#9776 HIGH 1.5-har: XO-1.5 B3 only boots when serial port connected

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#9776: XO-1.5 B3 only boots when serial port connected
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           Reporter:  wad        |       Owner:  wad           
               Type:  defect     |      Status:  closed        
           Priority:  high       |   Milestone:  1.5-hardware-C
          Component:  hardware   |     Version:  1.5-B3        
         Resolution:  fixed      |    Keywords:                
        Next_action:  reproduce  |    Verified:  0             
Deployment_affected:             |   Blockedby:                
           Blocking:             |  
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Changes (by wad):

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


Comment:

 Thanks for the pointers.

 After verifying (using a diode) that indeed it was the serial port back-
 powering the +3.3VSUS rail (to around 0.8 volt) through the pullup on the
 receive line that was making the motherboard boot, the debugging began for
 real.

 The next symptom was that +3.3V was asserted with VR_ON (not MAIN_ON).
 This led to the discovery that MAIND was always low.

 This was caused by one of the discrete MOSFETs (PQ18) attached to MAIND
 having a gate/source short of 200 ohms.   This was probably caused by ESD
 (that transistor is at the edge of the motherboard.)

 Closed as an isolated incident.

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Ticket URL: <http://dev.laptop.org/ticket/9776#comment:3>
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