video generator trouble

agimuhing

I recently got the propeller chip and I'm currently fiddling around with the video generator.
The code I'm using is from the tutorial in the propeller help files. I also changed the code to use a 20 mhz crystal

I can get a signal off the propeller with the setup I used below, but it looks like noise. the tv goes blank when I unplug the propeller, and displays something that looks like noise when I plug the propeller in.

I'm using a 20 mhz crystal and have a 1k resistor connected to pin 12, 470 resistor connected to pin 13, and 270 resistor connected to pin 14.
the rca jack has wires soldered to it and connected to the breadboard.

I can't figure out why I can't get a usable signal off the propeller. I thinks its something with the following: crystal, resistor values, rca jack

any help is welcome

Cluso99

agimuhing: Firstly, welcome to the fantastic world of the prop. Here you will get a lot of help so just feel free to ask.

Now for your circuit.

If I am not mistaken, 20MHz xtal is too high (PLL=4). This is because 20MHz is actually x16 then /4. Perhaps the PLL cannot lock properly and this is causing jitter resulting in video noise.

Breadboarding often also produces noise such as you have found. It may be good to place an 0.1uF capacitor and a 10uF tantalum capacitor across both sets of prop pins power. Phil has determined that a 191R should be placed across the RCA terminals (i.e. from the output of the resistor junction to ground. A 220R or even 270R may help. A cap across this (forget value) may also help. Search Phil Pilgrim threads.

potatohead

With video, it's very important to insure the baseline, reference implementations are working. Have you setup with a 5Mhz crystal, and the PLL setup properly?

Many of the drivers will not operate well under 40-60Mhz. A 20Mhz crystal isn't quick enough for anything but some basic, low resolution bitmap or text. 40Mhz may run a few of the drivers, but it's recommended to have 64mhz + to make the Parallax ones run. Running too fast is the same problem because the internal PLL will be driven at too high of a frequency.

The other important thing to do, particularly if you don't have a scope, is to change ONE thing at a time. Get a reference setup working, then modify it slowly, checking signal along the way. Multiple issues are very difficult to debug. If you have a scope, it's easier. You can measure the signal and check levels. Still not a walk in the park though.

If I were you, I would setup for 5Mhz crystal, then run the Parallax video driver to start, after configuring it for your particular pin setup. Verify signal, then explore options from there.

Joms

I just went back and looked at the TV Terminal demo, but I thought someplace I remember reading that the TV object would only work on the 5MHz crystal. Like I say, I can't find where that is in the documentation, but I thought I remember reading that somewhere... Maybe someone else remembers seeing that...

agimuhing

thanks for the info

Cluso99

Joms: The object needs adjusting if you use a different clock other than 80MHz (because, provided yo do things correctly, a 5MHz x16 = 10MHz x8 but 20MHz is too high a frequency for the PLL). I use 104MHz and provided I adjust the dependencies on clock freqency everything is fine - in fact we can get higher resolution.

ericball

Just to clarify, the system clock PLL always runs at 16x, then there are taps for lower multipliers. So although a 6.25MHz crystal x 16 = 100MHz clock is generally okay, you can't use a 20MHz crystal even with a 4x multiplier because 20MHz x 16 = 320MHz is way over the norms. You can run without the PLL (so 20MHz), but many video drivers won't work at that low a clock rate; and if they do work, it will be at a lower horizontal resolution.

agimuhing

I took out my spare propeller chip and used the 20 mhz crystal with a frequency synthesizer program to get 5 mhz

It works now