Power Usage

[chris_r]

Seeing as the clock speed can be changed now i see people saying how it will drastically shorten the battery life but nobody seems to know by how much. So can someone with a multimeter just test how much power the gp2x is using at various clock speeds so that we can see how much it will actually affect battery life

 

[Robster]

I've already done some testing, and found that running the 920T at 200MHz takes about 95mA more than running it at 100MHz. That is, almost 1mA/MHz consumed by the 920T. To put it in context, the whole unit draws about 400mA when sitting at the menu with 200MHz, against 300mA at 100MHz. Running at 100MHz provides a 25% reduction in current!

A few other things - the 940T is held in reset unless you're actually decoding video or whatever. When it is held in reset, the 940T is consuming basically no power. When the 940T is running at 100MHz, it consumes about 30mA, and at 200MHz, it consumes about 50mA.

Enabling the TV-out consumes about 200mA more current- this is obviously major, but we don't know how to properly set up the TV encoder chip so when that is done correctly, its current draw may decrease.

There is a clock signal that is sent to the SD card, which can be turned off. I don't know whether the GPH software ever turns it off, but if you know that you're not going to be doing any SD access, turning this signal off saves about 15mA.

Turning off the backlight saves about 50mA. Not a great option as it means you can't see anything on the screen, but for those applications (MP3 playback etc) where you don't need the screen, it's a useful power saving.

 

[DaveC]

I've already done some testing, and found that running the 920T at 200MHz takes about 95mA more than running it at 100MHz. That is, almost 1mA/MHz consumed by the 920T. To put it in context, the whole unit draws about 400mA when sitting at the menu with 200MHz, against 300mA at 100MHz. Running at 100MHz provides a 25% reduction in current!

A few other things - the 940T is held in reset unless you're actually decoding video or whatever. When it is held in reset, the 940T is consuming basically no power. When the 940T is running at 100MHz, it consumes about 30mA, and at 200MHz, it consumes about 50mA.

Enabling the TV-out consumes about 200mA more current- this is obviously major, but we don't know how to properly set up the TV encoder chip so when that is done correctly, its current draw may decrease.

There is a clock signal that is sent to the SD card, which can be turned off. I don't know whether the GPH software ever turns it off, but if you know that you're not going to be doing any SD access, turning this signal off saves about 15mA.

Turning off the backlight saves about 50mA. Not a great option as it means you can't see anything on the screen, but for those applications (MP3 playback etc) where you don't need the screen, it's a useful power saving.

​

Wasn't it said that power was leaking out of the TV-out? If so we could potentially save power by disabling it.

 

[BradN]

So we're looking at roughly 240mA for mp3 playing with the screen off (unless the cpu can be clocked lower than 100mhz and still play), and assuming the kernel can shut off the SD clock when it's not accessing it. It seems there's a lower penalty for using the 2nd CPU (probably because of the cache size), so maybe it's possible mp3 playback could be done on the 2nd cpu, and the main CPU could be clocked as slow as possible just to read the data and update the GUI. Not sure what impact the cache size would have on performance for something like an mp3 decoder though.

 

[Rayek]

Is it possible to adjust the intensity of the backlight? (I'm only thinking PSP because I'm used to it). That probably would save a few mAs. Plus, it could help a few users who think it's too bright...

 

[Minkoff]

So we're looking at roughly 240mA for mp3 playing with the screen off (unless the cpu can be clocked lower than 100mhz and still play), and assuming the kernel can shut off the SD clock when it's not accessing it. It seems there's a lower penalty for using the 2nd CPU (probably because of the cache size), so maybe it's possible mp3 playback could be done on the 2nd cpu, and the main CPU could be clocked as slow as possible just to read the data and update the GUI. Not sure what impact the cache size would have on performance for something like an mp3 decoder though.

​

I'm listening to MP3s at 60MHz right now.

 

[Robster]

Something really cool that comes out of the results above is that, say your application is running at 100MHz on just the 920 core, and you need more grunt. You could up the speed of the 920 at a "cost" of 1mA/MHz, or you could use the 940 at a "cost" of 0.25mA/MHz. Now, using the 940 is not as simple as just speeding up the 920, but for the skilful and creative coder, there are some great possibilities

Rayek: coincidentally, I've been looking at that. There is no hardware support for dimming the backlight, but it can be done (slightly untidily) with software and the current reduction is fairly linear so if you have half the backlighting brightness, you save 25mA. Might be good for eg. playing in aeroplanes without keeping your neighbours awake with the light!

 

[Robster]

DaveC: Sorry, I missed your post initially. I *think* the report about power leaking out of the TV-out circuit was mistaken. If it is true, then the base current drawn by the unit will decrease, but I think we just need to wait a bit before we can say accurately what effects the TV-out is having.

 

[Ninjia^]

In regard to this information has anyone tested the various applications at 100Mhz? Obviously most Emulators will be useless but just wanted to check if anyone has tried watching movies at this clockspeed? Any noticable slowdown etc?

Seven to Eight hours of movie playback on one set of batteries would be very impressive and more than enough for a international flight

Thanks

 

[sam fisher]

They cant test that yet as the program also turns off the second core.

 

[garteth]

so will slowing down the clock speed in the menus help at all?

 

[reallynotnick]

so will slowing down the clock speed in the menus help at all?

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Depends how much time you spend on the menus, but either way it would save some power.