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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 5304
Maxim > Design Support > Technical Documents > Application Notes > UARTs > APP 5304
Keywords: MAX3108, UART, RS232, RS485, SPI, I2C, WLP, wafer level package, FIFO
APPLICATION NOTE 5304
Interfacing to the MAX3108 UART
By: Micheal Scherrenburg
Mar 27, 2012
Abstract: This application note, the first in a series exploring the features of the MAX3108 high-
performance universal asynchronous receiver/transmitter (UART) in detail, explains the basic interface
between the MAX3108 and the controlling microprocessor. The application note briefly covers hardware
connectivity via 2, 4, or 6 pins; the implementation of 31 registers, which are accessible through a SPI or
an I²C interface; and three reset mechanisms. A more detailed explanation of SPI and I²C interfaces and
examples of pseudocode follows.
Introduction
The MAX3108 is a high-performance universal asynchronous receiver-transmitter (UART) in a wafer-
level package (WLP), ideal for low-power portable devices. Its advanced features range from individual
128-word transmit and receive FIFOs to extensive hardware-mediated flow control. However, before an
application can take advantage of these features, it must reliably communicate with the MAX3108 and its
31 internal registers. This application note covers the basics of MAX3108 communication. Software
conventions regarding the pseudocode appear in the Appendix.
Hardware Connectivity
Communication between the MAX3108 and the microprocessor (µP) occurs through 2, 4, or 6 pins. Two
dedicated pins, active-low IRQ and active-low RST, typically connect to microprocessor GPIO pins.
Active-low IRQ is an input pin to the microprocessor. As a dual-function signal, active-low IRQ both
indicates when a MAX3108 reset has been finished and notifies the microprocessor when certain
programmable UART events of note have occurred. Active-low RST is an output pin from the
microprocessor. It reliably forces a reset of the MAX3108 into a known state. Though it is possible to
implement a functional application without using these pins, it is preferable to connect them unless there
is a lack of GPIO pins.
The MAX3108 implements 31 registers, each a byte wide and accessible through either a SPI or an I²C
interface, which is chosen in the hardware. Because of its much greater bandwidth and simpler
implementation, the SPI interface is preferable. Tying the SPI/active-low I²C pin high implements a SPI
interface. Tying it low implements an I²C interface.
The MAX3108 SPI interface works best when connected directly to the SPI hardware engines commonly
available on microprocessors.
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