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alec(at)alecmyers.com
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 Posted: Wed Oct 18, 2017 6:44 pm Post subject: Constructing an automatic RCA video camera feed splitte |
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I had in mind that all outputs engage weak pull-ups and are driven either LOW or HIGH-Z at the PIC. I think that’s a better way of driving a CMOS gate from a PIC. Unfortunately I had forgotten my own advice there aren’t enough port pins with weak pull-ups in the 16F630, so that would require external discrete pull-up resistors.
However, the 16F18323 is the same 14 pin package as the 16F630, for the same price (it’s actually cheaper in bulk), and has configurable internal pull-ups on all port pins. It has a deeper stack and a bigger instruction set, and crazy amounts of peripherals. None of which we actually need, but it just boggles my mind what you can buy for $1.
So if we use that, we can bit-or to our heart’s delight.
| Quote: | On Oct 18, 2017, at 10:00 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
At 07:08 PM 10/18/2017, you wrote: | Quote: | | --> AeroElectric-List message posted by: Alec Myers <alec(at)alecmyers.com (alec(at)alecmyers.com)> I donât think thereâs an impedance problem - the inputs to the video switch IC are digital. So yes, I expect it would work fine to connect multiple pic outputs to one camera select input. You can have a camera that is displayed twice as often (or twice as long) if you really want, by using 2 PIC outputs connecting to one video select input. But youâd have to solder some green wires on the PCB to make it happen. |
Unless the outputs are open collector, active low with pull-up resistor, they cannot be wire-or'd. They have low impedance paths to either Vcc or Vdd at all times.
Bob . . .
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Eric Page
Joined: 15 Feb 2017
Posts: 242
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| Posted: Wed Oct 18, 2017 10:04 pm Post subject: Re: Constructing an automatic RCA video camera feed spl |
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Gentlemen, we have a problem. Unless I'm very much mistaken, the CD4066 is not going to work. I'll be quite happy if someone will tell me I'm wrong, but...
In the CD4066BE datasheet, at paragraph 8.1 it says,
CD4066B has four independent digitally controlled analog switches with a bias voltage of VSS to allow for different voltage levels to be used for low output. Both the p and the n devices in a given switch are biased on or off simultaneously by the control signal. As shown in Figure 17, the well of the n-channel device on each switch is tied to either the input (when the switch is on) or to VSS (when the switch is off). Thus when the control of the device is low, the output of the switch goes to VSS while when the control is high the output of the device goes to VDD.
I read this to mean that we can't tie eight outputs (from two ICs) together into a common VIDEO OUT bus because one "on" channel will be driving a video signal into seven others that are sitting at -5V. This IC is four independent analog switches; tying their outputs together looks like a recipe for silicon smoke.
It gets worse. At paragraph 8.2 (3) it says,
Normal operation control-line biasing: switch on (logic 1), VC = VDD; switch off (logic 0), VC = VSS. [VC is control voltage.]
So, the switches in this device don't turn fully off at 0V; they must be driven to Vss (-5V). Obviously, the PIC can't do that. At paragraph 9.2 it presents a typical application very similar to this effort, and suggests using the CD4054B CMOS LCD Driver to level-shift 0 to +5V digital signals into -5 to +5V signals. That's two more 14-DIP ICs, but it doesn't solve the first problem.
I think we're stuck with the AD8184, as it's a proper 4-to-1 multiplexer.
Thoughts?
Eric
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