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 Posted: Jul 20, 2007 - 07:24 PM |
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Joined: Mar 10, 2006
Posts: 74
Location: Johannesburg, South Africa
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I am designing a low power project which makes use of 8 accelerometers (max 0.18 mA each), external EEPROM (still spec'ing) and a RTC (0.15 mA max) - all low voltage spec.
I have read that for low power designs it is advisable to supply peripherals from IO pins of the microcontroller, the data sheet of which says the pins are able to sink/source 25 mA each - which is more than enough for the peripherals.
Is this a common practice? Is there any advice that someone may give in this regard? The addition of protection components perhaps?
If I need to step down the voltage from the IO pin is it enough to use a resistive divider or zener perhaps?
Also (in general) what happens to the overall supply current draw of the microcontroller if this is implemented? I have yet to receive samples and so I cannot test right now - this is just a preliminary design.
Thanks in advance.
Dino |
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Posted: Jul 20, 2007 - 07:39 PM |
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Joined: May 26, 2004
Posts: 2538
Location: Las Vegas, Nevada
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| As always, go look at the small print near the back of the data sheets - the actual DC specifications. The "feature list" on page 1 tends to be a bit optimistic. You'll find graphs headed "IO Pin Source Current vs. Output Voltage" for various supply voltages. These will tell you whether you can maintain a workable Vcc at whatever current the peripheral sucks. |
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Posted: Jul 20, 2007 - 08:05 PM |
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Joined: Mar 10, 2006
Posts: 74
Location: Johannesburg, South Africa
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Thanks for the tip! To clarify, I have a graph of output high current vs output high voltage, with a mean curve and +\- statistical deviations. Am I reading it correctly whereby if I draw x (mA), the corresponding peripheral supply voltage from the pin will be y (V).
Actually as I typed that out it made alot more sense!
Dino |
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Posted: Jul 20, 2007 - 11:51 PM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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It is NOT common in professional applications. It makes the peripheral have a lower logic voltage. Those peripherals may be compromised by having data inputs at a higher voltage level from the micro.
Use a nice P-channel switch driven by the micro. Their RDS(on) is a lot lower than the P-channel in an I/O driver of the micro. They cost only a few cents each (U.S.) and are available in small but easy to work with SOT-23 packages.
Jim |
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Posted: Jul 20, 2007 - 11:54 PM |
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Joined: Feb 19, 2003
Posts: 926
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Almost, if I'm understanding this thread at all. You are trying to power a device from the port pin of the Atmel. If the device draws less than what the pin can source at it's high voltage then the voltage will remain at the VCC of the Atmel. Said differently, if your Atmel's VCC is 3.3V and you turn on the output of a pin you'll get 3.3V (nom.) If you now load that pin down with your device, you will continue to get 3.3V(nom.) until you draw more than the pin can source (according to the graph you have). There after the voltage will drop or you'll kill the pin.
Notice I refered to 3.3V as the nominal voltage. Of course it will be lower than that even under the best of cases.
You should also note that there is a spec on the current draw from the Atmel's VCC pin. You can load individual pins to their spec but the effect is cummulative on the Atmel's VCC pin. In other words, you can't drive twenty 20mA loads even though each pin might be able to do it. |
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Posted: Jul 21, 2007 - 12:47 AM |
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Joined: Jun 28, 2005
Posts: 52
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| As above use a P MOSFET to turn peripherals on and off. Even if a single device hung from the MCU is OK, dont forget the collective current which will soon be maxed out. I presume you want to set different parts of the board to sleep. |
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Posted: Jul 21, 2007 - 12:52 AM |
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Joined: Sep 20, 2003
Posts: 4073
Location: Surrey, England
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| For what it's worth, I power a graphical LCD from an i/o pin, and it works fine. But obviously you need to check the current requirements. |
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Posted: Jul 21, 2007 - 01:46 AM |
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Joined: May 26, 2004
Posts: 2538
Location: Las Vegas, Nevada
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Sometimes, engineering is the art of knowing what you can get away with. Or as someone else has said, an engineer is someone who can do for 50 cents what any fool can do for a dollar.* If the current is light enough and you can get away with switching it straight off a port pin, and thereby save the cost (and space) of an external transistor, I say go for it. If it doesn't work you can always go back to the drawing board.
(* Or as yet someone else said, you know you're an engineer when you spend the weekend fixing a $10 radio) |
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Posted: Jul 21, 2007 - 04:28 AM |
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Joined: Apr 20, 2007
Posts: 6066
Location: Long Island New York
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Just one more thing I would like to toss into this. If the i/o pin that is powering the peripheral goes low. You will need to take into consideration power-up times of the peripheral when the pin returns high. Add into that make sure the peripheral does not load down other pins it may be attached to should the supply pin go low.
Jim |
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Posted: Jul 21, 2007 - 09:13 AM |
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Joined: Mar 10, 2006
Posts: 74
Location: Johannesburg, South Africa
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Thank you everyone for the excellent advice and tips. The time difference means I post a problem at night, and wake up to find all these great posts.
ka7ehk wrote:
Those peripherals may be compromised by having data inputs at a higher voltage level from the micro.
