Resistor, Capacitor, and 9v to 5v questions

[ibwev]

I am trying to recreate a "Hello World" experiment of Chuck Hellebuyck as published in the January 2006 article of "Nuts and Volts" (Nuts & Volts - January 2006). Any help with the following three questions would be much appreciated:

1) The datasheet of the 16K876 shows max current of 250mA on the VDD port. Why should the VDD port not be protected by a resistor of 36 ohms (R=V/I=9V/0.250mA=36 ohms)?

2) In the article, how is the 9V battery reduced to 5V?

3) In the hardware schematic, should a 20 microfarad capacitor be used instead of the 20 picofarad?

 

[MikeMl]

This post should be moved to the MicroController Forum.

I am trying to recreate a "Hello World" experiment of Chuck Hellebuyck as published in the January 2006 article of "Nuts and Volts" (Nuts & Volts - January 2006). Any help with the following three questions would be much appreciated:

1) The datasheet of the 16K876 shows max current of 250mA on the VDD port. Why should the VDD port not be protected by a resistor of 36 ohms (R=V/I=9V/0.250mA=36 ohms)?

That spec is the absolute max current allowed on any PIC pin. Unless you have a short circuit on some other PIC pin, the current flow from the 5V supply into the VDD pin on the PIC will be a few tens of ma, so no external resistor is need. Nor would the PIC tolerate one there.

The Nuts&Volts article is yet another example of a project created by a programmer who doesn't know that there should be a bypass capacitor of ~1uF connected + to the PIC VDD pin, and - to the PIC VSS pin.

2) In the article, how is the 9V battery reduced to 5V?

There is no mention of this in the article. PICs must be powered from <5.5V. Usual practice is to use a suitable LM7805 IC regulator if you are starting from a higher voltage.

3) In the hardware schematic, should a 20 microfarad capacitor be used instead of the 20 picofarad?

No. 20pF is right to create a Pierce Oscillator across the two PIC Pins.

 

[ibwev]

Thank you Mike ML for your response. You have given me some more things to learn about.