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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Guspaz - Wednesday, August 7, 2013 - link
"I feel like the more common use case in smartphones is to just lock your phone/display when you're not actively using it."Wouldn't you benefit from PSR even when you are actively using it? When I'm reading a web page in a browser on my phone, I'm not constantly scrolling. I scroll, read a bit, scroll, read a bit, etc. I might spend 10% of the time scrolling and 90% of the time reading a static screen. It sounds like PSR would benefit there too.
JlHADJOE - Wednesday, August 7, 2013 - link
+1 that.Majority of my tablet use is actually reading ebooks, so in my case I spend a lot of time looking at one static page until it comes time to read the next one. Whenever I check my battery usage log it's actually something like 90% screen, since I do so little that actually taxes the CPU/GPU.
JPForums - Thursday, August 8, 2013 - link
Does the screen percentage include the GPU power used in sending updates to the screen? If (as I suspect) it doesn't then your battery life improvements will be limited to <10%. This tech doesn't effect screen power consumption. It lowers the power consumed by the SoC by removing the need to send updates to an idle screen.JPForums - Thursday, August 8, 2013 - link
I should be clear that the <10% figure comes from the fact that your logs show 90% of battery usage is coming from the screen.JlHADJOE - Thursday, August 8, 2013 - link
I'm not expecting to see the screen power consumption go down at all. Just maybe see a wee bit more battery life out of my usage model.The point I was making is that people reading ebooks seems like the ideal scenario where PSR can make a difference.
ikjadoon - Wednesday, August 7, 2013 - link
Nice! Glad to see PSR getting some traction.What I'm *really* excited about, though, is combining PSR with the IGZO panels coming out in those 3200 x 1800 Ultrabooks. Apparently, IGZO can hold its active state longer, so here you actually ARE decreasing the panel's power consumption. Sharp's marketing says a 67% decrease in power consumption: http://online.wsj.com/ad/article/vision-breakthrou...
SodaAnt - Wednesday, August 7, 2013 - link
Well, I'd think an increasing amount of the energy would go to backlighting, which wouldn't be affected much by this.kpb321 - Wednesday, August 7, 2013 - link
The key here is going to be how quickly it and efficiently it can more in and out of PSR. For many types of active use outside of playing games/watching movies there are still stretches where the contents of the screen aren't really changing quickly. For example reading a web page/email/text msg/ebook/etc results in scroll, read for a while, scroll again. PSR can easily kick in for that read portion as it will be several seconds at least but even for more active things you can still find a couple seconds here or there or even parts of a second where PSR can kick in and while it might not seem like much it can add up to spending a significant amount of time in PSR and save a lot of power.MrSpadge - Wednesday, August 7, 2013 - link
Switch latency should be no problem, as the SOC just needs to push an updated image to the display and its memory. At 60 fps it's got 16.7 ms to do this - which is eternity in the world of electronics.juvosemi - Wednesday, August 7, 2013 - link
You typically call the part of the SoC driving the display the Dispolay controller or the LCD controller. You don't run the whole GPU which is typically a separate part of the SoC,