Flashback : Measurements

A project log for TI-57 Programmable Calculator Hardware Retrofit

A modern MCU-based hardware replacement for the venerable TMC1501 used in the Texas Instruments TI-57 LED calculator.

tomcircuittomcircuit 02/28/2024 at 22:114 Comments

I carefully opened a TI-55 donor unit and inserted a 27ohm resistor in series with one of the digit cathodes. I captured the display sequence on my trusty LeCroy oscilloscope and dumped to the built-in thermal printer - TI-55 LED timing and Vfwd measurement

There are several important observations that can be made from this plot:

Thank you, TI-55.  That didn't hurt a bit now, did it?


Ken Yap wrote 03/01/2024 at 00:31 point

I guess it saved them some ICs. I remember that the early one chip calculators needed driver ICs. One drawback is the need to have displays where all the corresponding segments are wired together rather than all the segments of a digit. But then TI also made displays at the time so no problem.

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tomcircuit wrote 02/29/2024 at 19:53 point

Ah, here it is - the PMOS for segment drive has its source connected to VSS (+9V) so it's a good high side anode driver, able to handle multiple anodes simultaneously.  The PMOS for digit drive is more of a "source follower" configuration so it's much happier sinking a smaller amount of current.  This is described well in column 4 lines 17-52 of the patent text.  Of course, a Common Anode display would be back to "digit scanning" vs. "segment scanning".

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tomcircuit wrote 02/29/2024 at 02:12 point

The patent hints at some advantage to segment scanning and integration. I can only imagine that it has to do with PMOS source drivers for the segments, versus lateral PNP sinks for the digits. But, then, just go from Common Cathode to Common Anode configuration. I agree that the 12.5% duty vs 8% duty is an advantage. Another thought: perhaps TI had to go with segment scanning because someone else patented digit scanning?   There’s a whole TI patent devoted to a 3-bit counter that counts in an unusual sequence 4,277,675, and for the life of me I can’t fathom what purpose this serves! 

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Ken Yap wrote 02/29/2024 at 00:59 point

Not sure what the advantage of muxing by segment is compared to muxing by digit. I suppose it limits the duty cycle to no less than 12.5%.

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