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I'm trying to connect to my atmega1284rfr2 using the basic application circuit in their datasheet. The red lines and letters are my own annotations.

Basic Application schematic

I'm having trouble understanding what all the T's mean. Do they mean hook them up to positive supply? Ground? Dangling?

I get the impression that some should be dangling, but others should be bridged to ground, and others to the positive supply rail.

What do the T's mean?

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Theoretically, it doesn't matter whether filter capacitors are bypassed to ground or a power rail, since both are supposed to be clean DC signals with 0 AC. Practically, power rails tend to be noisier than ground, so connecting to ground is better. And usually (when you have a board-wide ground layer) there's no routing advantage to connecting them to any other power rail. –  Ben Voigt Aug 25 at 6:27

3 Answers 3

up vote 5 down vote accepted

The bar symbols are analog ground and the triangle symbols are digital ground.

They would be tied together somewhere (not shown), probably directly at one point but it's possible there would be something like a ferrite bead.

The orientation of the bars is immaterial (as with the triangle grounds).

This particular device has sensitive 2.4GHz RF circuits inside (probably on two dies), hence the extra detail devoted to grounding, but similar concerns are present whenever analog circuitry is mixed with digital. Here is a reference more oriented to the latter that goes into a lot of detail.

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In the diagram in the original post

The bar symbol is analog ground (in this case).

gnd symbol The triangle stands for analog ground.

vdd symbol The lollipop is the Vdd supply rail.

However, this does not follow the normal schematic convention. That's why that notation is confusing, and we have to tilt our heads and guess from the context.

Standard convention

The following symbols are roughly ordered by decreasing voltage.

positive symbol Upright T, or upright lollipop (not upside-down, not on the side). Positive supply rail.

signal ground Signal ground. Sensitive ground.

digital ground Digital ground. Less sensitive ground.

enter image description here Upside-down T, or upside-down lollipop. Negative supply rail.

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They are ground connections. The triangle symbols are ground, as well. For good noise immunity different grounds are used for analogue and digital parts of the device (AVSS and DVSS).

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