Snapping, and the quiet disaster of off-grid points

You move a corner with the mouse, and you want it to land on a legal point instead of wherever the cursor happened to rest.

That pull onto a legal point is snap. It does not repair a gap that is already stored. A later snap toward the neighbor can turn a half-micrometer space into a sliver of 200 database ticks — 0.2 micrometers when one micrometer is 1000 ticks — or it can close the gap and short the two nets. With snap on, a drag hops and the status line shows a round coordinate. With snap off, the corner follows the cursor and can stop between crossings. The shape still looks fine. The coordinate is the part that changed.

The picture at the top of this page freezes the two landings. Orange dots mark a regular pitch. One rectangle sits on those dots: each edge runs from dot to dot. The other rectangle misses the dots and cuts through the spaces between them. The first is what snap is trying to give you while you draw. The second is what you get when the pull is off, or when the geometry arrived from somewhere that never saw these dots.

Pulling an edge onto a crossing

The second picture is what a bad landing does to the space between two shapes.

A teal rectangle and a navy rectangle almost meet. The white between them is a sliver, thin enough that a quick look can read it as a touch. That sliver is a spacing error you can miss at a glance. In this drawing the gap is 200 database units, the integer ticks the file stores. If one micrometer is 1000 of those ticks, a common choice, the white line is 0.2 micrometers. A minimum space of half a micrometer will fail it. An etch that eats a tenth of a micrometer may close it and short the two pieces of metal. A wide, obvious gap is the same pair of rectangles pulled apart until the space is easy to see and easy to trust. This picture is the one that does not look like a crisis until you measure it.

Snap prevents that kind of miss while you create new corners. It does not repair corners that are already stored. An imported mechanical drawing, a rotated cell, a scaled instance, or a boolean that invented a point can all sit between dots. A later “snap everything” command will pull those edges onto crossings. The pull has a direction. If the nearest dot is toward the neighbor, the gap shrinks. The sliver in the second picture is what that shrink looks like when it stops short of a touch, or when it leaves a gap smaller than the process can hold open.

Off grid means the corner is not on the grid that matters, usually the manufacturing pitch, not merely that it looks slightly crooked. A point can miss the orange dots and still be a whole number of fine ticks. A point can sit on a coarse dot and still be illegal if those dots are not the factory pitch. Off-grid is a coordinate fact. The fill will not confess it.

A space that closed under a cleanup

Two metal rectangles were drawn with half a micrometer of space, a wide enough gap to see and to pass the rule. During a cleanup, snap was off, and one edge was nudged into the space between dots. A following command snapped that edge to the nearest dot in the direction of its neighbor. The half-micrometer gap collapsed into a sliver like the one between the teal and navy rectangles. The design-rule check reported a spacing error on a pair that had been clean the day before. On a sister block the same snap ran the other way and the sliver closed completely. Etch joined the nets. Both failures started as a corner that did not sit on a dot, and both were finished by a snap that was asked to be helpful.

Use snap at the manufacturing pitch while you draw, so new corners are born legal. When you must inspect a coordinate, turn snap off, read the number, and turn it back on before the next edit.

After any import or any global snap, measure the gaps that used to be minimum spaces. Read the white line in ticks before you trust the glance. Two hundred ticks is the sliver in the second picture. A touch where that line used to be is the snap that finished into a short. Repair the missed corner on purpose, onto the dot you chose, before the file leaves your hands.

Half a micrometer that a snap ate

The pair started with half a micrometer of space, a gap wide enough to see and to pass. Snap was off, one edge moved into the space between dots, and the next command pulled that edge toward its neighbor. On a 1000-tick micrometer that half micrometer is 500 ticks. The sliver left behind can be the 200 ticks in the second picture, or it can close all the way and the etch joins the nets. Both started as a corner that did not sit on a dot.

After any import or any global snap, measure the gaps that used to be the minimum space. Read the white line in ticks before you trust the glance. Two hundred ticks is a miss you can still repair by putting the corner on the dot you choose. Use snap at the manufacturing pitch while you draw, so new corners are born on a dot. Turn snap off only to read a coordinate, and turn it back on before the next edit.