Interfacing startKIT to 5V Topic is solved

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Interfacing startKIT to 5V

Postby Ian » Sat Jan 14, 2017 6:44 am

Hi Guys,

1st post!

I've been playing around with the startKIT for sometime now and have finally decided to start interfacing it to some TTL level circuitry but am not having any luck with any output.

I'm feeding a 74AHCT125 5V on Vcc(pin 14) via a 5V(7805) power regulator from 12V. Triple checked the pinout and it all appears good. Vcc @ 4.99V. and grounded on pin 7.

- I've got pin 1(OE) on the 125 IC tied to ground.
- XS1_PORT1F is then connected directly to pin 2 on the 125 IC.
- probing pin 3 with a DSO

Checked the StartKITS's 1F port with a DSO and my 100Hz square wave is outputting correctly @ 3.36V, the 125 only has a tiny ~200mV voltage on pin 3 which echos the input on pin2. Shouldn't this be 5V? Tried a 2nd chip, same result.

Am I missing something really basic here?

Thanks in advance.

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Postby mon2 » Mon Jan 16, 2017 12:49 pm

Hi Ian. Welcome to the forum!

Your post sounds logical and should work as expected but can you confirm if you are loading down the pin # 3 of the 74AHCT125 device or is this pin free floating when you review with your DSO ?

Suggest to remove any connection on pin # 3 and review again. Also consider to just force pin # 2 (input) to be a logic high (3v3) by outputting a value of "1" using XMOS code and then measure the output value on pin # 3 which should be close to 5v0, unless being loaded down due to excessive current draw. Too much of a current draw can also damage the internal driver circuit of this component on pin # 3.

If you slow down your pulse train to say 1 Hz, what are your results ?

Please post your results.



Ian, please confirm that you have a common ground between the XMOS StartKit and the external level shifter IC (74AHCT125). This is required and may be the issue.

Anytime you have connect with external equipment like these ICs, you will need to have a common ground connection.
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Postby Ian » Mon Jan 16, 2017 1:03 pm

Thanks mon2 for the welcome!

I did manage to solve this. I was scratching my head thinking I was going crazy as I thoroughly checked the specs on the 125. Turns out it was my faulty breadboard.

I shifted the 74AHCT125 to another section of the breadboard, did a continuity test on all pins/rails. Tied any floating pins to GND powered on and hey presto! 100Hz 5v square wave.



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