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Author Topic: UM245R - EPROM bus current limit setting  (Read 409 times)


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UM245R - EPROM bus current limit setting
« on: January 18, 2021, 02:09:08 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'.

On the FT_Prog page where I change the power setting to "self powered" there is also a setting for the current that may be drawn from the bus.
It has a default of 100ma for the current draw from USB.

Does this actually limit the current draw by the device in some way - if so is there further doco and examples of how this is useful.

The documentaiton mentions "hybrid" power but doesn't give any indivation of how or where this mighty be useful.

Any further references?

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Re: UM245R - EPROM bus current limit setting
« Reply #1 on: January 19, 2021, 11:17:57 AM »


The MaxPower Descriptor defines the maximum power that may be drawn by the peripheral from the USB host. The USB host will read this value during enumeration to determine if it can provide the desired current draw the device is requesting. If for example you change this setting to 500mA and the USB host cannot provide this much power to the device, then the device will not enumerate.

The MaxPower Descriptor relates to Bus Powered devices, when using a Self Powered design you can set this value to 0mA.

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« Last Edit: January 19, 2021, 11:24:17 AM by FTDI Community »