I²C Simplifies Front-Panel Design

Interfacing a central processor to a user front panel is usually accomplished in one of two ways. The first method involves a power and a serial data link (typically RS-232), which are routed from the central processor to the front panel. At the front panel, a second processor handles all the I/O and updates the central processor via the serial link. In the second method, the central processor’s parallel I/O bus is extended via buffers and a ribbon cable to memory-mapped interface components located at the front panel.

The problem with the first approach is that the second processor adds significant software and hardware complexity. EMI also can present a significant problem, as the front-panel processor clock frequencies tend to find their way through the inevitable frontpanel openings.

The problem with the second approach is the relative bulk and routing difficulties of a ribbon cable. EMI can again be a problem as central processor clock frequencies find their way out to the front panel via the parallel I/O bus extension.

An alternative to these two approaches is to implement an I²C interface between the central processor and the front panel. Using I²C communications allows a simple two-wire serial interface and also eliminates the need for the second processor. I²Ccompatible I/O circuits provide a simple yet flexible hardware solution at the front panel. EMI concerns are minimized because no additional processor clock frequencies originate at the front panel.

The figure shows a generic centralprocessor to front-panel interface design implementing I²C. A PCF8584 I²C-bus controller allows bidirectional communications with the I²C bus. At initialization, the processor writes to the PCF8584 registers to set up the address and communications parameters. Following initialization, the I²C protocol is handled by the PCF8584. Standard I²C programmable bus speeds range from 1.5 to 90 kHz.

Connection to the front panel doesn’t require special cable or connectors. Standard RJ-11 jacks and a four-conductor modular phone cord work quite well. The 82B715 I²C bus extenders located at each end of the modular cable provide high capacitive drive, eliminating the need for special cables.

Available I²C devices number in the hundreds, so front-panel options can include LCDs, potentiometers, dot-matrix graphic displays, etc.