XS1-U12: DCDC2 (analogue supply) & Power when Asleep

Technical discussions around xCORE processors (e.g. General Purpose (L/G), xCORE-USB, xCORE-Analog, xCORE-XA).
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Redrat
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XS1-U12: DCDC2 (analogue supply) & Power when Asleep

Postby Redrat » Thu Jun 23, 2016 9:55 am

Hello,

I'm playing around with the sleep/suspend feature of the U12, and noticed that the SW2 1V8 supply is remaining high when the processor goes to sleep. I thought at first that the processor was getting stuck in Sleeping2 state. However upon inspection I've found that the Asleep Power Supply State (0x0C) register has the DCDC2 bit set high (0x108002), counter to what the datasheet says it should be.

The bit is read only so I can't change it. Am I missing something? Can the DCDC2 not be disabled?

Thanks,
Red.
Last edited by Redrat on Thu Jun 23, 2016 9:55 am, edited 1 time in total.
henk
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Postby henk » Thu Jun 23, 2016 4:01 pm

Hi RedRat - 1V8 is kept alive to keep the wake-up mechanisms running.

It shouldn't be taking a lot of power.

Cheers,
Henk
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Postby Redrat » Thu Jun 23, 2016 4:20 pm

Hi Henk,

Thanks for the answer.

So is the datasheet incorrect for the Asleep register?

Thanks,
Red
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Postby henk » Thu Jun 23, 2016 4:47 pm

Hi Redrat,

Yes, that looks like a mistake in the data sheet.

Cheers,
Henk
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Postby Redrat » Tue Jul 05, 2016 10:41 am

Hello,

We're having trouble getting the power consumption of the u12 down when it's asleep. Is there an app-note or guide for low power as the datasheet is a bit ambiguous?

So far changes from default we make are; for the sleep power state we disable the clock to the XCore Tile, set DCDC 1 & 2 to PFM modulation.

However the u12 continues to draw ~4mA when asleep.

Thanks,
Red
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Postby henk » Tue Jul 05, 2016 10:51 am

Hi - you will need to switch the silicon oscillator on, make sure that that goes to 32 KHz on sleep, and then switch the bias circuit and the crystal oscillator off.

Cheers,
Henk
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Postby Redrat » Tue Jul 05, 2016 4:37 pm

Hi Henk,

That's reduced the power consumption, however it is no longer entering sleep mode properly (our software pwm driven led continues to blink instead of shutting off). Here's the sleep function code:

Code: Select all

         unsigned da[1];
         char dc[1];

         //asleep power state
         read_periph_32(usb_tile,6,0x0c,1,da);
         da[0] |= 0x4300; // dcdc set to pfm + disable xcore clock
         write_periph_32(usb_tile,6,0x0c,1,da);


         //set to 32kh
         dc[0]=3;
         write_periph_8(usb_tile,4,1,1,dc);

         //select silicon osc
         dc[0]=1;
         write_periph_8(usb_tile,4,0,1,dc);

         //reset xcore
         dc[0]=3;
         write_periph_8(usb_tile,4,0,1,dc);

         //disable bias and crystal
         dc[0]=0;
         write_periph_8(usb_tile,4,2,1,dc);

         // init sleep
         read_periph_32(usb_tile,6,0,1,da);
         da[0]|=2;
         write_periph_32(usb_tile,6,0,1,da);
         while(1);
henk
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Postby henk » Thu Jul 07, 2016 4:43 pm

Hi Redrat,

I haven't got hardware to try it here, but from memory the problem is that switching oscillators is not a reliable process; it may cause the PLL to chase and cause the chip to get clocks too close together.

- when you switch the oscillator from an external Crystal oscillator to the 20 MHz Si- Oscillator, you have to issue a reset to the XCORE (bit 1 of register 0x00 of the oscillator peripheral).

- Early on in main, you check whether you are running on the Si Osc, and if so, you progress to do the rest of your switching off actions as before.

So the reset actions form two halves: one half switches you to the silicon oscillator, the other causes you to go to sleep in low power mode.

Cheers,
Henk
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Postby Redrat » Mon Jul 11, 2016 9:43 am

Hi Henk,

Thanks for all the help, but I'm still having trouble with switching it to the 32khz clock;

It'll sleep and wake fine on the 20Mhz clock, but it won't go to sleep if I attempt to switch it to the 32khz. I've tried switching it at various points in the process; before switching to the on-sil, after switching, after the reset, ect. But every time the chip won't reset or sleep properly. I've added idle loops to wait for the oscillator to stabilise first but those haven't helped.

I'm using register I.2 On-silicon-oscillator control and writing 3 to it (tried 2 as well). Is this correct?

I've also had a look at the a-series support on git and tried to dissect it, but I'm not sure where it switches to 32khz. I also noticed that it checks if the clock speed falls within range of the on-sil before switching it to 20mhz, is that perhaps the problem I'm having with switching to 32kz?

Thanks,
Red
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Postby henk » Mon Jul 11, 2016 9:57 am

Hi Redrat,

The switch to the 32 kHz is automatic; if the right bit is set; bit 0 of the general control register.

You cannot switch is to 32 kHz yourself, since it isn't fast enough to feed the PLL.

Cheers,
Henk

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