Circuit Notebook 79 - Using the I²C Bus Extender, Philips 82B715

Philips Semiconductors invented I²C in the late 1970s to provide a standardised interface to support a growing number of general purpose and application-specific ICs used in consumer electronics. I²C is a low rate serial bus initially designed to operate at 100 kHz (standard mode) and later upgraded to handle 400 kHz (fast mode) protocols and more recently to 3400 kHz. Two wires are required, one for clock and one for data. The beauty of I²C is that it offers a simple way for devices within a system to talk to one another. Several devices can be connected to the bus and addressed individually.

I²C was used in the 'Remote Control Modular ATV Station' described in the BATC publication 'An Introduction to Amateur Television' by Mike Wooding and Trevor Brown. This system forms the basis of a number of ATV Repeater Stations in the UK (including my local repeater GB3TM).

A limitation of the I²C system is the maximum length of cable that can be used. The I²C bus capacitance limit of 400 pF restricts practical communication distances to a few metres. This is not a problem when devices are located within the same piece of equipment.

Remote Receiver Tuning

At the recent BATC Convention I purchased from G1MFG a 13 cms ENG Receiver and its associated Receiver Control Panel with LCD frequency display. The receiver is controlled from the control panel through a two wire I²C bus. My intention is to mount the aerial and the receiver (in a box) on the mast and tune it remotely from the shack. The cable run is about 20 metres, which is too long for a normal I²C connection.

The solution is provided by the Philips I²C Bus Extender 82B715P (Farnell 550-258). Using one 82B715 at each end of longer cables reduces the cable loading capacitance on the I²C bus by a factor of about 10 times and may allow the use of low cost general purpose wiring to extend bus lengths.

In normal use, pull-up resistors are required on the open collector outputs of each device. If two Bus Extenders are permanently connected, (as in my case) the circuit can be configured with only one pull-up resistor on each line of the buffered bus. A method of calculating the resistor value is given in the 82B715 data sheet. The value for my system is 330 ohm. This will allow for a total cable capacitance of about 3000 pF, so for the cable I will be using, (capacitance = 110 pF / metre) it provides a maximum length of about 27 metres.

The general arrangement is shown in Fig. 1. Mounted on the mast will be the aerial, and in a waterproof box, the receiver module, the I²C bus extender and its 5 volt supply regulator (both decoupling capacitors are 220 nF). R1 and R2 are the pull-up resistors for the extended I²C bus. Separate connecting cables are provided for the 13-volt supply and the I²C bus with coax cables for video and audio.

In the shack are the 13-volt power supply, the control panel and its I²C bus extender. The 5-volt supply for this is taken from pin14 of the PIC socket; the control panel has an on-board 78L05 regulator with spare capacity (but check with G1MFG).

At the present time, DIL switches set the frequency of the associated G1MFG 2.4 GHz transmitter. This could also be remotely tuned from the control panel using revised software and a duplicate set of I²C bus extenders.

References

'An Introduction to Amateur Television', Mike Wooding G6IQM and Trevor Brown G8CJS, B.A.T.C, ISBN 0-9513779-2-2

I²C bus extender, Philips Semiconductors, Data Sheet 82B715, Jan. 1998

2.4 GHz ENG Receiver, G1MFG.com, http://www.G1MFG.com

Figures

Fig.1.Tuneable masthead receiver using the I²C bus extender