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Maxim > Design Support > Technical Documents > Application Notes > 1-Wire® Devices > APP 4024
Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 4024
Keywords: Serial, SPI, I2C, data rate, interface, comparison, DAC, microcontroller, 3-wire, 2-wire, 1-wire,
D/A, Digital to Analog, pullup resistors, noise, master, slave
APPLICATION NOTE 4024
SPI/I²C Bus Lines Control Multiple Peripherals
Jun 21, 2007
Abstract: This application note compares two common serial, digital interfaces used by most analog ICs:
SPI™ or 3-wire, and I²C or 2-wire. Each serial interface offers advantages or disadvantages for many
designs, depending on criteria such like needed data rate, space availability, and noise considerations.
This application note discusses the differences between these two serial interfaces and presents
examples to demonstrate each interface in detail.
Introduction
Although real-world signals will always be analog, today more and more analog ICs communicate
through digital interfaces. Serial interfaces communicate between a master, which provides the serial
clock, and a slave/peripheral. Today's SPI (3-wire) and I²C (2-wire) ports found on most microcontrollers
are popular means for transmitting and receiving data. Microcontrollers thus communicate over several
bus lines to control peripherals including analog-to-digital converters (ADCs), digital-to-analog converters
(DACs), smart batteries, port expanders, EEPROMs, and temperature sensors. Unlike data sent through
a parallel interface, serial data is transmitted with numerous bits in succession, usually over two, three,
or four data/timing lines. Although parallel interfaces offer speed, serial interfaces have the advantage of
much fewer control and data lines.
Serial Interface Basics
Serial interfaces are available in three varieties: 3-wire, 2-wire, and single-wire. This article focuses on
the 3- and 2-wire interfaces. The Serial Peripheral Interface (SPI), Queued Serial Peripheral Interface
(QSPI™), and MICROWIRE™ (or MICROWIRE PLUS™) standards communicate through 3-wire
interfaces. The inter-IC (I²C) and System Management Bus (SMBus™) standards communicate over 2-
wire interfaces. Both types of serial interfaces have their advantages and disadvantages. See Table 1
below.
The 3-Wire Interface
Three-wire interfaces use a chip-select line (active-low CS or active-low SS), a clock line (SCLK), and a
data input/master output line (called DIN or MOSI). These interfaces can also include a data
output/master input line (called DOUT or MISO), for which they are sometimes called 4-wire interfaces.
For simplicity, this article refers to both 3-wire and 4-wire interfaces as 3-wire.
Three-wire interfaces operate at higher clock frequencies and do not require any pullup resistors.
SPI/QSPI and MICROWIRE interfaces also feature full-duplex operation (data can be transmitted and
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