In circuit programming SX 5V & 3V

jb1311

I want be able to program an SX while it is in circuit but the rest of the circuit can only tolerate 3 Volts. The SX communicates with the rest of the circuit via I2. Would it be safe to use a dual power supply (SX on 5 Volts and the other chips on 3 Volts) and tie the I2 pull up resisters to 3 Volts?

RobotWorkshop

Since the SX-key only talks to the SX chip via the OSC pins it shouldn't matter what voltage you are running the rest of the circuit at. It should only be a factor if you are using the original SX-Key since that needs 5V. In order to use that you would need the old Key-Ring adapter or roll your own to power the SX-Key instead of the target system supplying power. If you are using the USB version you should be ok.

Robert

jb1311

I have the serial programmer that is powered from the circuit. The sx key ring is an adapter that supplies 5v and passes the OSC1 & OSC2 and ground?

Zoot

You can also make an adapter with some header pins, a piece of perf board, and some wires. The idea is that you want to feed 5v to the SX-Key *only* from an external power supply, let the SX board/circuit be powered by it's 3v, with common ground and pass throughs on OSC1 and OSC2, sorta like the attached.




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dkemppai

Zoot said...
You can also make an adapter with some header pins, a piece of perf board, and some wires. The idea is that you want to feed 5v to the SX-Key *only* from an external power supply, let the SX board/circuit be powered by it's 3v, with common ground and pass throughs on OSC1 and OSC2, sorta like the attached.

Be Very careful with this setup! Heed the warning.

ALWAYS Unplug your key from your target before you unplug power to your key or your target with this setup.

ALWAYS Plug in your Key power AND Target power before you connect your key to your target.

This is only a problem when you SX chip is not running at the same 5 volt level as your target device.

Depending on how your serial key and target device power up and power down, you could cause your target (SX) to go into cmos latch up, and blow your SX-Key or your target SX. I blew 8 Keys, and half a dozen targets before I found the cause of this issue. I worked with very closely with parallax on this issue, and we found a way to resolve the problem on the USB-Key.

The safest thing is to use the USB Key, otherwise follow the sequences outlined above.

-Dan

P.M. Me if you wish a longer explanation...

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