replacing crystal with oscillator

[stolenfat]

Standard bending technique... replacing the 8mhz crystal with a square wave on a yamaha keyboard.

Rigged up a 40106 to replace a crystal but I can't seem to get the output hz high enough to match the 8mhz crystal.

.1 uf got me some where fun... but crashes the cpu
.001 uf gets me even better results but not quite there... so i kept trying smaller and smaller capacitors.

1pf is the smallest I have, and I guess from some research thats about as small as it gets. the 1pf actually does get the cpu clocking well but it's at a notably slower clock speed as the sounds it produces seem 'bit reduced' from the slower clock speed.

is there a circuit out there I could use to create an 8mhz squarewave? I know that's incredibly high.

[Grumble]

http://www.ti.com/lit/ds/symlink/cd40106b.pdf

the 40106 just isnt fast enough.
how i would do this is described here: http://electro-music.com/forum/topic-69874.html
but you need an arduino for it....

[stolenfat]

Arduino you say? Hmm! I just happen to have some laying around. Didn't even consider.

(BTW, Your keen advice has been very helpful lately. Ty buddy)


Alright... I'm still new to coding but I'd imagine I could find a way to get the chip to vary a clock pulse speed while checking an analog value.

Something like...

Set a pin to out put a clock at speed x
Set a pin to check an analog value
If that value is at 100 Percent, 5v, then put out 8mhz
When the analog value drops lower the clock speed and limit it to 4mhz or something.

Possibly check another pin with a switch to change the lowest possible clock speed.

Edit: some googling before bed leads me to believe Arduino will have some trouble outputting 8mhz. Some things I read mention the chip runs at a 16mhz while another source tells me some code that simply tells a pin to on and then off in the loop as fast as possible will output 2mhz.

[Grumble]

(see my previous post)
I would have the arduino program the AD9833 to output 8MHz, and if I want a frequency of 8.01MHz I would have the arduino to program the AD9833 to output 8.01MHz.
When you buy a breakoutboard with an AD9833 like THIS, you can program the AD9833 to a frequency well over 8MHz

[PHOBoS]

Most of the time the I see the LTC1799 used for this purpose but I've succesfully used the 74HC4046 myself
which gives you a voltage controllled frequency. You do need the 74HC(T) version though as the standard CMOS
version isn't fast enough.

[stolenfat]

thanks for the replies mates - i think given the research i looked into and the knowledge you all provided i think I'll end up mangling something simpler together. Since it's a circuit bent project I dont really wanna buy/order more stuff to make it so. Takes the spirit out of it, no? I'll save my spare arduinos for something else as I think this project will be fun to just explore and see what happens.

I'm pretty determined to just add a switch to go between my best substitute clock circuit, the 40106 with a 1pf cap (the 4046 i tried didnt event get close to matching the rate of the 40106), and the crystal. It may result in the cpu crashing more but that's what circuit bending is all about. Thanks again for all the input.

There are two TC40H00P ic's on this sucker that seems to react interestingly to 0 and v+ voltages. The datasheet claims they are 4011 clones and I'm considering desoldering the chips and replacing them with something else.

@phobos: If my memory serves right, in the past didn't you wizard up a way using a muxer, maybe a 4051, that you could manipulate to act as a nand/nor/xnor or what have you based on some switch statuses? (I gotta do more digging)

[JovianPyx]

+1 for a 74HC4046. It is plenty fast enough to do 8 MHz, can be voltage controlled and outputs a proper square wave to be used as a clock.

[PHOBoS]