#10953 NORM -: XO-1.75 External Mic sense does not work

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#10953: XO-1.75 External Mic sense does not work
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           Reporter:  wmb at firmworks.com  |       Owner:  wad    
               Type:  defect             |      Status:  closed 
           Priority:  normal             |   Milestone:         
          Component:  hardware           |     Version:  1.75-A3
         Resolution:  fixed              |    Keywords:         
        Next_action:  never set          |    Verified:  0      
Deployment_affected:                     |   Blockedby:         
           Blocking:                     |  
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Changes (by wad):

  * status:  new => closed
  * resolution:  => fixed


Comment:

 There are two problems with this circuit.   The first is the voltage
 divider on the gate of Q9, which means that while we have 2.6V of bias
 voltage, we are only applying 1.3V of it to the gate.   This ensures that
 the output voltage will never be higher than 1.3V.

 A second problem is the voltage drop across the LED, roughly 1.8V when no
 current is flowing.   Even correcting the voltage divider, it is hard to
 get a voltage that I would feel comfortable feeding into a 3.3V GPIO.

 In 1.5, the signal was current sensitive.   Either it was tied to ground,
 or it was pulled up by another pullup.

 The solution for 1.75 A3 should be to add a 47 Kohm pullup resistor to
 +3.3V to the EXT_MIC_PLUG signal.   I can't quite explain why the pullup
 has to be so low --- 15K pulls up to +3.3V, 120K only pulls up to 2.1V ---
 I suspect the input impedance of the SoC is relatively low (150Kohm ?).

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Ticket URL: <http://dev.laptop.org/ticket/10953#comment:2>
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