Jim,

On Mon, Jun 29, 2009 at 01:17:11AM -0500, Jim Brain wrote:
>> The read, write and sense can be mapped to GPIO.  The C2N232 makes use of
>> the input capture and output compare timer features, but that is not
>> strictly necessary.  It would be good to map CASS WRITE to a pin that can
>> generate an interrupt.
>>
>> The motor voltage is not TTL but something between 6 and 9 volts.  In the
>> C2N232, this voltage is halved by a resistor divider (4k7+4k7 if memory
>> serves).
>>
>> If possible, it could be good to use distinct I/O lines for sense, read and
>> write on the pass-through port.  (Just connect these lines in the AVR
>> firmware, e.g., by using the Pin Change Interrupt.)  If this is not
>> possible or feasible, you should be able to connect the pass-through port
>> in parallel.  I'm not sure of CASS READ, though.  You'd better check the
>> schematic diagram of a tape drive.

> My apologies for the delay, but I'm jsut now getting to this.  Here's my  
> thoughts:
>
> http://www.jbrain.com/vicug/gallery/c2npower
>
> A simple 'carrier' board that I can immediately use as a power tap, but  
> has through-hole parts, so it can be stuffed by anyone and used for  
> various projects.

Makes sense.

> Now that I've put the schematic up, I realize I probably need to feed  
> MOTOR into the uC (via a voltage divider) so the uC would know when the  
> system is asking for data.

Yes, I would definitely connect the MOTOR signal as well.

> Any thoughts?

What are jumpers SJ1, SJ2, SJ3 for?  You could label all signals.
Also, Kicad <http://kicad.sourceforge.net> is a free (as in speech)
alternative to Cadsoft Eagle.  I haven't looked at yet, but if it is
good enough for a mobile phone <http://wiki.openmoko.org/wiki/Gta02-core>,
it should suffice for any hobby project.

What about a small prototyping area or a header for all (or unconnected)
pins of the microcontroller?  The circuit could be more useful if you
could connect it to something else (e.g., computer, or memory card and
at least one switch).  I think that a tape drive emulator requires a
PLAY button with which you can initiate the transfer (or select tracks).

If you boot up the computer with SENSE=0, it will "think" that you are
trying to rewind or fast-forward the tape, and it will be supplying power
on the MOTOR line.  That is why you need to control the SENSE line in some
way.

> I am trying to balance my immediate need (power tap) with the fact that  
> PCBs cost pretty much the same, regardless of whether there are holes  
> drilled or not.  I did not put Xtal or caps on the design, assuming that  
> one could trim the built-in oscillator for satisfactory operation, but I  
> could be persuaded to add them if folks think it would provide a lot of  
> benefit?

An external crystal should work without capacitors.  The internal oscillator
should be fine, depending on what you are trying to achieve.  It might not
be accurate enough for high-speed UART, but I would assume that SPI or I2C
or accessing external memory is not that sensitive.  My only experience in
using the internal oscillator is a infrared remote control receiver
<http://www.iki.fi/~msmakela/electronics/worc5/>.

	Marko

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