Cells and instances

You want the same small pad in four corners of a larger drawing, each one turned so a finger points a different way, and you do not want to redraw that pad by hand.

Draw the pad once and name the drawing. That named drawing is a cell. Each time you place it on the larger sheet, the placement is an instance: same drawing, new position, and a rotation or a mirror. Lengthen the finger in that one drawing by 0.2 micrometers and all four stamps grow the same extra length, each still aimed its own way. If one corner must differ, flatten that one stamp into polygons on the parent and leave the other three as instances. The four pads you see are not four sets of copied polygons. The picture at the top of this page is the parent with those four stamps, and a cross of routing that belongs to the parent.

Open the child alone

The second picture is that child by itself, drawn large, with no parent and no routing cross.

Count shapes here and you are counting the master. The frame is the device region. The finger is the polygon whose direction shows how the stamp was turned. The contact square and the tab sit on their own sheets, and the tab is placed so a mirror cannot hide it. Count shapes on the parent, without stepping into the child, and you are counting the routing cross plus four instance records.

Change the child and every stamp follows. Shorten the contact square and all four contacts shrink. The parent does not store a second outline. It stores, for each stamp, which cell, where the origin sits, and which transform applies.

A database unit makes the storage concrete. If one micrometer is a thousand ticks, a pad 1.0 micrometer on a side is one thousand by one thousand, stored once. Each instance adds a point and a turn, not another copy of those ticks. A hundred placements still share the one pad. That is why a chip full of repeated contacts stays a small file until someone flattens it.

What the stamp is allowed to remember

Moving an instance moves the origin. It does not stretch the pad, and it does not bend the finger. The insides belong to the cell. If one corner of the parent needs a longer finger and the other three must stay short, you cannot get there by editing the stamp in place. You flatten that one stamp into polygons on the parent, then edit those polygons. The other three stamps remain instances and keep following the child. Flatten the one you must specialize. Leave the rest alone. When the corners need the same pad in different sizes rather than different turns, a parameterized cell with knobs draws one cell per parameter set instead of a family of hand-edited copies.

Names are part of the mechanism. The cell name is the drawing, so make it boring and unique. pad_flag will still mean something next month. A name like "copy of copy of pad" will not. A stamp may also carry a placement name, so a connection list can name one use without meaning every use. Two instances can share a cell and still have different placement names. The placement name is not a second cell.

On a printed circuit board the same split is a footprint: one land pattern, many placements on the board. A pcb flow often turns those placements into raw copper before the shop sees the film. Chip files usually keep the stamps. Either way, the drawing and the placement are different objects, and the property line should say which one you are holding.

A stream file writes one stamp as a reference to the cell plus a transform. It does not repeat the child's corners. A regular grid can be a single array record instead of many references. These four stamps are not that grid. Each finger is allowed its own turn, which is why each one is its own instance.

The edit that reaches every corner

The failure shows up when you meant to fix one quadrant. The upper-right stamp sits near a strap, and the finger is too long on that side only. You enter the child, shorten the finger, and all four fingers shorten. The other three corners no longer meet the straps they were built to reach. The screen still shows four pads. The file had one drawing, and you changed it.

Deleting feels safer and sometimes is not. Remove the upper-left stamp and the child remains in the cell list. The other three quadrants still display it. You deleted a placement, not a photocopy. If that placement was the only connection from a net to the pad, the net is open, and the drawing you thought you discarded is waiting to be placed again.

A group will not save you from either mistake. Grouping is a way to select several shapes together in the editor. It does not create a named cell, and it does not make other groups follow when you edit one. Making a cell moves shapes into a named drawing and leaves an instance behind. Turning an instance back into polygons is a separate command from ungrouping.

Before you call the file done, change the contact square in the child by a tenth of a micrometer and look again at the four quadrants. All four contacts must change together, each still turned with its stamp. If one contact stays the old size, that stamp was already raw polygons. Select it and read the property line: instance means it still follows the child, polygon means it has already left. Repair the stray while the routing cross is still there to tell you which shapes belong to the parent.