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.

What you're wanna do? Comfortable prototyping and breadboarding (meaning fast development with no care about 50mA) or a final product where every mW counts? There are many energy friendly systems. But most of them are not that comfortable in programming.

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.

Interesting post.

I have similar issues to resolve. Having finally solved my mbed 1768 noise spikes on the DAC output for my application, I now want to reduce power for portable use, so looking at the LPC11U24 I thought this would be the logical next step..... OK it doesn't have a DAC, but that was a plus as I had already decided not to use a built in DAC and design in a good low noise solution. But...

Why no RTC? These are a core requirement for data logging (and how many adc channels are there?!). They are low power and so useful for battery powered applications! So I suppose I could also add an RTC chip.

Now I want to save my stored data and transfer it to a PC, but there doesn't seem to the simple filesystem/ memory stick module any more? Correct me if I'm wrong. This method of transferring data to a PC is an amazing useful and simple to use feature.

So where next? I came from a PIC environment, but loved the mbed design, love the simplicity and speed of development, but have I got to leave it behind?

Also can anyone explain the LPC11U24 power supply arrangements regarding how to use Vin and Vb. Does Vb now power the whole board when not using usb power? Why is there not a Vin current spec?

All comments on the above, especially if I have made errors will be very welcome.

Chris Styles has addressed the powersupply questions on this page http://mbed.org/handbook/mbed-NXP-LPC11U24

So you loose the magic chip when the lpc11U24 is powered through VB. This means you also loose the localfile system. You will have the localfile available when you power the mbed using Vin. However, this will obviously increase your batteryload in comparison to using VB only.

A solution could be to use an external eeprom or SD card for storage and use an mbed pin to switch on the power to this memory whenever you need it. You could then copy back that data to the localfile when the mbed stick is plugged back into the PC.

Wim,

Many thanks.

That is useful extra information, thanks.

I saw Chris's post, but it didn't all sink in at that time!

Thinking about this statement, could I power the Vb and use an output to switch on the main Vin using an external circuit? The bit I can't quite get my head round is yes I can power the module from Vin, I get lower power and sleep modes from Vb, but how does my application doing measurements, saving data, displaying data on an LCD etc., running on Vin enter a low power mode using Vb under processor control? I assume it must remove Vin by an external electronic switch cuicuit?

Great news about localfile system thoough, thanks

USB an VIN override VB. You can see how this works by looking at the schematics that are available on the hardware page. Using an electronic switch to disconnect the main power, Vin in this case, is the normal way to enter a standby mode in many devices such as tablets and phones. Depending on how much power you want to save a separate SD card may be better since the lpc11u24 can control it and wont need the magic chip. BTW note that you also loose Vout 3V3 when Vin is not present.

Wim, many thanks for your fast response and replies. Based on your feedback I shall now design my circuit around the LPC 11U24 and add on an SPI DAC and RTC. I'll investigate the sleep modes first and decide how best to use these for my application. Many thanks

http://www.atmel.com/Microsite/atmel_studio6/default.aspx

AVR studio has been around for a long time.

how do u know that is consuming such a current...??? any harware connection like where do we have to connect the pins of a multimeter to check how much current it is drawing in the sleep mode or wake up mode???