pico-mcp48x1

A C library for using a Microchip MCP48x1 digital-to-analogue converter (DAC) with the Raspberry Pi Pico

The MCP48x1 devices are single-channel DACs interfaced with SPI, and the family includes MCP4801 (8 bit), MCP4811 (10 bit) and MCP4821 (12 bit) DAC. The data sheet is available at Microchip's website (pdf).

Wiring

MCP48x1	Pico
(1) VDD	3V3(OUT)
(2) ~CS	Any GPIO
(3) SCK	SPIx SCK
(4) SDI	SPIx TX
(5) ~LDAC	GND
(6) ~SHDN	3V3
(7) VSS	GND
(8) VOUT	-

In addition, the data sheet (Fig 6-1, p.25) recommends adding a few capacitors:

• VDD (pin 1): Add decoupling capacitors to ground, 0.1 uF and 10 uF (tant.)
• Vout (pin 8): Add capacitor to ground, 1 uF (but see data sheet Section 6.3)

SPI max baud rate is 20 MHz.

Digital input to analog output

The analog output voltage is calculated according to this equation (Data sheet Eq. 4-1):

$$V_{out} = (V_{ref} \times D \times G) / 2^n$$

where

• D = DAC input code
• G = Gain selection (1x or 2x)
• n = Gain resolution (8, 10, or 12)
• Vref = Internal voltage reference, 2.048 V

The analog output, however, cannot be greater than the voltage supplied to the device ¹. This means that, when used with the Pico (3V3 supply), the maximum output is < 3.3 V. Thus, depending on the DAC resolution and the gain setting, the range of valid digital input codes and their corresponding output voltage is:

Model	n	Gain 1x		Gain 2x
		Input	Output	Input	Output
MCP4801	8	0~255	0~2.04 V	0~206	0~3.3 V
MCP4811	10	0~1023	0~2.05 V	0~825	0~3.3 V
MCP4821	12	0~4095	0~2.05 V	0~3300	0~3.3 V

Footnotes

1. The output swing is in fact 0.01 V to (VDD - 0.04), i.e. 0.01 to 3.26 V for a 3V3 supply (Data sheet Sec. 1). ↩