[Trac #1412] Switch from driving the panel from the Geode to the DCON when the display is idle.

Zarro Boogs per Child bugtracker at laptop.org
Tue May 8 01:22:35 EDT 2007


#1412: Switch from driving the panel from the Geode to the DCON when the display
is idle.
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 Reporter:  jg               |        Owner:  rsmith 
     Type:  enhancement      |       Status:  new    
 Priority:  high             |    Milestone:  BTest-4
Component:  x window system  |   Resolution:         
 Keywords:  power            |  
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Comment (by gnu):

 Replying to [comment:2 JordanCrouse]:
 > Bear in mind - it will cost us up to a frame of time to switch back into
 CPU mode.  The DCON always keeps a frames worth of data in its internal
 memory; so when we switch back to CPU mode, we have to wait for a frames
 worth of data to fill the memory before switching back to CPU mode,
 otherwise, the DCON will gitch, @56Hz, thats at most a 17ms delay.

 The DCON spec 0.8 says there's no delay to switch back into CPU mode.
 Whether that matches the chip remains to be seen.  If the CPU raises
 DCONLOAD, the DCON will continue the current frame til the bottom, then
 switch immediately to displaying data from the CPU video interface,
 starting at the trailing edge of the CPU's vertical retrace pulse.  After
 DCONLOAD is high and the DCON has switched, the internal memory is not
 cycling; the CPU video data flows straight through into the panel.  When
 DCONLOAD is lowered, the DCON will keep flowing CPU data straight through,
 til vertical retrace.  Then it keeps flowing data through for one more
 frame, while it writes an entire frame into its internal RAM.  Then at the
 second vertical retrace, it switches to displaying with the internal
 memory and internal timing (and it interrupts the CPU to tell it to turn
 off the CPU video).  Thus the "frame delay" happens at the shutoff end,
 not the turnon end (and we hope that when we're about to shut off the CPU
 video, each frame is identical to the previous one, so it doesn't matter
 which one the DCON grabs).

-- 
Ticket URL: <http://dev.laptop.org/ticket/1412#comment:4>
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