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level shifter boards not working correctly

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perigalacticon:

--- Quote from: digistump on January 30, 2017, 12:57:55 pm ---The TXB0108 used on the level shifter requires a load to produce a proper voltage - so if you put an LED, connect it to another IC, etc on a pin it should be correct and work right - but just a voltmeter will be misleading.

--- End quote ---

How much current can the pins source?  Because pins 0, 2, 13, and all the analog in pins can only source 3mA from the Due.

perigalacticon:
Hey this is an important question, I need to know the max voltage and current limits for your shield for all of the pins please.

PeterF:
Simply going from the datasheet, the TXB0108 chip that is doing the shifting is speced as a maximum input voltage of 6.5v, and a continuous output of +/- 50mA.

perigalacticon:
I tried a std. red LED between pin 10 and ground and it just stayed lit brightly at 2V although the program was commanding switching between high and low at 1 Hz. 

Then I tried a set of resistors.  I found if they weren't making perfect contact the voltage would be 2.1V.  The pin voltage increased as the resistance increased from 100 ohms up to 100k ohms.  I attached the plot of measured voltage vs. resistance.  It seems you need even more than 100k resistance to get 5V output, as I only got 4.3V with 100k ohms. 

I suppose this might work with high-impedance shields, I have been hesitant to try because I don't want to fry anything.  I need to know is this expected with your level converter shield? 

I also read this chip doesn't work with I2C so that eliminates a lot of shields I want to use it with is this true?

PeterF:
I haven't tried it myself with I2C, but that is my understanding... that because pullups are needed, it confuses the auto-direction sensing, hence I2C doesn't work (well). It is also referenced in the app note linked below:


--- Quote ---This architecture is designed to exclusively be connected and interfaced with a push-pull CMOS driver and is capable of driving a capacitive or high impedance loads in applications such as Secure Digital (SD) or Serial Peripheral Interface (SPI). The TXB010x devices are not intended for use in open-drain applications. For applications such as I2C where there is a need to connect and interface with an open-drain driver, TI offers TXS-type (i.e., "S" for Switch-type) translators.
--- End quote ---

It being 4.3v shouldn't be a issue for most 5v logic stuff, as I believe most parts consider something around 3v to be logic high (i.e. 1/2 of VCC +  a margin). The app note explaining how the parts work may help, especially the impedance table. These parts are intended for use with hi-impedance parts, specifically, for logic chip to logic chip translation. So having them drive parts that don't have that impedance (i.e. loads such as leds) is problematic. 40-50k seems to be the recommended impedance for 5v to 3.3v.

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