Understanding Panel Self Refresh
by Anand Lal Shimpi on August 7, 2013 1:47 PM EST- Posted in
- Smartphones
- Displays
- Mobile
- Panel Self Refresh
- Tablets
- SoCs
Earlier today Brian spent some time with the G2, LG's 5.2-inch flagship smartphone based on the Qualcomm Snapdragon 800 (MSM8974) SoC. I'd recommend reading his excellent piece in order to get all of the details on the new phone, but there's one disclosure I'd like to call out here: the G2 supports Panel Self Refresh.
To drive a 60Hz panel, your display controller must present the display with the contents of the frame buffer 60 times per second. Regardless of what's being displayed (static vs. active content), every second there are 60 updates pushed through the display pipeline to the display. When displaying fast moving content (e.g. video playback, games, scrolling), this update frequency is important and appreciated. When displaying static content however (E.g. staring at the home screen, reading a page of an eBook), the display pipeline and associated DRAM are consuming power sending display updates when it doesn't need to. Panel Self Refresh (PSR) is designed to address the latter case.
To be clear, PSR is an optimization to reduce SoC power, not to reduce display power. In the event that display content is static, the contents of the frame buffer (carved out of system RAM in the case of a smartphone) are copied to a small amount of memory tied to the display. In the case of LG's G2 we're likely looking at something around 8MB (1080p @ 32bpp). The refreshes then come from the panel's memory, allowing the display pipeline (and SoC) to drive down to an even lower power state. Chances are the panel's DRAM is also tied to a narrower bus and can be lower power than the system memory used by the SoC, making these refreshes even lower in power cost.
LG claims a 26% reduction in power when displaying a still image with PSR enabled. I'm curious to see the impact on overall battery life. There are elements of our WiFi web browsing test that could benefit from PSR but it's unclear how much of an improvement we'll see. The added cost of introducing additional memory into a device is something that panel vendors have been hesitant to do, but as companies look to continue to reduce platform power it's a vector worth considering. LG's dual-role as a component supplier and device maker likely made the decision to enable PSR a lot easier.
PSR potentially has bigger implications for notebook use where it's not uncommon to just stare at a desktop that's not animating at all. I feel like the more common use case in smartphones is to just lock your phone/display when you're not actively using it.
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MaxPowerTech - Thursday, August 8, 2013 - link
Exactly what i'm wondering.Why can't you just leave the panel at its current state, when no new data arrives?
Why would you need that extra buffer?
Ortanon - Thursday, August 8, 2013 - link
The internal refresh on LCD is probably necessary to maintain the integrity of the pixel state. There are "zero-power" LCD technologies that haven't been able to get out of the lab for years. They don't need to be refreshed, but I think right now the speed at which they can refresh is still too low for prime time.mort32 - Thursday, August 8, 2013 - link
How fast do the refreshes need to be to maintain a pixel's state? Does PSR refresh the screen at a slower rate that's just enough to not lose the state? Kind of like how DDR memory works where refreshes are only done when needed.Ortanon - Thursday, August 8, 2013 - link
I don't think PSR is LCD-specific, so I doubt it. Then again, it may very well be that LCD requires an internal refresh for compatibility purposes -- legacy software and paradigms. Lazy quick Googling has not shone much light on this. This looks like a job for anandtech.com!Laststop311 - Wednesday, August 7, 2013 - link
wheres the battery life tests and comparisons?tomba - Thursday, August 8, 2013 - link
I find it odd that PSR is being touted as something new. It's not new. Phones, at least Nokia's, have had self refreshing panels for ever. Both MIPI DBI and MIPI DSI video busses support this. And some phones take this even further, as they only update the changed portion of the display.Soulwager - Thursday, August 8, 2013 - link
Would PSR make it possible to dynamically modify refresh rate to match frame rate when gaming? This would allow V-Sync to be enabled without limiting frame times to multiples of 1/60 of a second.DesktopMan - Monday, August 19, 2013 - link
Variable frame rate LCDs with really high internal refresh rate (from PSR) is one possible solution. It would work very well with OLEDs as the pixel switching time is very low. Some TVs already do this in one way or other. This would make it possible to vsync to any refresh rate as you suggest.Visual - Thursday, August 8, 2013 - link
This does not make sense.It just moves the framebuffer to memory on the display itself, but that is essentially a noop, or even adds complexity and power consumption instead of reducing it, in the form of one added framebuffer layer. The link from this new framebuffer to the actual pixels still does essentially the same thing as what the link from the old framebuffer to the pixels should have been doing. If it does it in a more power-efficient way or whatever, why not just optimize the old link...
Visual - Thursday, August 8, 2013 - link
To clarify, I am referring specifically to embedded devices, where "memory on the display itself" has absolutely nothing that can make it any more special than normal system memory, because the display and the rest of the system are literally millimeters from each other.