I'm looking at a small design using the AT328P in a TQFP package. According to the datasheet, pins 3 and 4 are VCC and GND, and then 5 and 6 are also VCC and GND. There is also a separate AVCC and GND as per other packages, but why has this package got two sets of VCC and GND next door to each other?
I'm assuming this is nothing more than using extra pins this package has for flexibility in circuit board design and perhaps only one of each needs to be connected? Is this the case? So I can simply pick either one of the grounds and either one of the VCCs, what ever suits me.
The data sheet doesn't specify if only one of each needs connecting.
Not only do you want to have parallel supply connections to distribute the currents when approaching the specified limits (per supply pin) in current draw, but it simply makes sense to provide the lowest impedance to manage transients.
I suppose you can get away with using only one. Most of us (who advise here) believe that you should strive to do things as meticulously as possible.
On this topic, be sure to connect AVcc to Vcc. You don't need the low pass filter - it works fine without it. (I've never seen an dev board meant to be used with Arduino that includes it).
There are a number of guides online that say not to connect it. These are wrong - I suspect the authors who said that got it confused with AREF, which is not connected to Vcc.
Phil-D:
I'm assuming this is nothing more than using extra pins this package has for flexibility in circuit board design and perhaps only one of each needs to be connected?
Chip manufacturers generally do not do that - I don't think I've ever seen a chip that had two identical pins where only one needed to be connected ("need" as in, in order to function reliably and in a manner consistent with the manufacturer specifications - a chip with only one of the supply or ground pins connected might appear to work under some conditions)
multiple pwr/gnd are directly connected internally on these chips as can be verified with an ohmmeter. contrary to datasheet comments and many noobs and experts generally do not need external connection unless you require high total chip current or for liability issues. same for duplicate caps. considering the minimal cost it dont hurt to perform these rituals but not as important as some believe.
failing to connect avcc however is a big mistake. some avrs appear to work w/o but can exhibit flaky behavior when least expected. imo this is worse than 100% failure because of false sense of security.
john1993:
multiple pwr/gnd are directly connected internally on these chips as can be verified with an ohmmeter. contrary to datasheet comments and many noobs and experts generally do not need external connection unless you require high total chip current or for liability issues. same for duplicate caps. considering the minimal cost it dont hurt to perform these rituals but not as important as some believe.
failing to connect avcc however is a big mistake. some avrs appear to work w/o but can exhibit flaky behavior when least expected. imo this is worse than 100% failure because of false sense of security.
Many thanks, I did suspect they might be connected internally and were likely a belt and braces thing because the 28 pin version of the package or the DIP version does not have the duplicated VCC or ground connections, but otherwise have identical sink/source ratings as far as I know.
It costs nothing in my case to add an extra cap, I'm only making a few, so a few pence goes un-noticed.