PIC bypass capacitor necessary

[AlGeorge]

Hi all. In my earlier 8051 days, it was mandatory to bypass the supply pins on the micros with a .1 tant. I think this is not necessary now, as there is no mention in the application note for eg PIC10F series; just an inclusion of it in the brownout part, and the application notes do not make any specific requirement for it. So far in all my tests, the cap does not seem necessary. I would like comments from learned friends.
Its being powered by a 3v lithium cell, which I suspect has an inherant capacity of many picofarad, although I cant find any referal to this anywhere by Google.

 

[karesz]

Hi,
I think its more needed es every time!
You see, moder ICs are CMOS/High-speed CMOS = torugh signal transitions you will have 20-50x higher currents(in 1-5 nseconds), if you dont have 100 (or in case highest speed IC) 10nF; you will have lot of problems, but minimum EMC disturbances!
K.

 

[AlGeorge]

Hi,
I think its more needed es every time!
You see, moder ICs are CMOS/High-speed CMOS = torugh signal transitions you will have 20-50x higher currents(in 1-5 nseconds), if you dont have 100 (or in case highest speed IC) 10nF; you will have lot of problems, but minimum EMC disturbances!
K.

Thanks for your advice. I guess in hindsight, that the best test is to power up the micro with the lithium battery and put a scope across the supply leads. If it shows signs on r/f at levels > 1mv then it would make sense to include a cap. I will advise. Al.

 

[karesz]

Hi Al,
You will see some voltage drops/ripple, than dont forget the situation with EMI...
You dont place a smallest SMD C? Inexpensive are their too.
Relevant: if your PIC works yet, in your enviroment problemless, means not , that in all evironments has the same possibilities:-(
K.

 

[betwixt]

ALWAYS use a bypass capacitor!

Even if your battery impedance seems low enough now, it will increase as it discharges. The manufacturer always advises you add one. It's particularly important if you use a processor with an internal clock generator as the frequency tends to modulate with supply line noise.

Brian.