Use a buffer. By Lee Davison.

You know when you say things like ..

Even I can't mess that up

 .. well that is usually just what happens next.

Long, long ago when TTL was king there was only one supply voltage, 5 volts. Everything in the land of logic was 5 volts, processors were 5 volts, memory was 5 volts, all the myriad of chips in your design were 5 volts. It was only when you wanted your logic to communicate with the outside world that you ever had to think about other voltages. Now there are all sorts of logic families and all sorts of supply voltages from less than a volt, for ultra low-power devices, upwards.

The EPM3064 used on the 8 Bit Baby is a 3.3 volt device with 5 volt tolerant I/O. What this means is that you put a little 3.3 volt regulator in next to it and forget about it. Until that is you do something like put an LED on an output pin.

The output, while tolerant of 5 volts, is clamped at a little over 3 volts by protection circuits built into the chip. The result is that an LED connected to the 5 volt supply has just under 2 volts across it even when it is suposed to be off and so it glows ever so dimly.

What I should have done was either connect the LED from the output to ground, modern logic is much better at sourcing current than TTL ever was, or have used a transistor as a buffer as in the diagram above. Either way the sense of the LED would be inverted, on when the output is high, but as long as we remember this it isn't a problem

Last page update: 3rd March, 2006.

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