Thu Dec 11 21:56:19 CET 2008
demos for dorkbot
I'd like to do the synth + the TV.
For the TV I have 13.5 Mhz xtal + now there's the bitbanged monitor
freeing one of the serial ports for raw binary output. I don't
remember which chip can do this though..
The serial port seems to be this one: in 18f2550 datasheet, section
20.3 synchronous master mode.
The 13.5 Mhz is standard video pixel clock. Can probably use one of
the usb devices they have higher clock ratings.
So... A better TV out circuit. What i tried before is to use the 75
ohm impedance as a voltage divider, to not have to use a 75 Ohm output
impedance. This is a dirty trick yeah..
So, to do it properly, generating the 4 levels (0V : 0.33V : 0.66V :
1V) requires a buffer amp to isolate the load impedance from the
resistor divider. This frees resistor values a bit, but requires a
rail-to-rail amp if fed from a single supply. For demo purpose it's
probably best to stick with the two simple resistors.
Asked Bert: To do it properly, provide double levels + add a unit
buffer opamp with a 75 ohm resistor in series (this assumes line is
properly terminated), maybe also add an output transistor to deliver
Non-terminated: it's possible to get the line levels always correct if
the buffer has zero output impedance.