Cheapest MCU and power suplly.

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Dear all
I know that lately I'm asking wired questions.
But I want to know your suggestions about the cheapest MCU with integrated RTC which will accept a battery and have about 16 IO, and more than 4KB flash.
And what is your cheapest way of generating a 5v 250ma from a 230v ac source?
Regards

I love Digital
and you who involved in it!

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The minute someone gives an answer, then the addtional specs start arriving... yeah, that one is cheap., but I need 5V not 3.3V. Need it in a dip. Dont like the .5MM pin pitch. The chip is cheap but they want $3K for the compiler. We havent even narrowed it down to 8 16 or 32 bit yet. Maybe approach it by telling why just about any old avr wont do the job?

Imagecraft compiler user

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Because it needs an internal RTC, and Old AVR's do not have them, and recently I have seen cortex m0, they are achievable in 1usd in 1k qty.
The parts can be anything, but no BGA and QFN... and 3.3v and 5v both are acceptable.
I want to use the MCU for building a 7segment Clock.it has a few buttons and 4 7segments.

I love Digital
and you who involved in it!

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Quote:
Because it needs an internal RTC

What are the demands on this "RTC"? Is the RTC of the ARMs something like the separate RTC chips, temperature compensated, calibrated and all that?

Or are you simply after something that can count seconds, minutes, hours and days/dates? An AVR does not need a specific RTC unit to do the latter, but you already know that.

So, what is it you want to get out of this "internal/integrated RTC" that a quite straight-forward piece of C code in an AVR can't do?

The answer to that is the real specification of requirements - something that should have been in the first post. Right now this is a goose hunt.

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Quote:

What are the demands on this "RTC"? Is the RTC of the ARMs something like the separate RTC chips, temperature compensated, calibrated and all that?

Or are you simply after something that can count seconds, minutes, hours and days/dates? An AVR does not need a specific RTC unit to do the latter, but you already know that.

So, what is it you want to get out of this "internal/integrated RTC" that a quite straight-forward piece of C code in an AVR can't do?

The answer to that is the real specification of requirements - something that should have been in the first post. Right now this is a goose hunt.

The system should count time in absence of power, I can use DS1307 with an avr, but integrated RTC units support external battery pin which will maintain clock in absence of power.The RTC does not need anything fancy! it should just keep the time in absence of main power.

I love Digital
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Commercial or hobby? If commercial, what sort of quantity?

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Most suppliers like Digikey / Farnell / RS have parametric selectors. If one goes on 1-off cost alone then Farnell reckon the following are cheap for >=4K and >=16 I/O:

£0.32: http://uk.farnell.com/freescale-...
£0.41: http://uk.farnell.com/stmicroele...
£0.47: http://uk.farnell.com/fujitsu/mb...

Whether they do the RTC thing or not is another question - at this stage I'd just start digging out the datasheets and see what they can offer.

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You could pretty easily make a circuit that powers an AVR from a battery in absence of external power supply!?

It would only take a few components! And if you use sleep modes you could get away with a pretty low power consumption as well.

There is nothing wrong with switching microcontrollers though, but it's just so much easier to stay with what you already know! :)

- Brian

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Quote:
Commercial or hobby? If commercial, what sort of quantity?

of course commercial, it’s where the price always matters.In the order of 1k units. What are your suggestions about power supply design?

I love Digital
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Geronimo wrote:
You could pretty easily make a circuit that powers an AVR from a battery in absence of external power supply!?

It would only take a few components! And if you use sleep modes you could get away with a pretty low power consumption as well.

There is nothing wrong with switching microcontrollers though, but it's just so much easier to stay with what you already know! :)

The catch is, if you power the whole AVR from a coin cell it won't last very long. MCUs with integrated RTC and battery pins let you supply just enough current for the RTC to keep counting, not to power the whole board.

Personally I would go with the DS1307 or equivalent before thinking about providing backup power to the AVR.

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I still want to conduct an experiment with an AVR keeping time with the async T2. It sure will use more power then a dedicated RTC chip, but maybe it's good enough to run the AVR from a supercap or so for a few days. Depends on the requirements of course if this is sufficient or not.

If you want something quick and now, a dedicated RTC is the way to go.

The cheapest RTC chip I can find at Farnell is the MCP7940M from Microchip. €1.01 SQ.

The LPC2101 from NXP is a ARM7 with a real RTC with dedicated power pin, and costs €3.71 SQ.

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I recently finished work on a nixie clock. It uses a 32KHz crystal with an ATMEGA168 for the RTC. Normally it is wall powered and draws quite a lot of current. But when it loses wall power it switches over to using a battery and draws about 2.5ua. I see little need for a specialized RTC for most purposes.

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Ali_dehbidi wrote:
But I want to know your suggestions about the cheapest MCU with integrated RTC which will accept a battery and have about 16 IO, and more than 4KB flash.
Overkill for your application but an XMEGA256A3B or XMEGA256A3BU will have the RTC switch-over to battery built-in. nleahcim's design would be closer.
Ali_dehbidi wrote:
And what is your cheapest way of generating a 5v 250ma from a 230v ac source?
Not isolated: LinkSwitch-TN by Power Integrations. If can reduce the 250mA by about half then other options are available. The 2-transistor Black regulator by Roman Black might work (see "High voltage use" near the page's bottom) but I don't have the simulation of it readily available to get a better guess-timate.

"Dare to be naïve." - Buckminster Fuller

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Ali_dehbidi wrote:
... a 7segment Clock.it has a few buttons and 4 7segments.
LED, LCD, OLED? Sunlight readable?
I like OLED (very good contrast, emissive) but it's not sunlight readable.
Crystalfontz OLED modules
Too small? 0.96-in Color OLED display by LadyAda.

LCDs are good though the back light could be a problem. Could try with an XMEGA B series or mega 9-series (?).

"Dare to be naïve." - Buckminster Fuller

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Thanks for all your feed back.

I love Digital
and you who involved in it!