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WORKING WITH A LITTLE EE

by George Martin

Start • The Basics • Getting the Routine Down • Debugging Your Reads and Writes • Sources and PDF

Last month, I promised that I'd look at how to chart with this data-logger program, but I ran into some problems. I thought I could use Microsoft's Chart command, but I got nowhere and Microsoft offered little (no) help. I started looking for other shareware charting stuff, but I haven't had any luck there yet either. And, I didn't want to just write about my troubles, so I switched gears.

Have you ever been asked to design an embedded system that needed to remember some data or settings even after the power cycles off? How about saving calibration constants or system configuration?

If you've ever faced these or similar system requirements, you've probably considered electrically erasable (EE) memory. Certainly, a disk or battery-backed-up SRAM would do the job, but not every system is equipped with those devices.

EE memory is expensive on a per bit basis (you find 32-Kb devices for less than $2 in low volume) as compared to other memory devices and technologies. But you typically aren't storing large amounts of data in this memory. Some CPU devices now come with their own EE memory.

In this article, I'd like to present the overall picture of how you can add an external EE memory device to your design. I'll show you how to interface the device, along with the software, to support system requirements.

Let's start with the hardware interfacing details. I'll present the most involved design approach. If your hardware offers more support, you can simplify what I describe by using the delete key. I'm using Microchip's 24C32P. Figure 1 shows you the pinout for this 8-pin integrated circuit.

Figure 1—The Microchip 24C32P offers electrically erasable memory. Each chip contains three address lines that can be uniquely identified through values included in the address.