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I am working on a low power design, which will have a MCU powered by my regulated DC power supply from mains(220VAC). But when the Mains fail, I intend to have a battery connected to the chip that puts the MCU in a sleep mode with some specific peripherals working (RTC). It come out of the sleep mode either when the mains is restored, or when an external interrupt occurs (in which case, it has to service the ISR and goes back to sleep).

What MCU can I use for this? Actually, i do not need a very expensive/sophisticated one. Preferably, I will like an 8051 core, 6-8k Flash, at least 512RAM, UART, I2C basically. Internal EPROM will also be nice if available.

But any suggestion, circuit diagram, application notes will be greatly appreciated.

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    \$\begingroup\$ If it wasn't for I will like an 8051 core I would say to get an AVR. \$\endgroup\$ – BenjiWiebe Feb 28 '13 at 17:49
  • \$\begingroup\$ You have "some specific peripherals" with only RTC as an example. Is that actually your only requirement? If so, lots of chips have a built-in RTC which runs off an external battery which requires no other electronics. Unsure about getting something which fits all of those requirements neatly though. If you drop the 8051 core part there are a lot of ARM components which should do it. \$\endgroup\$ – Kit Scuzz Feb 28 '13 at 17:54
  • \$\begingroup\$ RTC can be over SPI or IIC I presume, so this is really just a sourcing problem... which intel chip has "6-8k Flash, at least 512RAM, UART, I2C basically" and supports low power sleep... shouldn't that be almost all of them? \$\endgroup\$ – Grady Player Feb 28 '13 at 18:28
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    \$\begingroup\$ like i see 18 pages on digikey... \$\endgroup\$ – Grady Player Feb 28 '13 at 18:32
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    \$\begingroup\$ Sounds like a PIC 16F1xxx can do this job. Note that you don't need real time clock hardware to have a real time clock. These parts can drive a 33 kHz watch crystal and wake up every second for a few us to update the RTC data. This new series is also very low power. \$\endgroup\$ – Olin Lathrop Feb 28 '13 at 19:12
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You seem to be looking for an MCU that has a power failover built in as a peripheral of sorts. There may be a rare MCU available that has this, but from my memory there aren't many in a general way. For instance, I know that Atmel CortexM3s have the ability to run their internal RTC off of battery power. However, that is about as much that you can do.

Instead of looking for a single chip solution, it may be easier to look at designing a low power system out of a few different pieces, which would be easier to do.

For a microcontroller, pick something that's specifically designed to be low power. Having fine grained power control on the peripherals and the availability of low power sleep states is a plus. I've seen TI's MSP430 line qualify well in those parameters. Olin, in his comment, suggests the PIC16F1xxx series. Neither is an 8051 based solution, though. There may well be 8051 based controllers which qualify. I don't know how amenable the 8051 core is to low power states, though to be fair a lot of these other cores also do most of their sleeping by slowing down system clocks and turning off peripherals, which doesn't really need a specialized core.

In order to switchover power, you could use an IC designed for doing that, depending on the sort of battery and topology you have in mind. For a non-rechargeable coin battery sort of thing, you could look at one of the many RTC and Watchdog and Reset controller ICs, a large number of which include some way to switch over to a battery. For a rechargeable battery, assuming its voltage to high enough to be close to the supply voltage you intend to use with mains power, you're better off using a battery tied bus topology. This means that the battery is always connected to the circuit. Imagine the battery and mcu being connected in parallel. When there is mains power, the battery is charged also (using appropriate charging circuits depending on the chemistry of the battery) and when its absent, the current through the battery reverses and powers the circuit.

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If you want to stay with the 8051 core, try the Silabs C8051F320

It fits all your requirements except for internal EEPROM, but I am sure you could find some derivatives with this also. However, the C8051F320 is easy to get, and still not to complex for soldering (LQFP32). Check for the C8051F320DK develoment kit, USD 70 for board and programmer/debugger (and there are functionally equivalent devices on ebay)

Their IDE is free and support all major compilers, including the Keil, IAR, SDCC...

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