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Discussion - Hardware / UM245R data bus pullup - can it be disabled or set to pull down?
« on: January 18, 2021, 02:22:09 pm »
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.
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.