You forgot to throw in the part where this signal crosses between
+3.3V logic and +5V logic, nixing a few
of the proposed designs... The biggest headache is that EAPD really
should be inverted, from a hardware
point of view, but changing a ten year old standard seems like
tilting at windmills.
I was under the impression that Arnold and I agreed the best fix was
to move the amp shutdown signal
to the EC (and eliminate pullup R335). No additional components
required, and the driver doesn't require
modification. There are alternative solutions, but most seem to
cost one EC output...
Comments ? Alternative suggestions ?
wad
On Mar 16, 2007, at 4:37 PM, Mitch Bradley wrote:
Here is the train of thought that led me to the inverted EAPD
conclusion:
(Assume for the sake of discussion that we want to use EAPD, not an
external GPIO)
a) There is a period of time after power up when EAPD is going to
be at the logic low level, because that is the hardware default
value, and it takes absolute minimum 8 mS (longer than the pop
time) for me to get the CPU into a state where it could even think
about talking to the CODEC.
b) To eliminate the pop, the amplifier shutdown pin must be in the
shutdown state while power is stabilizing. That is the opposite
state from the EAPD default.
c) Therefore, in order to eliminate the pop yet preserve the normal
sense of EAPD, it would be necessary to have an additional control
to force the amp shutdown state despite the fact that EAPD is
telling the amp to be on. And that control would then have to be
turned off after the software is certain that the EAPD pin has been
driven to the power-down state. Which might require more driver
complexity than inverting.
Jim Gettys wrote:
Let's see if Jaya Kumar has a heart attack ;-).
He can tell us if we're on thin ice, or whether there is
infrastructure
in the audio subsystem that will avoid any heartburn.
Jaya? This is best from a hardware standpoint.
- Jim
On Fri, 2007-03-16 at 09:46 -1000, Mitch Bradley wrote:
I have a simple hardware fix for the popping sound that happens
on power up, suspend, and resume. The net component impact is
the removal of one transistor. I have tested this fix and it works.
Details are in http://dev.laptop.org/ticket/977#comment:5
There is a software impact. With this change, in order to get
sound out, it is necessary to assert the EAPD pin on the CODEC.
That pin, deasserted by default, is nominally used to turn off
the amp when asserted. With the change, asserting that pin turns
the amp on, not off. Hardware engineers from Quanta and Analog
Devices considered, without success, several other hardware
changes to eliminate that pop. It boils down to the fact that we
have either invert the sense of that pin, or switch the amp
control to an entirely separate GPIO pin on another device (which
are in short supply).
To assert that pin, write 0x8000 to codec register 0x26.
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