AVR-Chat

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> 1) ISP, I have never used ISP yet. It appears as though I just place
> a 6 pin header on my board - wire that to the correct pins (SPI) and
> plug the cable in for programming.  I will NOT be using SPI on my
> project so it looks like those pins will only be used on my project
> as PCINT input pins.  Any comments or suggestions about ISP for a
> first-timer?


You can use the pins, just put your programming connector so that it's
directly connected to the chip, and resistors in the signals that feed other
devices.  You need to make sure that the programmer "wins" any contention,
easily.   Also watch that activity on those pins won't do anything bad in
your hardware, like activate motors or something hazardous.


2) Coupling, someone suggested there could be coupling problems with this
> chip and the mounting method. I don't know much about
> electronics so I have no idea.  Should I just add some 1 uF caps at
> every power entry point to prevent this problem?


Oh boy.. Power Decoupling is something you need to take seriously, and 1uF
caps are not a good choice.  This depends on your frequency of operation,
and the actual caps you use, but generally 0.047 at 8 mhz is about right.
If you have any choices, locate the caps at the ground pins on the chip, and
take VCC to the cap, and from there to the chip. (forms an L/C "tee" filter
using the track inductances)

3) Is there anything "Different" to watch out for when using this
> chip as compared to the mega48,88,168 chips that I am familiar with?


Something you might not be aware of, I mention it a lot because the
consequences are SO nasty.. The CKOPT fuse has a default state that enables
a "vittoz mode" oscillator.  You definitely do NOT want this fuse in the
default state!   If you scope the crystal with it in "vittoz mode", you'll
see that the amplitude is about 0.5 VCC.   The chip will skip cycles, baud
rates will be a bit off (always slow) and sometimes the chip will execute
instructions wrong.  Or, if you got "lucky", you'll have just enough
amplitude that everything will work.  Until you get a new batch of chips, or
crystals, or have 10,000 units in production.

Also, crystal caps are determined by the crystal, NOT the chip, and a 22pF
crystal does NOT use 22pF caps.  (2C)-5pF is a good starting point, see the
app notes at Fox for details and a more rigorous treatment.


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