Interfacing ATMega32 SPI to MCP4922 Dual 12-bit Digital To Analog Converter

Microchip MCP492X SPI Digital To Analog Converter

MCP492X is a 12-bit digital to analog converter (DAC) with SPI interface. MCP4921 is a single output DAC while MCP4922 has two configurable DAC. This SPI DAC could clock up to 20 MHz.

A sample of this programming example

In this post I use MCP4922 I possess. I bought this chip from Futurlec on-line store a few years ago. It comes with an easy to prototype 14-bit DIP.

MCP4922 14-pin DIP Package

MCP4922 14-pin DIP pin diagram

These are the pins description of this IC:

1. VDD  is positive supply voltage for internal device working (2.7 V to 5.5 V).
2. NC - No Connection
3. CS - Chip Select Input
4. SCK - Serial Clock Input
5. SDI - Serial Data Input
6. NC - No Connection
7. NC - No Connection
8. LDAC - is active low. It transfers DAC setting from serial latches to output latches.
9. SHDN - Hardware shut down input
10. VOUTB - DAC_B Output
11. VREFB - DAC_B Voltage Input (AVSS To VDD)
12. AVSS - Analog ground
13. VREFA - DAC_A Voltage Input (AVSS to VDD)
14. VOUTA - DAC_A Output

The data format transfers to this device is 16-bit wide. One upper nibble is the DAC configuration bits while the remaining 12-bit is the DAC output value. The output value to DAC is up 4096 maximum.

DAC output voltage - VOUT lists as follow:

Where,

• VREF is DAC voltage reference input. It could be DAC_A or DAC_B
• G is output voltage gain, 1X or 2X.
• DN is 12-bit DAC data input value
• 2^N is the DAC resolution. In this case, the resolution is 12-bit yield the decimal value to 2^12 = 4046

The 16-bit SPI data representation lists below.

MCP4922 Write Command Register

Each bit functions as below.

• Bit 15 - A/B - Write '0' for DAC_A, otherwise DAC_B
• Bit 14 - BUF - Write '0' for Unbuffered, otherwise Buffered.
• Bit 13 - GA - Write '0' 1x Gain, otherwise 2x Gain.
• Bit 12 - SHDN - Write '0' to disable output buffer, otherwise enable the output buffer.
• Bit 11:0 - D11:D0 - 12-bit digital data DAC output ranges from 0 to 4095 (4096 in total).

Data transfer to this device is 16-bit, we need to send 8-bit twice with only on latching pulse.

Writing the command to MCP4922. External wiring is not include here.

Interfacing And Programming With Atmel Studio 7 In C

In this example, I use both output DAC. Both DAC outputs the maximum 12-bit value, 0xFFF or 4096. With 2X gain, DAC A generate the output voltage nearest to 10 V. Similarly, using 1X gain, DAC B generate output the voltage nearest to 5 V. The sample of this program lists below. The reference voltage for both DAC is 5 V.

A sample of this programming example

C source code:

Schematic Diagram

Schematic Diagram for simulation 

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