While looking for a circuit for an Altera ByteBlasterMV, I found this one for an original BB:
The MV uses a 74HC244 instead of the LS one, and that means that the unused inputs need to be pulled high. But with the '244 running at 3.3v, for example, won't the parallel port cause latchup by pulling the inputs too high? Is it ok to connect the inputs using series resisters to avoid that? What value should I use?
Any other changes needed to make this work at 3v3 with an EPM3064?
Clifford Heath.
I've designed a similar unit using a 74HC244. In theory, there could be a problem, but mine works fine with 3.3V devices. I've sold a few of them, as well. I think that a 74AC244 might be better.
Leon
A LOT of target-powered parallel port programming dongles use the HC244 with 3V targets. I've used the Byteblaster you mentioned, as well as the Kanda AVR programmer (w/ 3V target).
If you look at a datasheet you'll see the input clamp current is 20mA, so you could work it out from that.
In practice, though, most of those dongles use a diode between target VCC and power, so the target can't actually sink any current at 3V, so the 5V that gets through the protection diodes on the HC244 probably just raises its supply rail above the target voltage. So really it'd be the target that would see higher voltages.
Anyway, I'd leave out the resistors. The last programming dongle I made used a 1k resistor for the return (between the buffer and the parallel port). Looked fine on a scope, didn't work. Shorted it out, worked fine.
-- Ben Jackson AD7GD http://www.ben.com/
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