5 ms·
Transistor sizes are easy to extract from photos. What's hard is doping levels, which can make a transistor enhancement or depletion. The type is usually obviou
by kens 6y ago
Transistor sizes are easy to extract from photos. What's hard is doping levels, which can make a transistor enhancement or depletion. The type is usually obvious from context. However, the Z-80 designers famously used the "wrong" doping for a few transistors. The purpose of this was to create traps for companies copying the IC from the die, since a straightforward copy would yield a few gates that didn't work right.
I've found a few of these traps in the Z-80, and they were placed very maliciously. The circuit appears reasonable and correct, but fails in subtle ways.
- bonzini 6y agoHave you blogged about it?
- kens 6y agoI haven't blogged about the Z-80 traps. I was waiting until Sergei wrote about the transistors, but then didn't get around to it.
- raphlinus 6y agohttps://www.cl.cam.ac.uk/~sps32/Z80proj/Z80_traps.pdf https://www.cl.cam.ac.uk/~sps32/Z80proj/Z80_traps.pdf has very high quality imaging of the doping levels, by Sergei Skorobogatov. https://retrocomputing.stackexchange.com/questions/11143/in-the-original-z80-layout-where-are-the-traps-located-and-what-are-their-ef https://retrocomputing.stackexchange.com/questions/11143/in-... references "Zilog oral history" and poses the question, but there isn't a strong answer other than the link to the above. The oral history document referenced is: http://archive.computerhistory.org/resources/access/text/2012/07/102658025-05-01-acc.pdf http://archive.computerhistory.org/resources/access/text/201... . The discussion of traps begins on page 101.