 volt reference.
+
 20190710: The z_hop option was removed from the [firmware_retract]
 config section. The z_hop support was incomplete and could cause
 incorrect behavior with several common slicers.

klippy/extras/mcp4728.py

@@ -7,7 +7,7 @@ import bus
 
 class mcp4728:
     def __init__(self, config):
-        self.i2c = bus.MCU_I2C_from_config(config, default_addr=0)
+        self.i2c = bus.MCU_I2C_from_config(config, default_addr=0x60)
         scale = config.getfloat('scale', 1., above=0.)
         # Configure registers
         for i, name in enumerate('abcd'):
@@ -17,7 +17,7 @@ class mcp4728:
                 self.set_dac(i, int(val * 4095. / scale + .5))
     def set_dac(self, dac, value):
         self.i2c.i2c_write([0x40 | (dac << 1),
-                            (value >> 8) & 0x0f, value & 0xff])
+                            ((value >> 8) & 0x0f) | 0x80, value & 0xff])
 
 def load_config_prefix(config):
     return mcp4728(config)