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  • Joined: 19 Aug 2011
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Sony CXA1145P
Post Posted: Sun Apr 01, 2018 10:35 am
Trying to better understand the CXA1145 better as I want to utilise it to create an RGBS to composite converter.

Where I'm lost is the subcarrier / oscillator / crystal section.

1) The datasheet species that the CXA1145 contains its own internal oscillator for the subcarrier so do I even need to use an external crystal?

2) If I do need to utilise an external crystal, do I leave pin 5 (XO OUT) disconnected and connect a crystal to pin 6 (XO IN)?

Have everything hooked up but getting nothing on screen. Have supplied power/ground to the chip. Fed RGBS to pins 2, 3, 4, 10. Running pin 20 (composite out) to the usual 220uF cap and 75R combo leading to composite input of my TV. Have the reference pins 13 and 14 grounded via necessary caps/resistors. Pin 7 is going to +5V for NTSC mode.

Not sure what to do with pins 5 and 6. Tried connecting them to each other to hopefully get the internal subcarrier going to no avail. Guessing this is where I'm going wrong. Thinking maybe I need to grab a 3.58MHz crystal and run that to pin 6?

I'm lost. A little help please.
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Post Posted: Sun Apr 01, 2018 11:50 am
Oscillator means circuit that uses crystal to oscillate. A crystal on its own doesn't do anything without an oscillation circuit (usually an inverter). CXA has an oscillator in it and you just connect a crystal to the relevant pins as the datasheet shows.

Now, the CXA1145 requires lot of external parts to give you a composhit output, there are two application circuits later in the datasheet, one for PAL, one for NTSC, those show minimum required configuration to get video output. Delayline and bandpass filters are what you're missing right now, without those you're not gonna get any sort of signal on the composhit output.

CXA1645 requires much less parts to function as do all later encoders, most even with almost same pinout.
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Post Posted: Sun Apr 01, 2018 1:01 pm
Thanks! I'll look into a more modern IC. Just have a few 1145s on hand and they're cheap/easy for me to get a hold of so thought I'd give em a go.

Out of curiosity, I hooked up only rgbs to a 1145 within an MS2 console. I get a clear black and white image. Don't even have the MS2 powered on so not sure how it's working.

Turned the MS2 on and get a colour picture but it's very unstable.

Think I'll try to follow the MS2 schematic relating to the 1145 and see if I can get it running outside of the console.
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Post Posted: Sun Apr 01, 2018 9:09 pm
In the SMS you can use the 3.58 MHz clock going to the Z80 to drive pin 6 of the CXA1145 for NTSC video. For PAL video you need a 4.48 MHz clock source. If you have a PAL SMS2 then that signal is available somewhere, perhaps from the I/O chip.

If there is no clock signal provided you'll get the black and white video you described earlier. That's OK, it's part of how color TVs and black and white TVs are compatible. But it's an indicator that the clock is either not present or is of the wrong value on pin 6 if you are aiming for color output.

This following schematic shows a lot of external parts needed to generate the CXA1145 clock, but I think that's all integrated into the I/O chip in later PAL consoles so it isn't really necessary:

http://www.smspower.org/uploads/Development/SegaMasterSystemServiceManual-schema...
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Post Posted: Mon Apr 02, 2018 12:21 am
Thanks for all the help guys.

Think I'll use a CXA2075. Requires fewer components and has its own internal trap filter.

Still not sure what to connect to pin 6, sub carrier input. Is this where the crystal connects? One pin of crystal to 5V the other pin 6?
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Post Posted: Mon Apr 02, 2018 4:47 am
CXA2075 has no internal oscillator you need to feed it a proper clock signal.
Crystal + inverter will work : https://en.wikipedia.org/wiki/Pierce_oscillator
Or a premade 3.579545 (NTSC) or 4.433619MHz (PAL) metal can oscillator
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Post Posted: Mon Apr 02, 2018 5:50 am
So I can use a 4 pin 3.58MHz oscillator and that's it? The square metal can thingy.

One pin to GND, one to +5V and one as the output which goes to pin 6 of the CXA2075? One pin is NC. That's what I gather from looking at the datasheet.
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Post Posted: Mon Apr 02, 2018 7:43 am
Yup, exatly. Freq must be exact or there's no color. 3.58MHz will not work, 3.579545MHz does.
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Post Posted: Mon Apr 02, 2018 8:27 am
Got it, thanks.

Re the freq numbers, was just rounding up for ease of typing. :)

Well I have a PCB design all finalised, followed the datasheet to the letter.

Will report back here in a few weeks time to let you know how/if it works. Fingers crossed.

Thanks for all the help. Truly appreciated.
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Post Posted: Tue Feb 19, 2019 6:07 pm
Hi...If there is no clock signal provided you'll get the black and white video you described earlier. That's OK, it's part of how color TVs and black and white TVs are compatible. But it's an indicator that the clock is either not present or is of the wrong value on pin 6 if you are aiming for color output.
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