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David Prentice
03/17/12 02:34
Read: 563 times

#186744 - Well add a pull-up resistor then
Responding to: Michael Karas's previous message
Michael Karas said:
David Prentice said:
Most 8051 systems have a 10k resistor array as pull-ups on a port.

Not So!! On yours maybe but not on mine and 100's of others.

Michael Karas

Ok, so Mehdi was using P2.

You can't get more bog-standard than a AT89C51:

Port 2
Port 2 is an 8-bit bi-directional I/O port with internal pullups.
The Port 2 output buffers can sink/source four TTL inputs.
When 1s are written to Port 2 pins they are pulled high by
the internal pullups and can be used as inputs. As inputs,
Port 2 pins that are externally being pulled low will source
current (IIL) because of the internal pullups.

So you need to match the 'internal pull-up' of P2.
Likewise if you use the 'weak internal pull-ups' of other ports.

Since these are not closely controlled values, an external stronger resistor can simply swamp it. e.g. Mehdi's selection of 1k0 resistors.


List of 21 messages in thread
Making a pulse with 3 different voltage level!      MEHDI      03/15/12 10:57      
   No best but consider a voltage divider        Per Westermark      03/15/12 11:02      
      derivative with a DAC?      Erik Malund      03/15/12 11:07      
   derivative with configurable ports        Erik Malund      03/15/12 11:10      
   Thanks Per And Erik      MEHDI      03/15/12 11:44      
      which derivative      Erik Malund      03/15/12 12:12      
      with an OPamp      Erik Malund      03/15/12 12:16      
         Thanks Erik      MEHDI      03/16/12 01:57      
      So, you need to make -15V, 0V and +15V?      Kai Klaas      03/16/12 06:38      
         Hi Dear Kai      MEHDI      03/16/12 09:45      
         74hc series      MEHDI      03/17/12 12:50      
            FPGA      Kai Klaas      03/17/12 14:58      
      I think you can do it wih one resistor.      David Prentice      03/16/12 11:09      
         Most 8051 Systems?????      Michael Karas      03/16/12 18:51      
            Well add a pull-up resistor then      David Prentice      03/17/12 02:34      
               Would prefer to not have to match internal resistors      Per Westermark      03/17/12 06:48      
               You cannot match the weak internal active pull-up!      Kai Klaas      03/17/12 07:02      
                  Being pedantic.      David Prentice      03/17/12 09:12      
         Hi David      MEHDI      03/17/12 12:43      
   maybe a couple of mosfets ...      Richard Erlacher      03/15/12 16:30      
      Thanks Richard      MEHDI      03/16/12 02:02      

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