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General Category => Discussion - Hardware => Topic started by: johnlon on January 18, 2021, 02:06:56 PM

Title: UM245R self powered - what is the impact of the EPROM 'self powered' setting
Post by: johnlon on January 18, 2021, 02:06:56 PM

The UM245R documentation says that if I want self powered mode then I need to make a few specific wiring changes over the default USB powered.
In addition the doco says I also need to use the FT_Prog utility to change the EPROM from 'Bus Powered' to 'Self Powered'.

I would like to understand what is the actual impact in the internal behaviour of the device when making this change?

If I make the wiring changes but forget to make the EPROM change to "Self powered" then will the device fail to work properly if I put it on self power?
And if so then what is it that actually causes the failure?

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Can someone confirm that this eprom setting is purely informational to the host. If I leave it set to "Bus powered" but in fact run in self powered mode then what are the negative consequences if any. Are there in fact zero negative consequences???

Title: Re: UM245R self powered - what is the impact of the EPROM 'self powered' setting
Post by: FTDI Community on January 19, 2021, 11:34:54 AM

Hello,

This setting does in fact alter the USB descriptors presented to the USB host on enumeration, altering the host that the device is either Bus or Self powered. The USB spec requires that a device correctly identify itself to the USB host, so please ensure you have selected the correct setting for your configuration.

Forgetting to update the setting to reflect if you are using a bus powered or self powered design may result in enumeration issues for the device.

Best Regards,
FTDI Community