28 Feb
2017
28 Feb
'17
10:10 a.m.
On 2/28/2017 11:43 AM, Tony Duell via cctalk wrote:
I fail to see how anyone can be a good digital
designer and not
understand analogue
electronics.
A couple points:
* It is entirely possible I am not a good digital designer
* I believe I have a basic understanding of analog, but I may not be
competent.
To get back to the problem. What do you _actually_
want the output to become? An
analogue signal or a digital one?
A digital one. The current goal is to feed the
incoming data stream
into an Atmel AVR running at 5V for parsing. The final goal is to do
the same, but run the AVR at 3V on two coin cells.
If you want a digital signal then look at a comparator chip like the LM339. This
compares the voltages on the 2 inputs, the output changes state as they pass
each other (if you see what I mean).
I understand the comparator function, though I
will admit I have rarely
used one. As I had some success on the AVR internal comparator circuit,
I was able to play around with the bias and see how it affected the output.
So you cassout signal to one
input, a stable
voltage of, say 0.5V on the other. There are couple of gotchas with
this chip. The
first is that it has open collector outputs, so you need a pullup
resistor. The second
is that it tends to oscillate at switchover. You can add hysteresis
with a couple
of resistors to prevent this.
I saw the notes in the datasheet about the
oscillation, and the
recommended way to overcome this. I also noticed the Coco 1 schematic
includes this exact recommendation. I then wired up the exact circuit,
using the LM339 from my Coco1, and the exact resistors and other
passives as specified in the schematic.
Where do you get the 0.5V reference from? Well, if your 5V supply is stable you
can use that, divided down with a couple of resistors. Say 9.1k and 1k in series
across the supply, comparator input to the junction.
The Coco1 creates a 1.05V
reference via the 15K and 56K resistors. After
I noticed that the analog signal was centered around .5V, I quickly
added another 15K resistor in parallel with the first one, to create a
56K/7.5K divider, which yields .591V as a reference. I am using a lab
PSU, running at 5.00V, but the resulting circuit did not respond to
changes in the Coco cassette data signal. I rechecked my circuit last
night, but will recheck it tonight in case I made some stupid wiring
mistake.
Jim
--
Jim Brain
brain at jbrain.com
www.jbrain.com