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  • Who knows about capacitors?
  • DavidB
    Free Member

    I have a schematic for a USB keyboard interface I want to hack together. I know **** all about proper electronics but have a matrix board and a will to experiment. This is going to connect an old Sinclair Spectrum keyboard to a Raspberry Pi

    The schematic has 3 x 100 pF capacitors on it, but no mention made of their voltage. The circuit is 5v DC (I guess as it is USB)

    The schematic is here on page 21 any idea which ones I should buy? In Maplin today they had loads but differing voltage ratings..or does it not matter?

    ps. How do I wire up the MCUVDD line?

    peterfile
    Free Member

    IanMunro
    Free Member

    As a rule of thumb go for about 2x the maximum voltage you need – more doesn’t matter though.
    BTW There’s no 100pf caps on that schematic that I can see.
    The 0.1uf capacitors are 100nf.
    You want ceramic capacitors.
    Something like this

    http://uk.farnell.com/multicomp/mcrr25104x7rk0050/cap-ceramic-0-1uf-50v-x7r-rad/dp/1216438?Ntt=1216438

    The X7R part refers to the accuracy of the component(+/10%). Not really an issue on the 100nf parts, but possibly important on the 33pf caps.
    The 10uf doesn’t have to be accurate, it’s just a big bucket for holding electricity local to the circuit.

    Anything labeled MCUVDD gets wired together, so in the case of p21 the ‘MCUVDD’ coming out of the 22R resistor gets connected directly to pin 15 VDD of the chip. The reason they are not shown wired together on the schematic is just to make it clearer to see the other signals. You can see the same being done with the 0v/gnd lines.

    IanMunro
    Free Member

    I just looked on maplin site, but it’s too retarded to use.

    Try proto-pic they are far better for this sort of thing

    33pf
    http://proto-pic.co.uk/capacitor-ceramic-33pf/
    100nf
    http://proto-pic.co.uk/capacitor-ceramic-0-1uf/
    10uf
    http://proto-pic.co.uk/electrolytic-decoupling-capacitors-10uf-50v/

    Good luck!

    DavidB
    Free Member

    IanMunro, I will (if this works) owe you pintage. Thanks!

    Waderider
    Free Member

    Just to echo IanMunro a bit, I recommend Farnell rather than Maplin. They have a minimum order of £20, but that just encourages you to get keen and do more electonic tinkering!

    DavidB
    Free Member

    The proto-pic site is the business. No minimum order size. Soon the RaspberryPiSpectrum shall arise.

    maxtorque
    Full Member

    As mentioned above, those 0.1uf caps are just “decoupling” capacitors for the ASIC. There ratings is really fairly unimportant (and the circuit would probably work without them btw, they are generally put in for “good practise”). As the whole system is powered from the USB socket, this makes large input voltage spikes somewhat unlikely. So, any 10v rated caps should do the trick. Ideally, you want low ESR (Equivalent Series Resistance) ones so they can supply the sudden “Bursts” of power the ASIC might require without the supply voltage sagging. However, pretty much any small 0.1uf cap will have a relatively low ESR anyway. More important is that they are mounted /wired as close to the ASIC pins as possible to avoid parasitic inductance in the wiring.

    The 22ohm/33pf resistors and caps are “Low pass” filters, designed to help stop any noise picked up on the USB data line from being interpreted as signal edges and hence mucking up the serial transmission etc. Again, you will probably find these aren’t needed.

    One thing not shown in that schemeatic is any ESD (Electro Static Discharge) protection for the data / power input lines. The ASIC datasheet may (i haven’t read it fully) specify what overvoltage protection is internal to the device, if none, then you may want to add some TVS (transient voltage suppressors) to the input lines to prevent you zapping it when you handle the USB connector etc (the human body carries a lot of static electricity!)

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