66MHz PCI
 

Wikipedia says about conventional PCI:

The M66EN pin is an additional ground on 5 V PCI buses found in most
PC motherboards. Cards and motherboards that do not support 66 MHz
operation also ground this pin. If all participants support 66 MHz
operation, a pull-up resistor on the motherboard raises this signal
high and 66 MHz operation is enabled.

OK.

The pin is still connected to ground via coupling capacitors on each
card to preserve its AC shielding function.

I don't understand that part. Anyone?

66MHz PCI
 

T. Ment wrote:
Wikipedia says about conventional PCI:

The M66EN pin is an additional ground on 5 V PCI buses found in most
PC motherboards. Cards and motherboards that do not support 66 MHz
operation also ground this pin. If all participants support 66 MHz
operation, a pull-up resistor on the motherboard raises this signal
high and 66 MHz operation is enabled.

OK.

The pin is still connected to ground via coupling capacitors on each
card to preserve its AC shielding function.

I don't understand that part. Anyone?

It means the pin has an RC on it, and the C serves
to shunt high frequency signals attempting to get
to ground, to their intended path.

+5V
| Presence of 33MHz card stops 66MHz operation
Rpullup C shunts AC content to GND, if 33MHz card pulled
| M66EN
+---------------+--------------------+
66MHz | | x
Mobo C | 33MHz PCI card 66MHz PCI card
| |
GND GND GND

+5V
|
Rpullup No possibility of 66MHz operation (mobo no comprende)
| M66EN
+---------------+--------------------+
33MHz | | x
Mobo | | 33MHz PCI card 66MHz PCI card
| |
GND GND GND

The "logic function" is defined by the DC conditions.

The RC charging curve of the circuit, might benefit from
a schmitt trigger logic input (on the mobo chipset),
when something on the motherboard monitors M66EN, so
that the input does not sit in mid-rail for too long.
I doubt anyone wasted mental energy on this. It would
depend on the value of C (which could be a 0.01uF
or a 0.1uF ceramic). There would not be a huge electrolytic
or something, on that pin. It'll be a tiny bypass cap if
anything.

http://www.ti.com/product/SN7414

Paul

66MHz PCI
 

On Mon, 15 Jul 2019 14:03:00 -0400, Paul wrote:

It means the pin has an RC on it, and the C serves
to shunt high frequency signals attempting to get
to ground, to their intended path.

+5V
| Presence of 33MHz card stops 66MHz operation
Rpullup C shunts AC content to GND, if 33MHz card pulled
| M66EN
+---------------+--------------------+
66MHz | | x
Mobo C | 33MHz PCI card 66MHz PCI card
| |
GND GND GND

+5V
|
Rpullup No possibility of 66MHz operation (mobo no comprende)
| M66EN
+---------------+--------------------+
33MHz | | x
Mobo | | 33MHz PCI card 66MHz PCI card
| |
GND GND GND

The "logic function" is defined by the DC conditions.

The RC charging curve of the circuit, might benefit from
a schmitt trigger logic input (on the mobo chipset),
when something on the motherboard monitors M66EN, so
that the input does not sit in mid-rail for too long.
I doubt anyone wasted mental energy on this. It would
depend on the value of C (which could be a 0.01uF
or a 0.1uF ceramic). There would not be a huge electrolytic
or something, on that pin. It'll be a tiny bypass cap if
anything.

http://www.ti.com/product/SN7414

Paul

I understand the diagram. Thanks. Some of the terminology escapes me
though.

"the pin has an RC on it"

RC? What does that mean?


"the C serves to shunt high frequency signals attempting to get
to ground, to their intended path"

C means capacitor I guess. But how does C prevent high frequency signal
going to ground?

66MHz PCI
 

T. Ment wrote:
On Mon, 15 Jul 2019 14:03:00 -0400, Paul wrote:

It means the pin has an RC on it, and the C serves
to shunt high frequency signals attempting to get
to ground, to their intended path.

+5V
| Presence of 33MHz card stops 66MHz operation
Rpullup C shunts AC content to GND, if 33MHz card pulled
| M66EN
+---------------+--------------------+
66MHz | | x
Mobo C | 33MHz PCI card 66MHz PCI card
| |
GND GND GND

+5V
|
Rpullup No possibility of 66MHz operation (mobo no comprende)
| M66EN
+---------------+--------------------+
33MHz | | x
Mobo | | 33MHz PCI card 66MHz PCI card
| |
GND GND GND

The "logic function" is defined by the DC conditions.

The RC charging curve of the circuit, might benefit from
a schmitt trigger logic input (on the mobo chipset),
when something on the motherboard monitors M66EN, so
that the input does not sit in mid-rail for too long.
I doubt anyone wasted mental energy on this. It would
depend on the value of C (which could be a 0.01uF
or a 0.1uF ceramic). There would not be a huge electrolytic
or something, on that pin. It'll be a tiny bypass cap if
anything.

http://www.ti.com/product/SN7414

Paul

I understand the diagram. Thanks. Some of the terminology escapes me
though.

"the pin has an RC on it"

RC? What does that mean?


"the C serves to shunt high frequency signals attempting to get
to ground, to their intended path"

C means capacitor I guess. But how does C prevent high frequency signal
going to ground?



An RC is a two element network consisting of
a resistor and a capacitor.

In the diagram, the capacitor *passes* high frequency
content (noise), while it blocks DC from flowing.
Thus, it will not pull down the logic signal we
want to float up to the +5V potential.

The Rpullup established a 5V level, when no card signals
it is 33MHz only. Any card which grounds that
signal, or the motherboard itself can ground that
signal, indicating 66MHz ain't gonna happen.

*******

For background on RC time constants and the
actual waveform to be expected on a 66MHz
system, try here.

https://www.eecs.tufts.edu/~dsculley...al/rc/rc3.html

If you pass that signal to a schmitt trigger,
it "sharpens up the edge" and makes a square wave
out of it. In the old days, we'd use a 7414.
I think on my first job, one of the other guys
used some of those on his board. They can clean up
noisy signals or signals with slow risetimes. The
CMOS versions of those had an even wider window,
and could do a better job on really crappy looking
signals.

A canonical usage for those, might be as a reset
generator, where the reset circuit consisted of an
RC. To automatically reset a board at power up,
we might use a 0.5 second to 1 second RC charging
time constant. The R would have to be small enough
to ensure the logic gate leakage current was met, which
meant the C was larger than anyone would like. It
was sometimes set up with a diode (for fast recovery
from VCC removal).

https://www.maximintegrated.com/en/i.../3227Fig04.gif

Paul