This application note describes the interface between an HC705J1A and a multiplying digital-to-analog converter (MDAC) to digitally control the amplification or attenuation of an operational amplifier circuit. The MDAC used is Analog Device’s DAC8043, a 12-bit, 8-pin serial device. By using the MDAC, a mechanical potentiometer can be replaced by a more reliable and robust solution. The microcontroller unit (MCU) interface must be able to “talk” to the DAC8043 using a serial communication link. The serial peripheral interface (SPI) is one of the most widely used serial transmission methods for communication between an MCU and a peripheral, although not all HC05 Family members have SPI modules. An HC05 MCU without an SPI must interface with the DAC8043 using a software input/output (I/O) driver. This method uses software bit programming to communicate with the DAC8043. Even though it is not as efficient as the hardware SPI method, it provides the MCU with a means to send data to the DAC8043.