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As far as the RTC. I don't think the LPC11U24 has one, Although it might be implemented in firmware.
I have had a very good look at the M0 schematic, And there is no separate I/P for battery, or a separate crystal.
But as a solution, the DS1302 RTC Chip is good contender. And I am sure Sparkfun has a BOB for it.
I must admit, this is my first test, early next week. To measure the current, and try to get it to goto low power. And most important, wake up, and do something usefull, While keeping context.
Keep me, us posted
Ceri
From the demo post in the blog it looks like it might be 15mA operating current and 1.5mA sleep. An acceptable price for the advantages of having the SDK and other ease of use.
The watchdog timer can be set to use the programmable internal watchdog oscillator, which can run really quite slowly (<10kHz). I seem to recall you can get times of about 2hrs between interrupts - they aren't accurate but it is reliable and easy.
Do you have any idea how inaccurate that would be? If the drift is large enough for the time to need correcting every few weeks then I'll have to add an external RTC.
It's not going to be very accurate at all, unfortunately. It's derived from the external clock signal and isn't temperature compensated.
Some new questions: the datasheet for LPC11U24 CPU mentions a "high drive output pin", with 20mA *min* output current. It indicates 4mA for standard pins (min). I'm a bit confused about what that actually means in terms of performance, but I presume this means that I can't drive my white LEDs from GPIOs anymore; am I right, and is the high power pin accessible on the mbed?
As I'm not too sure about the capabilities of the LPC11U24 mbed I'm planning on trying AVR chips instead. Rather than risk spending a relatively large amount of money on a device that may cause me more problems than it solves, I'm going to attempt to use an ATmega1284p with an ATtiny85 providing a USB interface (with the help of V-USB). I'm hoping that this will provide the features I want (uA's of standby current, 20mA current through GPIOs, plenty of IO pins), and even if it doesn't work out it won't cost as much, so I can try something else.
AVR is cheap, and very popular , as are PIC's As with both a programmer is required, as well as an IDE.
I know that the PIC IDE is free, and truly powerfull, because I have used it for about 20 years on and off
I don't know of an IDE for AVR's. (free anyway)
but you are still going to have to spend best part of £100 on a programmer.
which is where MBED wins on low device count.
but when the bare LPC11U2 chip is much less than an equivalent 8 bit USB enabled device
there is a clear winner
hope that rant was informative.
Ceri.
I'm not sure if there any good IDE's around for AVR, but there is a GNU compiler toolchain that supports them, and the chips themselves can be programmed via an SPI serial connection. I'll use some parts I already have to build my programmer (I still have an LPC1768 mbed I can extract from another project). As I don't have access to industry grade kit I'm limited to devices I solder by hand, which makes small devices like AVR's and PIC's a good choice.
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After an impressive performance from the LPC1768 mbed for prototyping last year, I'm thinking of using the LPC11U24 mbed for a prototyping project. Before committing to using a fairly expensive processor (in comparison to AVR devices) I want to make sure it will do what I want.
I'm intending to build a custom USB peripheral with some stand-alone capabilities, powered by an internal 1Ah lithium-polymer battery. As you might expect I need very low power consumption, support for deep-power-down, an RTC (with wakeup interrupt) and flexible USB slave functions.
The new mbed should be ideally suited to this, but I'd like some more detail. What is the power consumption of the board as a whole? The 1768 was terrible for battery power, I think it needed 100mA, and had no support for sleep. Is the full range of low power modes, like deep-power-down, now officially supported? Is there an internal real time clock? The 1768 supported real-time quite well, but I haven't seen any mention of an RTC in the 11U24. If there is an internal RTC, does it support interrupts to wake the CPU from deep-power-down?
I'm hoping for another enjoyable and succesful mbed powered project. Any constructive comments welcome.