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Author Topic: UM245R data bus pullup - can it be disabled or set to pull down?  (Read 203 times)

johnlon

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I believe the UM245R has 200k pull ups on the D0-D7 data pins - is this correct?

When neither the read nor write control lines are enabled then I expect the data lines to go into a high impedance state with this weak 200k pull up  - is this all correct?

The bus that I am connecting the UM245R to already has a weak pull down, therefore I want to avoid a situation where the UM245R is pulling up and the surrounding circuit is pulling down - it's an irritating tug of war at the very least, but there is some more irritating impact in my circuit (see the post script).

Is there a means to either disaable the pull up entirely OR change the UM245R so that it pulls down instead of pulling up.

What are my options?

Thanks



PS. The impact of the 200k pull up in the UM245R is that an LED array that is directly attached to the bus lights up when nothing is asserting on the bus.
200K is enough on 5v to allow 15 to 20uAmp to flow and these LED's are really efficient and glow nicely at that current. I would rather thay were dark.
Of course I can use a 10k pull down to overcome the UM245R pull ups but that's a bit irritating.
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Re: UM245R data bus pullup - can it be disabled or set to pull down?
« Reply #1 on: January 19, 2021, 11:49:17 AM »

Hello,

Yes that is correct when used in Input Mode, the input pins are pulled to VCCIO via internal 200kΩ resistors, so they will gently pull high unless driven by some external logic. These pins can be programmed to gently pull low during USB suspend (PWREN# = “1”) by setting an option in the internal EEPROM. But unfortunately you cannot disable the pull up entirely.

Best Regards,
FTDI Community
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johnlon

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Re: UM245R data bus pullup - can it be disabled or set to pull down?
« Reply #2 on: January 19, 2021, 05:57:23 PM »

Thanks.

So they can be programmed to pull low during USB suspend.

But can they be programmed to pull low when neither RD nor WR are asserted, ie when it's relinquished the bus?
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Re: UM245R data bus pullup - can it be disabled or set to pull down?
« Reply #3 on: January 20, 2021, 12:42:34 PM »

Hello,


Yes that is correct, they can be programmed to pull low during USB suspend.
Unfortunately however they cannot be programmed to pull low when neither RD or WR are asserted.

Best Regards,
FTDI Community
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