| ??? 12/27/05 15:33 Modified: 12/27/05 15:47 Read: times |
#106015 - a word on pull ups Responding to: ???'s previous message |
Shivani Saini said:
although the point was not clear to me shivani sorry for not being descriptive i was thinking to use pins as inputs i need pull up. the pin of your micro is just a peice of metal and voltage appearing on it is just the nett effect of all the drives. On normal pins there are transistors that can drive(pull) the pin to the ground and also transistors that can drive the pin to Vcc. the later are the pull ups and they are very weak in comparision to the pulling down transistors (this facilitates the quasi bidirectional ports). when Pulling down FET is ON (conducting when out put is '0') it sinks. the current to the ground. sources of this current are the internal pull up transistos and the input being driven. at this time your pin is the junction of pull ups and pull downs and nett voltage is near zero. when output is one the pulling down FET is turned off and pullups source the current so nett voltage that appears is almost Vcc (if your input eats more current than the internal pullups can source, then that input may read it as zero). now these outputs without pull ups, when output is zero, pulling down FET conducts and it sinks current from the digital input it is connected to and nett voltage (if sinking capability is more than inputs sourcing capability) is zero. if output is one, pulling down FET is turned off showing very high resistance towards ground (almost open). At this time you pin is neither high(no pull ups) or low( lower FET is off) but floting. that means its just a metal foil in the air/space. now you can't measure the voltage in reference of ground because the pin is not in the circuit. using as ports as inputs is done by writing ones to latches. Infact by doing this we turned off pulling down transistor, and leaving the pin pulled up by internal pullups(if they exist). This is just like outputting a one. when you read the ports it reads the nett voltage on the pin and interprets according to the specifications(Vil,Vih). Now if you have internal pull ups you will read a one when nothing drives it or somthing drives it High(as we are already high). If you dont have internal pullups then its responsibility of the driver to pull it high when driving high and low when driving low. suppose you have connected and configured a pin without internal pullups as input(1 in the latch) and driver is open drain, open drain can ony pull you down when o/p is zero. so when open drain outputs an one it will leave the pin just floting and so did your MICRO! in this case you will need an pull up. Abhishek |
| Topic | Author | Date |
| this code is not working | 01/01/70 00:00 | |
| You solved it! | 01/01/70 00:00 | |
| overreaction | 01/01/70 00:00 | |
| Humility ? | 01/01/70 00:00 | |
| Call it cultural difference... | 01/01/70 00:00 | |
| Sounds familiar | 01/01/70 00:00 | |
| thanks Jan Waclawek | 01/01/70 00:00 | |
| Keil's startup.a51 | 01/01/70 00:00 | |
| hex seems OK | 01/01/70 00:00 | |
| re hex seems OK | 01/01/70 00:00 | |
| And the switch? | 01/01/70 00:00 | |
| and switch | 01/01/70 00:00 | |
| Try a pull-up | 01/01/70 00:00 | |
| switch function | 01/01/70 00:00 | |
| Not at all | 01/01/70 00:00 | |
| reset cap | 01/01/70 00:00 | |
| yeah | 01/01/70 00:00 | |
| It can! | 01/01/70 00:00 | |
| points | 01/01/70 00:00 | |
| Some suggestions | 01/01/70 00:00 | |
| nope | 01/01/70 00:00 | |
| Use LEDs | 01/01/70 00:00 | |
| ok let me follow | 01/01/70 00:00 | |
| That sounds good ! | 01/01/70 00:00 | |
| try this code | 01/01/70 00:00 | |
and what good will that do? | 01/01/70 00:00 | |
| How? | 01/01/70 00:00 | |
| capital | 01/01/70 00:00 | |
| Thanks! | 01/01/70 00:00 | |
| I did the same thing recently | 01/01/70 00:00 | |
| At least... | 01/01/70 00:00 | |
| HORSEFEATHERS | 01/01/70 00:00 | |
| i think yopu didn't got the message | 01/01/70 00:00 | |
| of course it is not | 01/01/70 00:00 | |
| It's easy to fix the code | 01/01/70 00:00 | |
| no, he does not | 01/01/70 00:00 | |
| wrong suggestion | 01/01/70 00:00 | |
| it DOES NOT write to R0 | 01/01/70 00:00 | |
| RE: it DOES NOT write to R0 | 01/01/70 00:00 | |
| why the continued discussion | 01/01/70 00:00 | |
| Time zone | 01/01/70 00:00 | |
| just another guess | 01/01/70 00:00 | |
| She is most likely from India | 01/01/70 00:00 | |
| Irony | 01/01/70 00:00 | |
| Help me | 01/01/70 00:00 | |
| re help me | 01/01/70 00:00 | |
| What happens if? | 01/01/70 00:00 | |
| i will see | 01/01/70 00:00 | |
| "mov @r0,#00h" by "NOP, NOP"? | 01/01/70 00:00 | |
| A good idea | 01/01/70 00:00 | |
| nothing postive | 01/01/70 00:00 | |
| RAM fill ? | 01/01/70 00:00 | |
| re ram fill | 01/01/70 00:00 | |
| Simulator. | 01/01/70 00:00 | |
| Is the CPU really running? | 01/01/70 00:00 | |
| mov p1,r0 | 01/01/70 00:00 | |
| last hope | 01/01/70 00:00 | |
| Tell this first | 01/01/70 00:00 | |
| let me clear | 01/01/70 00:00 | |
| pull ups | 01/01/70 00:00 | |
| re pull ups | 01/01/70 00:00 | |
| a word on pull ups | 01/01/70 00:00 | |
| What to say for that ignorance? | 01/01/70 00:00 |