Devices that monitor and report system current can either provide an analog output that is proportional to the sensed current, or communicate the current to a host processor digitally. Use of a digital current sense amplifier is sometimes preferred because the integrated analog to digital conversion allows data to be sent directly to the host controller or processor. Interaction with a host processor requires a digital interface that can both allow sending and receiving of instructions as well as data. For current sensing applications, the most commonly used digital interfaces are I²C, SMBus, PMBus, and SPI. Each interface has different strengths and weaknesses; selecting the correct interface for a given application can allow better system optimization, faster system response time, and a reduction in software development time.

I²C, SMBus, and PMBus all utilize open drain clock and data lines that require pull-up resistors to an external power supply. SMBus and PMBus compatible devices feature an active low alert output to notify the host processor of fault conditions. I²C, SMBus, and PMBus devices commonly exist on the same physical bus; however, differences exist. Table 1-1 highlights some of the key differences among I²C, SMBus, and PMBUS devices.

I²C, SMBus, and PMBus devices can easily co-exist on the same bus since all logic low thresholds are at 0.4 V. Differences in the logic high thresholds usually are not an issue since the open-drain clock and data lines will go up to VDD when not held low. SMBus expanded on the frame work laid by I²C by adding a bus timeout requirement that prevents a device from holding data lines low for extended amounts of time. SMBus also clearly defined many different types of transaction protocols that support the transmission of data from the bit level to blocks of bytes. The SMBus and PMBus specifications are very similar because PMBus leverages the electrical characteristics and communications protocols as defined by the SMBus specification. PMBus incorporates the SMBus electrical specification, while also standardizing the address locations for common commands that are used in power systems. The address/command standardization allows one software driver to support many devices without the need to be completely rewritten to support new devices or devices from different manufacturers.