Just to bring some closure to this old thread, I finally picked up a working TMS9980 cpu chip (after getting one faulty one off of eBay -- it was even more dead than the one it was replacing). And it appears to work properly in the 990/189 board, with the 9.3V supplied on the CPU socket.
Thanks, Josh On Thu, Nov 17, 2016 at 3:05 AM, Eric Smith <space...@gmail.com> wrote: > 9.3V might actually work fine for a TMS9980, even though it's below spec. > It's not going to damage the part, so it may be worth a try before > modifying the board for 12V to the CPU socket. > > In NMOS digital parts that predate depletion loads, Vdd needs to be > significantly higher than the most positive logic level in order to bias > the enhancement nFET used for the loads (pull-ups). The Vdd voltage > doesn't have to have a precise value, but it needs to be somewhat more than > the FET gate threshold above the most positive logic level, and below the > breakdown voltage. The higher the Vdd voltage (below breakdown), the > faster the pullup will operate, so running below spec will reduce the > maximum speed at which the part will operate. This is also dependent on > temperature. The part is spec'd for operation over a fairly wide > temperature range (even if only "commercial" rated). Since the logic high > level is no more than 5.0V, and generally somewhat less, a Vdd of 9.3V is > probably more than adequate at room temperature, but may fail at > temperature extremes. > > The MP9529 is a "selected" TMS9980. In most contexts, a "selected" IC is > one that has been tested and found to meet specifications more stringent > than the normal specifications. However, in this case I think the MP9529 > might actually be "selected" in the sense of being tested to *lower* > specifications than a standard TMS9980. It's unclear why they would want to > use the lower Vdd, except possibly to reduce power consumption. > > With the introduction of depletion loads in later NMOS ICs, generally > starting around 1976, and becoming ubiquitous by 1980, the requirement for > a supply above +5V was eliminated. Similarly, by adding an on-chip > substrate bias generator, the need for an externally supplied substrate > bias voltage (Vbb, typically -5V) was removed. >
