23 Sep
2023
23 Sep
'23
9:06 p.m.
On 09/23/2023 3:25 PM CDT Warner Losh via cctalk
<cctalk(a)classiccmp.org> wrote:
On Sat, Sep 23, 2023, 12:31 PM Paul Koning via cctalk <cctalk(a)classiccmp.org>
wrote:
What's too fast in absolute terms?
Warner
paul
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>
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I can't answer for Paul, but I can tell you how to do approximate calculations for
yourself.
You need to know two things: the equivalent parallel resistance (Thevenin equivalent) of
the two series resistors and the input capacitance of the driven pin (including stray
capacitance.)
If you assume the input+stray capacitance is 10 pF (a reasonable first approximation) and
the resistance is 1000 Ohms, the time constant (time for the signal to go from 0 to 63% of
max) will be 1000 * 10E(-12) seconds or 10,000 picoseconds = 10 nanoseconds. That assumes
the 5V output can switch instantaneously into the 5 mA or so load. I would double that
for the real world. So the signal gets delayed around 20 nS and the output has to drive 5
mA or so.
A more realistic divider might be 5K or 10K resistance for less power consumption and less
strain on the output. With 10K you are looking at a delay of 100 to 200 nS. If the
timing isn't critical you "might" be able to go as high as 5 MHz that way.
But probably more like 1 MHz is more realistic.
I've used voltage divider level translation at about 1 MHz, using something like 2200
and 4700 ohms for the divider (Thevenin equivalent 1500.)
ymmv
Will
If you want to build a ship, don't drum up people to collect wood and don't assign
them tasks and work, but rather teach them to long for the endless immensity of the sea.
Antoine de Saint-Exupery
On Sep 22, 2023, at 3:07 PM, Chuck Guzis via
cctalk <
cctalk(a)classiccmp.org> wrote:
On 9/22/23 11:34, emanuel stiebler via cctalk wrote:
I used an Arduino Feather that way, for my PS-2 to LK201 converter. And
while not 5V tolerant, a Raspberry Pico is a particularly powerful and
cheap option; one of my projects could run DDCMP at 10 Mb/s including the
"integral modem" compatible signaling.
As I mentioned, 5V tolerant inputs can, at least for not too high speeds,
be done simply by resistive voltage dividers. There are still some 84pin chips out there(Altera
& Xilinx). Sometimes
they are pulls, or some 5V tolerant xilinx xc95xxxxl
I still have a few 84 pin PLCC XC95108 5V CPLDs Originally, I did a
tape controller design with one before Xilinx discontinued them. I
figured that using a discontinued part was not the way forward, so I
dropped the project. Xilinx did/does have its ISE design suite, which
is fairly easy to use.
Eventually, it turned out that using a reasonably fast MCU with 5V
tolerant I/O worked just as well and avoided the "mystery in a chip" of
a CPLD.