XU4 shifter shield Pullup/Pulldown hints

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qxtgo
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XU4 shifter shield Pullup/Pulldown hints

Unread post by qxtgo » Mon Nov 06, 2017 4:35 am

Hello HK

discovered a weird behavior when using a port from the shifter shield as digital output with external pulldown or load below ~5K.
Using any port (Images are from 209) with a pulldown the same: High level rings with ~ 3MHz

This begins with Pulldown < ~ 5K:
pulldown.3k33.png
pulldown.3k33.png (13.81 KiB) Viewed 624 times
with 10K not seen :
pulldown.6k66.png
pulldown.6k66.png (12.4 KiB) Viewed 624 times
The ringing signal:
pulldown.error.signal.png
pulldown.error.signal.png (15.72 KiB) Viewed 624 times
Reading the Data Sheet of the TXS0108E
(TI, SCES642D) 8.3.5 Pull-up or Pull-down Resistors on I/O Lines
they talk about internal RpuB of ~4K, which could explain the ringing with ~4k load.

Tested with a potentiometer, the ringing starts exactly at 4.4K load.


Found an AN regarding this: http://www.ti.com/lit/an/scea054/scea054.pdf
1.3.2 Conclusion writes not to use Pullups/Downs below 50 K with TXB Devices.


Perhaps you could/should add this information to your wiki/Product page.

Br
Tom

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odroid
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Re: XU4 shifter shield Pullup/Pulldown hints

Unread post by odroid » Tue Nov 07, 2017 3:36 pm

Really appreciate your valuable information with detail tests.
We've updated the WiKi about the 50Kohm story.
https://wiki.odroid.com/accessory/add-o ... ift_shield

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