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It looks to me that the DS1811 performs only the power-on reset.
Additional circuitry would be needed to add the reset button
function. Availability of this chip is limited to just a few
commercial suppliers who prefer to sell in large quantities.<br>
<br>
It all comes down to personal preference.<br>
<br>
--John<br>
<br>
<br>
<br>
<br>
On 03/04/2014 11:00 PM, neilbradley wrote:
<blockquote
cite="mid:dcd8f54e-2ea1-48cb-b671-667c6b90d0eb-/***@public.gmane.org"
type="cite">
<div dir="ltr">On Tuesday, March 4, 2014 5:58:37 PM UTC-8, John
Coffman wrote:
<blockquote class="gmail_quote" style="margin: 0px 0px 0px
0.8ex; padding-left: 1ex; border-left: 1px solid rgb(204, 204,
204);">
<div text="#000000" bgcolor="#ffffff"> On 03/04/2014 03:31 PM,
neilbradley wrote:
<blockquote type="cite">
<div dir="ltr">
<div>I've noticed that the SBC V2's reset circuit
seems... wonky? And by 'wonky' I mean sometimes when I
hit the reset button, sometimes I get double and even
triple resets, and sometimes the reset button doesn't
take at all if I hit it too fast</div>
</div>
</blockquote>
<br>
Mechanical switches bounce. That is, a single push can
result in several make/break/make cycles.<br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div> Most of the time the reset button works just fine,
however.</div>
<div> </div>
<div>I'm curious - why was the design choice made to go
with a standalone reset circuit instead of something
like a DS1811 reset IC?</div>
</div>
</blockquote>
<br>
DS1812 = more board space. Add a socket. = higher cost.
RC = simplicity + low cost.<br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div> Not enough current somehow or some other
electrical issue that I'm not internalizing?</div>
</div>
</blockquote>
<br>
The RC circuit <u>must</u> drive an input with hysteresis.
74LS14 <u>must</u> be used. Other logic families cause
trouble. <br>
<br>
The biggest problem I've ever seen with this type of reset
circuit is a capacitor that is way less than its marked
value. A flaky switch could be a problem, too.<br>
<br>
--John<br>
<br>
<br>
</div>
</blockquote>
<div>BTW: From my experience with the Mini-M68K, I noticed that
Motorola goes to great pains to specify the Reset input timing
to the 68000 in terms of number of clock cycles. I've seen
boards with hairy circuits to meet these timing specs. On the
Mini-M68K board, I used the same reset circuit, but R=10K, and
C=47uF. These values meet the 68000's timing requirements
with a reset delay of about 5X that of the SBC v1/v2. Signal
rise and fall times are handled with a chip with a
Schmidt-trigger input: 74LS14. Simplicity is bliss.<br>
<br>
I'm a bit confused... the DS1811 is a reset IC which is in a
TO-92 case which takes considerably less of a footprint than
the RC circuit and the LS14 logic and needs no external
circuitry. The D1811 gives a guaranteed 150ms pulse and even
senses external (button based) resets. With the circuit on the
SBC V2, if I thwack the reset button repeatedly, sometimes I
get double resets, and other times the SBC V2 hangs
completely. Most of the time it resets properly. My own Z80
board never has this issue when using a DS1811. Insofar as
cost goes, they are 46 cents via Digikey. Anyway, something to
consider for the future. I've never found RC reset circuits to
work very well. Not sure how it behaves in the 68K world, but
I've cleared up many reset problems in the pinball machines
and video games in the arcade I run in 6809, 6800, 6502, and
Z80 games simply by replacing their reset circuits with a
DS1811/12.</div>
<div> </div>
</div>
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