SparkFun Forums

[roach]

If Olimex is around, maybe he can shed some light: On the Atmega128 header board, there is a pullup resistor on the PEN pin. According to the datasheet, this pin is pulled up internally, and is used to enter SPI programming mode when pulled low. So is that resistor really necessary? Or is it just there so that, when I pull the pin low I don't blow up the battery

Also, the datasheet says that, if I want to use the ADC, I need to connect AVCC to VCC through a "low-pass" filter. 1) what the heck is a "low-pass filter"? and 2) is there a way to do this with the header board? It looks like AVCC is already connected to VCC...

[andrey]

FYI, I believe the PEN pin is for ATmega103 compatibility mode and that resistor is needed if the compatibility fuse is enabled, so they probably put it there just in case you choose to use that mode. I noticed they got rid of the PEN pin entirely on the AT90CAN128 as they removed ATmega103 compatibility mode from that chip.

With all due respect, you may want to read up on some basic electronics before you work with microcontrollers. A low pass filter for the AVCC pin is a filter that will pass lower frequency signals, cutting out higher frequencies. It is made by using an appropriate capacitor (which, if I recall correctly, Olimex already provides on your board). Please see this page for some info:

http://en.wikipedia.org/wiki/Low_pass_filter

-Andrey

[roach]

With all due respect, you may want to read up on some basic electronics before you work with microcontrollers.

heheh. You really don't know me . Look around the forums a bit. This is far from the dumbest question I've ever asked here (and there's lots more to come!)

It is made by using an appropriate capacitor (which, if I recall correctly, Olimex already provides on your board)

Well, the schematic says that VCC and AVCC are both connected directly to the regulated 5V supply, so I fail to see where the filter comes between them. Of course, as has been previously established, I'm a bit thick when it comes to electronics, so be patient.

[transcendentnb2]

Well, the schematic says that VCC and AVCC are both connected directly to the regulated 5V supply, so I fail to see where the filter comes between them.

That's simply a reference point, not the regulated power. The power comes through the 78L05 regulator, and runs along the parallel capacitors (!!), which act as your low-pass.

[roach]

The power comes through the 78L05 regulator, and runs along the parallel capacitors (!!), which act as your low-pass.

Okay, NOW I get it. Thanks!