1-pin black and white TV?
Dennis Ferron
Posts: 480
With a high enough frequency and a small integrator cap, could the propeller generate black and white NTSC with just one pin? I have friend who was asked about the possibility of animating multiple screens for a mock-up of a 1960's NASA control board, with many monitors. I told him the simplest thing to do would probably to get five or six $20 black and white TV's, and use the Propeller to control it all. People have said that you cannot really use all 8 of the video hardware that comes on the cogs due to a lack of pins. With 1-pin black and white output, you really could get use of all of the video on all of the cogs (8 monitors). (I realize there are four groups of four, but you can also mask out bits and use the lowest bit for four and the second lowest for the others.)
Comments
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker
Propeller Applications Engineer
Parallax, Inc.
One could of course make deductions along the value of the resistors...
Sync, Black, White.
With two resistors you would also get Gray.
The third pin (with the highest value resistor) is used to create color.
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Teacher: What is the difference between ignorance and apathy ?
Student: I don't know and I don't care
Teacher: Correct !
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
www.hittconsulting.com
·
There's a brief description of how it works here.
Post Edited (ciw1973) : 7/13/2007 3:39:36 PM GMT
Clever though...
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Teacher: What is the difference between ignorance and apathy ?
Student: I don't know and I don't care
Teacher: Correct !
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
www.hittconsulting.com
·
You didn't explain that circuit, but my guess is Pic pin 13 would be exercised
between outputs HI, LO and input (=essentially tri-state), given the diode and
transistor biasing. Am I close?
I didn't study the levels, but might be usable at 3.3v drive.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Harley Shanko
h.a.s. designn
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Teacher: What is the difference between ignorance and apathy ?
Student: I don't know and I don't care
Teacher: Correct !
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
www.hittconsulting.com
·
It's not my circuit, just one I found a while ago.
You're spot on with how it works though, and if you follow the link I provided (click the word "here" in my post above) there's a brief description of how it works from the guy who designed it.
Are WE cheating using any resistors?
Yes, would be neat if NO external components were required. (Man, am I lazy?)
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Harley Shanko
h.a.s. designn
Again, agreed, but the COGs run fast enough for you to comfortably create the signal in code. In fact, I'd bet you could manage a couple of simultaneous TV outputs from a single COG using this circuit and some fiddly timing sensitive coding.
Now that'd be fun - 16 TVs running from a single PropChip!
This scheme would still work with just a voltage divider to set the black level when the I/O pin is open. The values in the voltage divider would have to be high enough to limit the maximum I/O pin current. A minimum resistor value of 1K would be more than adequate. The sync level would be a logic low output while white would be a logic high output.
With the video config register in VGA mode you could maybe generate your syncs from two pins and the luminosity for 6 different TV's off of each of the R, G, and B outputs. You could do all 6 TV's from one cog, and each bit of RGB in the bitmap would actually correspond to a black or white pixel on one of the 6 different televisions.
' Set up the output pins for white level, ' but leave in input mode for black level MOV DIRA,#0 MOV OUTA,videoOut :loop RDLONG DIRA,counter ' Hub timing sets pixel rate ADD counter,#1 DJNZ pixelCnt,#:loop ' At this point, you'd output a horizontal sync pulse ' and get ready for the next scan lineThen you could generate 7 tv outputs with the video hardware in VGA mode.
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Teacher: What is the difference between ignorance and apathy ?
Student: I don't know and I don't care
Teacher: Correct !
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
www.hittconsulting.com
·
Based on my own experience with the PropCAM grayscale output, I think you can do what you want -- with one pin and no extra semiconductors. To get grayscale, I use a counter's DUTY mode, which gets applied to (i.e. internally ORed with) one of the video output pins. What you need is a voltage divider that divides 3.3V down to 1V with an overall 75-ohm impedance (i.e. R1||R2 = 75 ohms). Between the tap (video output) and ground, install a 220pF ceramic cap. (Since there's no color, you might get by with a higher value than this.) For black-and-white (no grayscale) you will use the DUTY mode to set your backporch level, and it will be on all the time, except during syncs, so you have to bit-bang the DUTY for those. Within the displayed portion of a line, you can then use the video circuitry to output a 1 for light, and a 0 for dark. The ones will get ORed with the DUTY output, thus masking it, so the output will be 1V. When a 0 is output, the DUTY level takes over, and the output will be at the blanking (backporch) level. Don't worry about the difference between "black" and "blacker-than-black"; the TV monitor won't care.
If you want grayscale, you'll have to bit-bang it, instead of using the Propeller's video circuitry. But you should still be able to get resolution that's limited only by the memory available for the image.
Good luck!
-Phil