TI 990/189 debugging

Josh Dersch derschjo at gmail.com
Fri Dec 30 20:53:42 CST 2016

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.


On Thu, Nov 17, 2016 at 3:05 AM, Eric Smith <spacewar at 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.

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