Generating three voltages with a MCU

Question

I'm going to try and generate black and white NTSC video, using a micro-controller. According to this:

https://instruct1.cit.cornell.edu/courses/ee476/video/

It is possible to do it using two pins of Atmel Mega644. I looked at the datasheet and noted that the IO is 1.8 V. For NTSC you just need three voltages sent at the correct time, 0V, 0.3V, and 1V.

What I don't understand is how does this circuit generate the 0, .3 and 1V. When I do the calculations it doesn't really add up.

I simulated this using TI TINA simulator, 2 batteries and switches inplace of the IO ports, and trying all four combinations of the IO, I can get 0, .3V but not 1V.

Can someone using math explain how you can get 0 V, .3 V, and 1 V using the circuit shown in the link above. I also, I guess what I don't understand is the uC IO equivalent circuit for simulating in a SPICE program, is it just a battery set to the IO voltage? Or maybe an OP AMP with the output set to the IO voltage can someone please explain this.

Any help is appreciated.

Answer 1

Your calculations are then incorrect.

The ATmega644 works fine with 5 V (which is also close enough to the output voltage of the microcontroller which itself is rated at minimal 4.2 V on page 316 of the datasheet) and at that voltage using a \$ 330 \mbox{ } \Omega \$ and \$ 75 \mbox{ } \Omega\$ voltage divider, you get 0.926 V which is close enough to 1 V for most purposes.

Using the \$1 \mbox{ } k\Omega\$ and \$ 75 \mbox{ } \Omega\$ voltage divider, you get around 0.349 V which is for most purposes close enough to 0.3 V. And when both pins are off, you get 0 V.