| ??? 08/02/06 04:57 Read: times |
#121538 - Benefit of additional pull-ups Responding to: ???'s previous message |
Richard said:
Yes, there are often undesirable effects associated with attaching one output to another. It's not advisable practice, but INPUTs to outputs, in this case WEAK pullups, if available, will cause no damaging effects. Yes, the weak pull-up which looks roughly like a resistance of about 10...20kOhm (depending on actual derivative) will not cause any damage. But there's some reason, why even with the weak internal pull-ups additional pull-ups might be useful, like it's shown in the schematic of Michael by the way: When pulling-down a port line emitting high state, the weak pull-up is turned-off and only the very weak pull-up remains active. As this very weak pull-up looks like a resistance of 100...200kOhm (depending on actual derivative), this can result in slow charging of stray capacitance and contact problems with older or cheap switch contacts, which need a contact current of more than several 100µA. Richard said:
With push-pull, I'd be tempted to use a series resistor at the column port bits. Yes, that's a good idea, as long as the low level noise margin isn't eroded. So, eventual additonal pull-ups and series resistors must be carefully chosen. Kai |
