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usb8451 open_drain DO 3.8V i.s.o. 5V

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Hey,

we want to use the usb8451 device for DO 5V level. (open_drain)

We only get 3.8V.

 

How to ?

 

regards,

Koen.

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Do you use external pull-up resistors ?

See this discussion for more details.

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Hello,

i'm using an external pull up resistor of 10kOhm.

I tried just between +5V of 8451 and P0.7 and also between an external 5V power supply and P0.7 (even with GND of power supply connected to one of the GNDs of 8451).

 

Always obtaining that 3.8V.

 

regards,

Koen.

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Hi Koen,

 

I will try to answer your questions and provide a solution to your problem, can you tell me a bit more about your application? If I understand correctly, you're using a USB 8541 but you're only able to read an output of 3.8V, both from the digital output ports and from the 5V source built into the device, is this correct?

 

As per my research, the external +5 V is not a true 5 V signal. This is VUSB taken through a current limiting circuit so it cannot be considered true +5 V. It may be that your USB is not outputting the full 5V that you desire.

 

I hope to hear back from you soon,

 

Jake

 

Applications Engineer

National Instruments

Jake A

Applications Engineer
National Instruments UK and Ireland
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Hi Jake,

thx for Your input.

It can be that Vusb is not 5V but 3.8V. That I will measure some day if I work on that subject and that PC again.

But what do You mean by Vcc(5V) built into the device? Our device, or the usb8451? If I remember it well, I measured correct 5V on the usb8451 5V output (where the pull up resistor of 10k was pulled up to)! But still measuring 3.8V i.s.o. 5V on the digital output (High). I also tried with an external 5V voltage supply for the pull up with 10k without success.

 

regards,

Koen.

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Solution
Accepted by topic author kdCMC

 

I was referring to the 5V output on the USB 8451, but if that output is the expected 5V then it's likely that the USB port is not the source of the issue. Have you tried using a different pull up resistor, possibly one of lower resistance?

 

I can link you to both the USB 8451 manual and the OEM specifications for the device, you may be able to find more information there.

 

Regards,

Jake A

Applications Engineer
National Instruments UK and Ireland
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Hello,

i had also opened an SR and received just a hint to try out. I will do this when i get back to that issue on the same PC in the future.

 

Link: http://digital.ni.com/public.nsf/allkb/D9B4018C3E540CE4862570F30068C0DB Link: http://digital.ni.com/public.nsf/allkb/0C5091E9099059BC86256FC1007947AA?OpenDocument

regards,

Koen.

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my pull up resistor was 100kOhm. I used 3kOhm instead and it works now. Don't completely understand why.

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