ADC and DAC Interfacing with a Microcontroller

ADC interfacing

• On-chip ADC (PIC18F): 10-bit, multiplexed analog pins. Steps: set pin as analog, select channel, set Vref and acquisition time, start conversion (GO/DONE), wait for completion, read ADRESH:ADRESL.
• External ADC: (e.g. MCP3208 over SPI) used when higher resolution or more channels are needed.
• Digital value: $D = (V_{in}/V_{ref}) \times (2^n - 1)$.

DAC interfacing

• Parallel DAC (DAC0808) connected to a port, or serial DAC (MCP4921 on SPI, MCP4725 on I2C).
• Microcontroller writes the code; output $V_{out} = V_{ref}\cdot D/2^n$; an op-amp buffers it.
• Uses: waveform generation, motor speed, calibration.

SPI vs I2C

Point	SPI	I2C
Lines	4 (MOSI, MISO, SCK, CS)	2 (SDA, SCL)
Speed	Faster (10 MHz or more)	100 kHz, 400 kHz, 3.4 MHz
Addressing	Chip select per slave	7-bit address in protocol
Duplex	Full duplex	Half duplex
Devices	Few (more CS lines)	Many on one bus
Complexity	Simple hardware	Protocol overhead, ACK
Use	ADC, SD card, display	EEPROM, RTC, sensors

Exam tip

Explain the ADC steps on PIC18F and finish with the SPI vs I2C table.