How I use the input with pull up/down internal resistor?

[nailuy]

How to build an application using the input with the pull up/down internal resistor from the microcontroller or microchip.
Can I use that and analog inputs?

[Ttelmah]

'Resistors' are generally pull-up only, not pull up/down.
They are actually programmable current sources, rather than resistors, so for an analog application might give rather different results than a resistor. Also currents involved vary significantly from pin to pin. The data sheets are generally very unclear whether these can be enabled on a pin configured as analog. But does in most cases say these are for digital inputs only. A search finds the 16F688, with a block diagram for the pin logic, (Figure 4-1), which clearly shows that the pull up is _disabled_ when the port is set to analog, for either comparator, or ADC.
So I'd say you cannot use the pull-ups with pins set for analog.

Remember also the impedance requirements for analog inputs. The leakage current on an input pin would be over 1/10th the available current on these weak pull-ups, so any form of accuracy would be lost....

Best Wishes

[nailuy]

Thanks Ttelmah
So no each controller has a system pull up resistance.
I worked with PIC12F509 and pull up resistor is activated automatically if you read input pin B0, B1, B3, B4 for B2, B5 don't have.
If the algorithm operates with pull up separately for 16F688 do you know an example?

For Atmel have pull up resistor and of course pull down.
Atmega16 is a example.

[SherpaDoug]

You really have to read the data sheet for the chip you are using. There are digital inputs, analog inputs, and Schmitt trigger inputs. Common outputs are digital, PWM, and open collector or open drain, and any output may or may not have a pull up resistor or current source as well.

The are far too many types to cover them all in a BBS posting.

Read your data sheet and get back with specific questions about specific chips.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[nailuy]

Be sure I read datasheet.
But if you know all ATMEGA controllers have internal pull up/down system.
Of course the current is about 10-20uA for some devices.
Look for this example:

[Ttelmah]

Seriously, _what chip_.
You need to read the data sheet for the specific chip.

The point is that the PIC's _DO NOT_ have universal resistors. Most smaller/older ones only offer them on 'port_b' (port_b_pullups(TRUE); is the command to enable these). Latter chips with them still on port B, often allow then to be individually enabled (port_b_pullups(0b11110000); - turn them of for the high four bits). Some of the latter chips have them on other ports as well, and on these you get 'port_x_pullups', where x is the port name.
Then a _very few_ chips do have pull down resistors as well. On these you have:
port_x_pullups(upmask,downmask);
and here you could have:
port_a_pullups(0b11110000.0b00001111);
to enable pull 'down' on the low four pins, and pullup on the high four pins.

You need to talk about your specific chip. There are currently about 10* as many PIC based 'models' as there are Atmel, and the range of features is much larger, with the smaller chips in particular being much more 'cut down'.

Best Wishes

[SherpaDoug]

PORTA = 0xFF; //enable all pull-ups
//or next line
PORTA = 0x00; //enable all pull-down

On a PIC with the CCS compiler this would not enable pull-up and pull-down RESISTORS, but it would use the pull-up and pull-down TRANSISTORS of the logic output stage. Make sure you know the difference.

Pull-up resistors (or current sources) are meant for "wired OR" situations where multiple devices may pull a line low, and the pull-up brings the line high when none of the other devices are active. Pull-ups can also be used to set the state of an unconnected pin.

If you use a pull-up transistor (the usual logic output set to "1") in a wired OR situation with another device pulling the line low you get a bad situation called "bus contention" which causes high current to flow, and if you do it for a whole byte the heat generated may even perminately damage the processor port.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[nailuy]

yes Ttelmah

[Ttelmah]

Yes.

To SherpaDoug, this is a big difference with the Atmel chips. On a port set (via their equivalent of TRIS), as an _input_, the direction the pull-up/down's operate, is controlled by the contents of the output latch.
This has it's advantages, but also disadvantages. Obviously it does mean that every port can offer pull-up/pull-down, without needing separate registers, but you can't do the equivalent to the PIC on things like I2C, setting the TRIS as an input, and the driver to 'pull down', and then giving effectively 'open collector' drive, without having the pull-down resistor present, if you are using this on any other bit. It is a very different approach.

Best Wishes

[bkamen]

[Ttelmah]

Some of the new 24E/33E chips have programmable weak pull-downs.
On older chips the only pins with this feature (that I know of), are the USB pins, where it is automatically enabled.

Best Wishes