[Coco] TC-9

[Chuck Youse]

On Mon, 2008-07-07 at 23:35 -0400, Chuck Youse wrote:
> On Mon, 2008-07-07 at 23:29 -0400, jdaggett at gate.net wrote:
> 
> > The MC6809E, no internal oscillator,  the E and Q clocks are inputs and 
> > sink very little current.   
> 
> I believe you are mistaken.  The E clock on the 6809E is not a simple
> TTL signal, it has different voltage and current requirements because it
> directly drives the internal circuitry.  Look at the data sheet.
> 
> C.

Hmm, so I just looked at the data sheet myself; it lists Vih for the E
clock (separately) as VCC-0.75V (nominally 4.25V).  I can't see any
specific information on the current draw, but the sample clock circuit
shows the E to the CPU separated from the E to the system and pulled up
with 680ohms.  I don't know if they're just trying to keep the rise
times short or if they're trying to ensure a lower-impedance supply to
the pin.  

ANYWAY, point still is that you can't drive those with a standard TTL
output and certainly not a 3.3V FPGA IO pad.  I'm pretty sure both the
GIME and the SAM were designed to meet the E clock requirements without
any external assistance.

But now we're just drowning in details.. :)

C.