understanding current sink/source in a I/O pin

hi, can any one clarify about the following things WRT to I/O pins on 89s52 micro controller. kindly refer attached image.

1, What is the meaning of IOL = 1.6mA, with respect to VOL(0.45 V max)? is it minimum sink current when no output is connected?

2, similarly for VOH, IOH = -60uA? how is it related with VOH minimum of 2.4V? is it minimum source current when no output is connected?

3, How are the above values VOL & IOL and VOH & IOH related, If i am not wrong can we use this to determine output pin resistance?





General questions:

5, what happens when more current is sourced from or sinked into the I/O pin?

6, what happens when a high level output pin is shorted to low level output pin of same controller?

7, what happens when different controller(atmega8 and 89s52 ) are interfaced as output, like ATM8 as logic high O/P and S52 has logic low O/P?


thanks in advance

regards
kj

1 & 2. the outputs are not perfect voltage sources, a high will drop in voltage under load and a low will increase in voltage if current is passed into it. The data sheet shows 'conditions' at which the voltages are expected. The voltages would be different if other currents were specified but some standard condition has to be specified to help with design.

3. 'Resistance' is not really applicable for two reasons: firstly, there is an active current source or sink at the pin which is almost certainly not linear so the resistance would seem to change under different operating conditions. Secondly, some pins may have internal pull-up or pull-down circuits behind them that are made of weak current sources rather than physical resistance.

4. someone mis-counted!

5. The logic level voltages will suffer up to the maximum current allowed. Beyond that, internal heating of the output driver may cause damage to the IC.

6. Think of the circuit behind the pin as being a switch to supply and a switch to ground but only one at a time can be closed. If you link a switch to supply with a switch to ground, you get a short circuit across the supply. In practice, the IC will overheat until one or both output drivers burns out.

7. The golden rule is NEVER link two active outputs together. It doesn't matter whether the clashing logic levels are from the same device or another one. It IS permissible to link outputs together is you can guarantee only one of the devices will provide current at a time. this is a common scenario in bus contructs.

Brian.

Hi,

2.4V and 0.45V most probably are used for "more than safe" TTL levels.

5) usually there is a chart in every datasheet that shows the V - I relationship of the outputs.

Klaus

1) 1.6 mA is the maximum sink current for which they still guarantee that the output voltage is 0.45V or lower.
2) 60 uA is the maximum source current for which they guarantee that the output voltage is 2.4V or higher.
3) The output resistance in not linear, so don't try to calculate or use it.
5) The output voltages are not guaranteed. In extreme cases you can destroy the output.
6) The combined output will be low since the sink transistor is much stronger than the source transistor. For the same reason the current will not be very high, so probably no damage. Don't use it intentionally in a design.
7) Check the data sheets. The generated heat is proportional to the resulting current.