TXB0108 level shifter current-limiting question

[smindinvern]

Hi,

I'm fairly new to electronics, so forgive me if this is a stupid question. I'm looking to interface two systems, one of which is a 3.3V microcontroller, and the other a system running on an unknown voltage (almost guaranteed to be <5V though). For this I wanted to use a dual-supply level-shifter, and the TXB0108 looks like a good fit, especially with the breadboard-friendly breakout board. I need to be careful about controlling the input and output currents, though, and I'm a little unsure as to how to do that. My requirement is that the microcontroller side be <20mA, and the other side be <8mA. Can I just put a series resistor on each of the lines? As I understand it that would reduce the slew rate of the signal, which I definitely don't want. Or can I just put a resistor on VCC to reduce the amount of current that the level-shifter can sink? And according to ohm's law, wouldn't each of these solutions drop a certain amount of voltage over the resistor, leading to a voltage level less than VCC? So I guess in short, the question is, how do I limit the amount of current flowing through the data lines of a level-shifter?

Thanks in advance,
smindinvern

[adafruit]

why limit it?

[smindinvern]

I'm implementing an EJTAG probe. The EJTAG specification imposes an upper limit of 8mA for the off-chip data lines of the EJTAG port.

smindinvern

[adafruit]

You might need to read the TXB0108 datasheet to figure this out - we've never used it in that way

[smindinvern]

haha, ok, well, thanks anyway upon reviewing the EJTAG spec though, i think i can get away with just using resistors. I was afraid that the voltage drop across the resistor would mean the DUT would never see a logic high, but since the DUT is the current source, that shouldn't be a problem. So, false alarm I guess

Thanks again for the help,
smindinvern

[adafruit]

great, please let us know how it works - we'd love to know if it is good for JTAG!

[smindinvern]

Well, I tried using it with my EJTAG probe, and it doesn't seem to work. I'll have to do some more testing to find out why, but I know the probe works, as just connecting series resistors without the buffer works just fine. It could be that VCCA and VCCB are too close together. They're both about 3.3V (in this case). idk, more testing to come...

smindinvern