MCP4725 D/A Converter

Distinct Library Functions and The Write Access Time

The best way to define the sample rate to send samples to MCP4725 is to use a timer hardware interrupt, as instance, the Timer1 Overflow Interrupt.
Considering a constant sample vector, it is possible to store the samples in the program memory, where there are more space, or keep them in RAM memory, if the application uses a small amount of data.
In some applications the samples are calculated in real time, but this implies a significant computational burden. As the Atmega328P is a simple fixed point microcontroller, only simple calculations can be executed for a given sample rate. In the presented example, the samples are calculated in real-time, implying a limited sample rate (2kHz).

Library or Function	ISR Code Example (Only the write code snippet in the MCP4725!)	Time Duration Pulse
Adafruit_MCP4725 Library	`ISR(TIMER1_OVF_vect) { : sei();//enable TWI interrupt recognition inside Timer 1 ISR PINB \|=0x01; //pulse for time measurement dac.setVoltage(sample, false); //send sample PINB \|=0x01; //toggle PB0 : }`	Fig. 2. The square pulse generated by the version 1 code during the sample writing into MCP4725
MCP4725 Library	`ISR(TIMER1_OVF_vect) { : sei();//enable TWI interrupt recognition inside Timer 1 ISR PINB \|=0x01; //pulse for time measurement MCP.setValue(sample); //send sample PINB \|=0x01; //toggle PB0 : }`	Fig. 3. The square pulse generated by the version 2 code during the sample writing into MCP4725
Wire Library	`ISR(TIMER1_OVF_vect) { : sei();//enable TWI interrupt recognition inside Timer 1 ISR PINB \|=0x01; //pulse for time measurement DA_Wire_Write(sample); //send sample PINB \|=0x01; //toggle PB0 : } void DA_Wire_Write(int sample) { Wire.beginTransmission(MCP4725_address); Wire.write((sample & 0x0FFF) >> 8); //send MSB:command+upper nibble Wire.write(sample); //send LSB Wire.endTransmission(); }`	Fig. 4. The square pulse generated by the version 3 code during the sample writing into MCP4725
Direct Register Access Model	ISR(TIMER1_OVF_vect) { : PINB \|=0x01; //pulse for time measurement error = DA_Fast_Write(sample); //send sample PINB \|=0x01; //toggle PB0 : } int DA_Fast_Write(int sample) //direct register Access { sample &= 0x0FFF; //saturation in case the argument > 4095 uint8_t LSB = (uint8_t)sample; //get LSB of data uint8_t MSB = (uint8_t)(sample>>8); //get MSB of data //D7-D4=0000 => Power Down Normal Mode and Fast Mode command TWCR \|= 0xA0 ; //set TWI START Condition Bit while (!(TWCR & 0x80)); // wait until the start condition has been sent if ((TWSR & 0xF8) != 0x08) return 1; //mask the prescaler bits => status TWCR &= 0x5F ; //clear TWSTA TWDR = MCP4725_Address<<1 //SLA+W = Slave address+write bit command (0) TWCR \|= 0x80; // start transmission of address while (!(TWCR & 0x80)); // wait until SLA+W has been transmitted if ((TWSR & 0xF8) != 0x18) return 2; //mask the prescaler bits => status TWDR = MSB; //load MSB data do TWDR TWCR \|=0x80; // start transmission of MSB data while (!(TWCR & 0x80)); // wait until MSB has been transmitted if ((TWSR & 0xF8) != 0x28) return 3; //mask the prescaler bits => status TWDR = LSB; //load LSB data do TWDR TWCR \|= 0x80; //start transmission of LSB data while (!(TWCR & 0x80)); // wait until LSB has been transmitted if ((TWSR & 0xF8) != 0x28) return 4; //mask the prescaler bits => status TWCR \|=0x90; // set TWI STOP Condition Bit return 0; // return zero if writing process was succeeded }	Fig. 5. The square pulse generated by the version 4 code during the sample writing into MCP4725

Notes

The square pulses present in the oscilloscope figures are relative to the signal on pin PB0 (Arduino UNO), and were obtained considering a sampling rate of 2kHz and an I2C clock of 400kHz. The pulse widths are proportional to the duration of the process of writing a sample to the MCP4725. See the instructions to set and clear (toggle) the PB0 bit in the code snippet.
The libraries "Adafruit_MCP4725.h", "MCP4725.h" and "Wire.h" use the hardware interrupt of TWI, then if a MCP4725 write function is included in an ISR (Timer 1 Overflow ISR, as instance), it is necessary to enable the global interrupt flag inside of the ISR, otherwise the code won't work. The Atmega328P disables the global interrupt flag when an interrupt is acknowledged, as default.
The proposed function presents the lowest write cycle time among the presented options. In this case, it is not necessary to enable the global interrupt flag within the ISR.
The default clock speed for the TWI is 100kHz in the libraries "MCP4725.h" and "Wire.h", so change it to 400kHz, if it is the case.
The proposed function uses the "Fast Mode Write Command" of the MCP4725 (see MCP4725 Datasheet). Some modifications are required to perform other write or read commands.
Links to the complete codes are provided below.

Waveform Generation - Sampling Rate of 2kHz

Assuming a 50Hz sine waveform, any of the writing options on the MCP4725 can be used for a 2kHz sampling rate. The sampling rate can be higher if the sinusoid samples are previously calculated and inserted into a vector in memory. In the presented example, the samples are calculated by the ISR.
All codes generate the same sine waveform shown in Fig. 6.
The square pulses shown in Figs. 7 to 10 represent the time duration of the ISR (calculations plus MCP4725 writing). The sample calculation takes around 170 microseconds to be executed (CPU clock at 16MHz).

Fig.6. Sine waveform generated by the code in the MCP4725 output

Result using the Adrafruit_MCP4725 Library Fig.7. Square pulse used to measure the time duration of the ISR - Version 1 Code	Result using the MCP4725 Library Fig.8. Square pulse used to measure the time duration of the ISR - Version 2 Code
Result using the Wire Library Fig.9. Square pulse used to measure the time duration of the ISR - Version 3 Code	Result using TWI Direct Register Access Fig.10. Square pulse used to measure the time duration of the ISR - Version 4 Code