Relation or difference between Input clamp current and Sink current

[Prakash123]

I want to take signal from external peripherals to PIC micro-controller(PIC18F66K22) through buffer IC(74HC541). My problem is o/p of buffer source current is +/-35mA, output clamping current +/- 20mA and i/p of PIC18F66K22 PORTF pin is 2mA. And another case input clamp current of PIC18F66K22 is +/-20mA. We may directly interface these two IC's to PORTF pin? And in what way input clamp current and sink current related or differs?

 

[Harald Kapp]

You'll have to sort this out:

• buffer source current is +/-35mA -> This is the max. current a single output stage can source or sink without violating the limits for low or high output voltage.
  
• output clamping current +/- 20mA -> This is the max. allowed current into the output stage when an external voltage < 0V or >Vcc is applied. This current is caused by the protection circuit which directs current to GND or Vcc if the external voltage is too high or too low. This is a protective function and should never be active in normal operation. If there is a risk that this operation happens, resistors may be used to limit the current.
  
• i/p of PIC18F66K22 PORTF pin is 2mA -> This means that the input of the PIC will draw max. 2 mA under normal operating conditions.
  
• input clamp current of PIC18F66K22 is +/-20mA -> same as output clamping current of the buffer, see item 2 of this list (above).

The input of the PIC will draw only as much current as required, typically much less than 2mA. The buffer will easily be able to drive this input.

 

[BobK]

Don't know where you got 2mA for the the input current. Looking at a datasheet for a similar PIC, i see that the input current is ±100nA. 2mA is ridiculously high.

The parameter is called "input leakage current". What was the name of the parameter that gave you 2mA?

Bob

 

[Prakash123]

Sink current of PORTF pins are 2mA.

 

[Harald Kapp]

Sink current of PORTF pins are 2mA.

That would be in output mode, not as an input.

 

[Prakash123]

That would be in output mode, not as an input.

Hi,
Sink current means ability to receive current. I' confusing with "output mode". Please explain.

 

[BobK]

You are looking at this spec:

Maximum output current sunk by PORTA<5:0> and any PORTF, PORTG and PORTH I/O pins ............................2 mA

That is the ability of an output port to act as a ground to a current coming in. For instance, if you were trying to run an LED by connecting it to V+ and to the port which is set low, it would only be able to handle 2mA.

The one you are interested in is this one:

IIL Input Leakage Current(1)
D060 I/O Ports ±50 ±200 nA VSS VPIN VDD, Pin at high-impedance

Bob

 

[Harald Kapp]

Typically "sourcing a current" is used to describe a current that is driven from the positive rail (Vcc) through a load to ground.
"Sinking a current" is the other direction: a current flowing from a high potential (Vcc) through a load into a pin to ground.

The idea behind this nomenclature is the conventional direction of electric current from "+" to "-" where the current flows out from the positive potential through a load and back into the negative potential of the source.