Proposed fix for the suspend/resume pop

[Jim Gettys]

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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-- 
Jim Gettys
One Laptop Per Child