What is the intent for the use of a weak pullup?

[javida14]

Posted on December 09, 2006 at 00:16

What is the intent for the use of a weak pullup?

[javida14]

Posted on December 08, 2006 at 01:01

I'm looking at the 735 data sheet which indicates that if a weak pull-up is defined for an input pin, then its typical value is 120K. I'm planning on scanning a keypad where 4 signals from the keypad are inputs and 5 signal to the keypad will be outputs. Normally I would pull each of the inputs signals to Vcc through a 10K resistor. Any suggestions on the best approach. What are the weak pull-ups good for?

Thanks, Javi

[kleshov]

Posted on December 08, 2006 at 14:30

If I'm not mistaken, after reset port pins are configured to have weak pull-ups. This is certainly better than hi-Z floating state. You can use weak pull-ups to provide a definite logic level when no external signal is applied. The datasheet for my STR710FZ2 says that its weak pull-ups and pull-downs are between 110 and 700 kOhms. If such resistance suits your needs, you can omit external pull-ups. But 10K seems less susceptible to external interference.

Best regards, - volius[ This message was edited by: volius on 08-12-2006 19:01 ]

[javida14]

Posted on December 09, 2006 at 00:16

That way my conclusion. Use external pull-ups on inputs signals that require pull-ups and use the weak pull-up for all unused pins.

Thank Javi