power usage

Erik Friesen

I Just got a new battery powered project together.· I am powering it with 2 nimh AA batteries and using a tps61070 stepup converter.· I am also powering a pic16f688, a quantum touch chip(25ma), a 4k fram,· a 512 eeprom, and a small lcd with no backlight.·

My multitester says· I am using 150ma which is more than·my 100ma target.·(measured at the battery)

I am operating at 96 mhz for the benefit of·my usb interface(propeller driven).· If I understand the data sheet I am using 96ma·with all 8 cogs in use.

Can I change the pll on the fly?

In which configuration does a cog use the most or the least power? (driving no loads)

Does anyone else have some MA power usage numbers with a similar project?· Is this in the ballpark or do I have something out of wack here?·

Mike Green

A cog uses the least power when it's stopped or waiting for something (like WAITCNT, WAITPNE, WAITPEQ).

You can change the PLL "on-the-fly", but it takes a little while. Normally, you switch first to the built-in RCFAST clock, then re-configure the clock and PLL to allow it to stabilize, then switch to the new clock. Page 29 of the Propeller Manual discusses the necessary 10ms delay.

At the speed you're running, the Propeller takes about 100mA, your quantum chip takes about 25mA, the PIC, FRAM, EEPROM and LCD certainly could take another 25mA together.

Paul Baker

Mike Green said...

At the speed you're running, the Propeller takes about 100mA, your quantum chip takes about 25mA, the PIC, FRAM, EEPROM and LCD certainly could take another 25mA together.

Not to mention the losses of the step up converter, while lower than a linear regulator are not 0. Also with step up, you have to get the extra voltage somewhere, and it gets it through extra current from the input. Po = Pi * Eff, or Eo * Io = Ei * Ii * Eff, so the current consumed from the input is Ii = Eo*Io/(Ei*Eff)

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Erik Friesen

Mike, right after the 100 us it says you can switch freely among all clock sources by changing clkselx bits. Do I misunderstand this or is there some other reason for switching back to the rc clock?

Does the ma usage actually drop in half at pll8 vs pll16?

Mike Green

I may be wrong, but I think the issue is whether or not you know that the previous clock setting had the oscillator running. If not, you need to wait a while (10ms) using the RC clock for the crystal oscillator to stabilize before switching to it.

I just go by the charts on the datasheet. Whatever it says regarding current demand vs. PLL setting (clock speed) is what I accept until someone with some experience tells me otherwise.

Fred Hawkins

Erik, have you looked at the tps63000? It does seem to provide more current (800ma in boost mode), but doesn't work with batteries as low as the tps61070. (I think ...)

Erik Friesen

Battery Current supply at 150ma @ 2.5v should be giving me around 115 to 125 @ 3.3v which is whithin 61070 specs. I just naively assumed that the prop didn't use that much power. My first board stepped up to 3.8v, then used a ldo to drop it to 3.3. It used about the same current(at 80mhz). When I set up my last board I set it up to output 3.3 directly because I figured the power usage would be much less. When the current showed about the same it caused me to start figuring things out. I haven't had time to experiment with on the fly clock changes but even at 150ma It still gives me around 15 hours of battery time using alkalines, partly because current use is less at alkaline voltages.