| ??? 03/17/12 08:34 Read: times |
#186744 - Well add a pull-up resistor then Responding to: ???'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. |
| Topic | Author | Date |
| Making a pulse with 3 different voltage level! | 01/01/70 00:00 | |
| No best but consider a voltage divider | 01/01/70 00:00 | |
| derivative with a DAC? | 01/01/70 00:00 | |
| derivative with configurable ports | 01/01/70 00:00 | |
| Thanks Per And Erik | 01/01/70 00:00 | |
| which derivative | 01/01/70 00:00 | |
| with an OPamp | 01/01/70 00:00 | |
| Thanks Erik | 01/01/70 00:00 | |
| So, you need to make -15V, 0V and +15V? | 01/01/70 00:00 | |
| Hi Dear Kai | 01/01/70 00:00 | |
| 74hc series | 01/01/70 00:00 | |
FPGA | 01/01/70 00:00 | |
| I think you can do it wih one resistor. | 01/01/70 00:00 | |
| Most 8051 Systems????? | 01/01/70 00:00 | |
| Well add a pull-up resistor then | 01/01/70 00:00 | |
| Would prefer to not have to match internal resistors | 01/01/70 00:00 | |
| You cannot match the weak internal active pull-up! | 01/01/70 00:00 | |
| Being pedantic. | 01/01/70 00:00 | |
| Hi David | 01/01/70 00:00 | |
| maybe a couple of mosfets ... | 01/01/70 00:00 | |
| Thanks Richard | 01/01/70 00:00 |