Replacing an oscillator circuit with a clock source? - Page 1
Forum section: electronics / beginners

author: zment
author_rank: Newbie
timestamp: November 01, 2020, 11:18:43 pm
content: So, I'm doing a 8bit homebrew computer/console project as you are wont to, and I recently had a little setback (my CRT finally gave out and won't turn on anymore) which requires me to add an RGB to Composite encoder chip to my setup to continue working with the project while I either fix the CRT or source another one. I thought I had all the needed parts to do it and had figured it all out, but apparently I'm missing 4.43 MHz crystal oscillator it expects for PAL output. I need the composite out to connect to my capture card to continue seeing the GPU design output.I however do have a 17.73 MHz crystal, and I could drive an ATtiny84 (that's already involved with generating the RGB signals in the first place) with that and create a 4.43 MHz clock source with it. Would it be possible to drive the chip with a clock source (ie. square wave) instead of the expected oscillator? The chip in question is an MC1377 (Datasheet) RGB-to-Composite encoder chip.I tried searching google for replacing a crystal oscillator with a clock source, but didn't really get anything. My google-fu must've failed me or something. Oh and if it matters, according to the datasheet the OSCin and OSCout in the MC1377 are expected to be connected to a Colpitts oscillator, whatever that means, unless I misunderstood something. In the reference designs the crystal sits there with a pF scale capacitor on either side like in an AVR.I'm a bit bummed by this to be honest, I just had got to a part where my current "hardware design" was adequate and I was just writing software for sprite handling and how to access the GPU etc.Thanks for any and all help and hopefully I managed to give enough information!

author: ledtester
author_rank: Super Contributor
timestamp: November 02, 2020, 02:55:07 pm
content: My guess from looking at the datasheet that you can in the same way that microprocessors and microcontrollers can be clocked from oscillator modules, an Arduino GPIO pin or even a 555 timer.

author: Ian.M
author_rank: Super Contributor
timestamp: November 02, 2020, 03:20:51 pm
content: See datasheet page 9. It needs an AC coupled sinewave, between 0.5V and 1V pk-pk, so you'd lowpass filter the 4.43MHz squarewave from your logic level clock generator, choosing the filter components to get the correct level at the OscIn pin.

author: zment
author_rank: Newbie
timestamp: November 03, 2020, 08:03:44 pm
content: Thank you both for answering.Quote from: Ian.M on November 02, 2020, 03:20:51 pmNow that I read the page 9 of the datasheet again, I realize the oscillator is not for timing the chip, but for generating the color subcarrier wave - either as a reference or used as is, I'm not sure which. So what you are in essence saying is I could drive this with a square wave from an AVR, but I'd need to filter it to generate sinewave and lower the voltage to an acceptable level? Hmm. I wonder how precise I need to be with the sine wave, perhance only a single pass would work, or maybe even straight up square wave when brought down to 1Vpp.Thanks for the help!