I will typically be measuring inputs on the accelerometers that will correspond to values lower than the required supply voltage. I understand the implications of your comment, and will look into the P-channel switch.
sgomes wrote:
You should also note that there is a spec on the current draw from the Atmel's VCC pin. You can load individual pins to their spec but the effect is cummulative on the Atmel's VCC pin.
Ideally only one peripheral will be supplied at any time, thus total peripheral current drawn at any time shouldn't be more than that drawn by my hungriest device (the accelerometers -> 0.18 mA).
However in a further effort to reduce power consumption I will be running at a low clock frequency, and further reading of the data sheet shows how the max supply current is directly proportional to the clock frequency. I will have to either re-think the peripheral supply, or trade-off for a higher clock frequency in order to support the cumulative current draw.
jgmdesign wrote:
You will need to take into consideration power-up times of the peripheral when the pin returns high. Add into that make sure the peripheral does not load down other pins it may be attached to should the supply pin go low.
It should be added for interest that consideration for such effects are taken into account by using the microcontroller to switch capacitor values in and out of the transducer circuit at optimum times (just before power-off and just before power-on). Hmmm now how will that affect the current profile. |
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Posted: Jul 21, 2007 - 12:11 PM |
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Joined: Nov 01, 2005
Posts: 6323
Location: Hilversum - the Netherlands
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| Peret wrote:
Quote:
.. you know you're an engineer when you spend the weekend fixing a $10 radio
... and feel good about that  |
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Posted: Jul 21, 2007 - 01:27 PM |
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Joined: Jul 01, 2003
Posts: 55
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| Hi! Please check the AVR butterfly schematic, it uses one I/O pin to power the temp-senser. |
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Posted: Jul 22, 2007 - 04:20 PM |
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Joined: Jun 01, 2004
Posts: 351
Location: Brisbane, Australia
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Quote:
However in a further effort to reduce power consumption I will be running at a low clock frequency, and further reading of the data sheet shows how the max supply current is directly proportional to the clock frequency. I will have to either re-think the peripheral supply, or trade-off for a higher clock frequency in order to support the cumulative current draw.
You are confusing cause and effect here. There is a reduction in supply current at lower frequencies due to lower switching losses, however this will not affect the maximum current available on an I/O pin, which is a totally separate issue.
Jim |
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Posted: Jul 23, 2007 - 08:33 AM |
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Joined: Mar 10, 2006
Posts: 74
Location: Johannesburg, South Africa
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sgomes wrote:
You should also note that there is a spec on the current draw from the Atmel's VCC pin. You can load individual pins to their spec but the effect is cummulative on the Atmel's VCC pin.
I assumed from this information that current drawn from the IO pins will relate to current drawn from the supply (Idd).
I was looking at a graph entitled "Maximum Idd vs Fosc over Vdd" and I assumed that meant that for certain supply voltages there was a maximum current that could be drawn by the micro depending on the clock frequency. |
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Posted: Jul 23, 2007 - 06:24 PM |
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Joined: Feb 19, 2003
Posts: 926
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| Chicken before the egg... The graph you are looking at is the Maximum Idd drawn by the Atmel itself. To this you must add the draw from each of your pins. Where do you think the pin currents come from? hehehe |
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Posted: Jul 23, 2007 - 07:10 PM |
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Joined: Mar 10, 2006
Posts: 74
Location: Johannesburg, South Africa
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Yes I do think I confused myself with that graph, thanks for the help.
I do see under the Absolute Maximum Ratings that there is a parameter entitled "Maximum current into Vdd pin: 250 mA".
I think this is the upper current limit that can be pulled through the supply, which is what I thought the previously mentioned graph was showing me.
This value is much higher than anything I can expect to draw so in this regard I'm safe.
Dino
P.S.
FYI (for those who refer to my micro as an AVR or "the Atmel") I am using a PIC and not an AVR (please dont throw rocks at me). The sponsor company (this is for a university engineering design project) is a Microchip Consultant Program Member and so obviously recommends Microchip products - hence me posting this in General Electronics and not in the AVR section. I'm finding out for myself that AVRs are WAY more newbie-friendly. |
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Posted: Jul 23, 2007 - 07:21 PM |
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Joined: Nov 01, 2005
Posts: 6323
Location: Hilversum - the Netherlands
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Posted: Jul 23, 2007 - 07:28 PM |
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Joined: Mar 10, 2006
Posts: 74
Location: Johannesburg, South Africa
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Well theres no (written) rules against asking questions on the AVR arch nemesis, and I felt a pang of guilt everytime someone said "Check the AVR's data sheet..."
Hopefully my good deed will go unpunished! |
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Posted: Jul 23, 2007 - 09:29 PM |
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Joined: Jan 03, 2006
Posts: 4416
Location: Hemel Hemsptead, UK
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Mwahahaha! *No* good deed goes unpunished! 
Neil  |
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