As we prepare for the New Year, we wanted to leave you with a piece of logic taken out of an older PIC16C series microcontroller. We want you to guess which micro(s) this gate (well the pair of them) would be found in. After the New Year, we'll right up on the actual micro(s) and give the answer :).
An AND gate in logic is basically a high (logic '1') on all inputs to the gate. For our example, we're discussing the 2 input AND. It should be noted that this is being built from a NAND and that a NAND would require 2 less gates than an AND.
The truth table is all inputs must be a '1' to get a '1' on the output (Y). If any input is a '0', Y = '0'.
The photo above shows the schematic layout using P and N type FETs. A P-FET is conducting between the source and the drain when a logic '0' is presented on its gate. The N-FET is the exact opposite (a '1' conducts).
As seen above, there are 2 signals we labeled 'A' and 'B' routed in the Poly layer of the substrate (under all the metal). This particular circuit is not on the top of the device and had another metal layer above it (Metal 2 or M2). So technically, you are seeing Metal 1 (M1) and lower (Poly, Diffusion).
It's quickly obvious that this is an AND gate but it could also be a NAND by removing the INVERTER and taking the '!Y' signal instead of 'Y'.
The red box to the left is the NAND leaving the red box to the right being the inverter creating our AND gate.
The upper green area are PFET's with the lower green area being NFET's.
After stripping off M1, we now can clearly see the Poly layer and begin to recognize the circuit.
This is a short article and we will follow up after the New Year begins. This is a single AND gate but was part of a pair. From the pair, this was the right side. We call them a pair because they work together to provide the security feature on some of the PIC16C's we're asking you to guess which ones :)
If you have Photoshop, here is a link to a Photoshop image we created for you that you can control the layer opacity to see the remove the top metal to see how the poly and M1 layers connected virtually.
Happy Holidays and Happy Guessing!