Email: Password: Remember Me | Create Account (Free)

Back to Subject List

Old thread has been locked -- no new posts accepted in this thread
David Prentice
03/17/12 02:34
Read: 540 times
Wormshill
England


 
#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.

David.

List of 21 messages in thread
TopicAuthorDate
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      

Back to Subject List