HP-15C Speed-up

[matthiaspaul]

(01-18-2016 11:17 AM)jebem Wrote:  So you have added a inductor in parallel with the existing one, hence decreasing the final inductance value.
That's a good idea as in this way the modification job is fast and easy to implement.

This reminds me of my old Sharp PC1403. ;-)

One of the modifications I did for this calculator was a "turbo-mode". By default, the processor was clocked by a 768 kHz ceramic resonator and there were patches circulating replacing the resonator by one with a higher frequency or by an external oscillator, so I knew chances were high that the processor would continue to run stable at twice the speed or more. However, I didn't want to add a separate active oscillator for size reasons [*] and to not increase the power consumption more than necessary (about linear with clock). Also, I didn't want to permanently replace the resonator in order to be able to switch back to the old speed in case there would be baud rate problems with serial communications. Finally, switching between two resonators was no option for me as well as it would have required a double-throw switch, when all I could still fit into the case was a recessed miniature single-throw switch (operated by a paper clip sidewards through the opening for the default interface connector, as I didn't want to cut a visible hole into the case just for the switch).
By looking at the resonator's equivalent circuit diagram I figured it should be possible to make it resonate at a much higher speed by adding an inductor in parallel (and in series with the miniature switch - something that wouldn't work for much higher speeds, but still worked fine at those low frequencies). After experimenting with a few inductors from my spare parts collection, even the baud rates matched nicely at a factor of 2, but I found it more reliable to switch back to the default speed when doing longer serial communications.
(I don't think I had to add a capacitor as well, but I'm not completely sure about that any more after all these decades - unfortunately, I can't find my old notes right now.)

Greetings,

Matthias

PS. [*] I had managed to fit several other extensions into the case like a bank-switched 72 KB memory expansion (with corresponding software extensions hooked into the Sharp's OS I had reverse-engineered and for which I had written various development tools like a debugger, disassembler and assembler) as well as a second serial interface (with complete hardware handshake), so there really was little space left.