LZ: TI 85 Memory Expander

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LZ: TI 85 Memory Expander

On Wed, 11 Sep 1996, Mel Tsai wrote:


> I thought about this, but I figured it would be easier to do it the
> way the TI-85 does it.  When you send a file to the expander, you send
> a checksum byte to it (and then read it back to make sure there were
> no errors in the checksum byte).  Then, when you want to reload the
> file back to the TI-85, compare the checksum of the file with the
> checksum byte, and if there was a difference you should read it again.
> Of course, the file itself may have been corrupted during
> transmission, so the parity checker may be necessary!  Anyone have any
> suggestions to do a hardware parity check?
> 




> >Now that I think about it, a toggle flip-flop might do the
> >trick.  
> >
> 
> What exactly do you mean by this?


This is what I was thinking for a parity check in hardware.


Since we might be checking theparity of 15+ bits (I am not sure exactly of
the number of bits.  I would check the docs, but they are at home and I am
at school) and the fact that the data is coming in serially, I figure a
T-F/F would be the easist way of checking even/odd parity of the incoming
bit stream in hardware.  


I would connect the T-F/F's clock to the clock (of course), and the input
to the current incoming bit.


Before any data is received, clear the F/F


Then if the bit it high, toggle the F/F, if the bit is low, don't toggle
it.  If the data was odd parity, then the F/F would be high at the end of
the bit stream and if the parity was even, the F/F would be low.


Basically a T-F/F's equation is:


Q(T+1) = TQ' + T'Q


I am not sure if this would be a "correct" way to do this, but from what I
see, it should work.  Since we are talking about low clock speeds,
propigation times shouldn't be a problem.


There should be IC's with T-F/F's.  You could also do it with And/NOR
gates.  The circuit that I have in my digial logic book has 2-Three input
AND's and 2-2Input NOR gates.  If you need the block diagram, ask me to
post it.


Anyone else have a suggestion?




> 
> >Another comment:  The power requirements for the board should not be much.
> >I have powered a 68HC11 circuit w/ various IC's with a 9V battery and a
> >voltage regulator. I'd say try it with a 9V Battery.  
> >
> 
> Remember, though, the 68HC11 is a new generation of extremely low
> power microcontrollers, so the power requirement may be a bit
> different.  However, it will probably work on a 9V battery, but it may
> not have extremely long battery life (as compared to using AA's or
> AAA's.).
> 
True, the battery life may not be so hot.  I never let my circuit run for
long periods of time.  I usually loose the battery/use it somewhere else
between times I power up my project.  In my final project, I used a 5V DC
power supply.


> 


--- Jonathan F. Freeson 
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