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I want to use a 555 timer (as I have them at hand) to give me a 2MHz clock for an AY-3-8500.

I have two questions really:

  • While the 555 may not be ideal, is it possible?
  • How to I calculate the required R1/R2/C values by using the frequency required? All I have found so far you have to provide the values to get the frequency.
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  • \$\begingroup\$ What do you mean "While the 555 may not be ideal"? why it is not ideal? Also, can you attach a link of the datasheet you are looking for the 555 IC? \$\endgroup\$
    – Christianidis Vasileios
    Dec 6, 2022 at 10:09
  • \$\begingroup\$ 2MHz is right at the limit of a standard 555 timer (as metnioned in this question: electronics.stackexchange.com/questions/558445/…) so in theory it is possible, but there are lots of different 555s out there with subtly different specs \$\endgroup\$
    – Puffafish
    Dec 6, 2022 at 10:19
  • \$\begingroup\$ If your tag ne555 is correct, 2 MHz will be really really at the upper end of what your chip can do, and you might get less reliable amplitude, less stable frequency and generally insufficient performance. If you have a CMOS 555, for example a TLC555 or similar, this might be less of a problem. Nevertheless probably wiser to actually use your colorbust clock to generate a pixel timing that works for your TV set – assuming that's not a 1980's fully analog TV. Plus, if your 555, whichever you use, wanders, your TV becomes a visualizer for that inaccuracy: diagonal "vertical" lines. \$\endgroup\$
    – Marcus Müller
    Dec 6, 2022 at 12:43
  • \$\begingroup\$ Ok, thanks, it's ne555 chips that I have. But I get the feeling from what is suggested that it will not be stable enough and/or on it's limits. thanks all :) \$\endgroup\$
    – Simon
    Dec 6, 2022 at 14:09

2 Answers 2

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  1. Yes, sure it is possible to generate 2 MHz with the CMOS 555 version.

But your chip is likely intended to have more precise and stable clock than what is possible to provide with 555 so it is very likely that you are not happy about the result if you try this. There are much better ways. Just buy a suitable crystal oscillator for example.

  1. Read the data sheet of your specific 555 version. It will have the instructions and formulas to calculate the best values within suitable range for your specific chip.
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  • \$\begingroup\$ Perfect, thanks for the clear answer, much appreciated :) Have decided to go down the 2MHz Crystal Oscillator route as shown here : eleccircuit.com/simple-crystal-oscillator-circuit Mainly because it was the first I found, and I only have to wait for the crystal, as I have everything else :) \$\endgroup\$
    – Simon
    Dec 6, 2022 at 14:14
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You should probably think of ~100kHz as maximum for NE555 and ~500kHz for the CMOS versions (assuming suitable values are chosen for both the capacitor and resistors. Above that it starts to become very part and manufacturer-dependent. And maybe 1/10 of that if you want the output frequency to be very predictable and stable (by 555 standards).

Since the antique AY-3-8500 runs from a 6-7V Vcc it's not convenient to use off-the-shelf oscillators. But it will accept a 0/5V CMOS input if you use a 6.5V Vcc and sub regulate it down to 5V for the oscillator, using an LDO. Alternatively, there's a crystal oscillator circuit using a 4001 which I would not recommend since 4001s are hard to find (4001B is not the same) though you could use a 4007, and an LC oscillator using a 90uH inductor, both on the datasheet. 90uH is a bit large for an air-core coil, but totally do-able if you have a few square inches to spare. It will work with any general-purpose NPN transistor such as 2N2222:

schematic

simulate this circuit – Schematic created using CircuitLab enter image description here

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