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Review: mbed NXP LPC1768

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alm:

--- Quote from: NiHaoMike on October 08, 2010, 02:26:38 pm ---My friend Brittany Benzaia does a lot of work on OpenWRT routers and APs and she thinks 8MB is small. Some of those routers are as little as $10 retail.

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But these run something like Linux, which is a different class. I wouldn't use an ARM M3 to route 100MBit/s or run Linux either. $10 is probably old stock, even the ones with less RAM/flash, which can't even run Linux (2MB flash or so), are like $25 here. Plus they are sold in much larger quantities.


--- Quote from: NiHaoMike on October 08, 2010, 02:26:38 pm ---Back when the mbed was $100, the Gumstix boards and the Beagleboard seemed to offer much more features for just a little higher price. But now that it's $60, the higher end boards are in a totally different category.

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Yes, the mbed is not particularly cheap, but I see it as a development board for prototyping. I would just stick an LPC17xx on the board for production work, and they're pretty cheap. A higher-end micro with external memory would be more expensive in similar quantities, and would be harder to route (four layer is probably mandatory).

TheDirty:
Ya, there seems to be a disconnect as to what the MBed is about.  It's just a low quantity hobby development board for the LPC1768.  It doesn't even have a JTAG header, which makes it unusable for professional development.  The LPC1768 is not going into any routers, it's not meant for that in any way.  The reason routers are cheap is because they are mass quantity consumer items.  512KB/64KB is actually the top of the line LPC17xx chip and I can't imagine a lot of the embedded ethernet applications that this chip is meant for will be using the 512KB/64KB part.  Most likely the 128KB or even the 32KB part will be more in demand.  It's really supposed to be an end point device.

NiHaoMike:
It has Ethernet and I2S, so some applications could be a networked music player. But with only 64kB of onboard RAM, it would require external RAM for the audio buffer.

It actually takes very little CPU to run an AP. As a layer 2 device, all the CPU does is keep track of MAC addresses and configure the hardware. The CPU in most modern switches is about the same level as a PIC. Routing does take a fair amount of CPU, especially if you do stateful packet inspection. A 96MHz ARM would be just barely enough for a typical broadband connection, so it would not be a good choice for a router. But it would have no problem acting as a bridge in an AP.

--- Quote ---Ya, there seems to be a disconnect as to what the MBed is about.  It's just a low quantity hobby development board for the LPC1768.  It doesn't even have a JTAG header, which makes it unusable for professional development.  The LPC1768 is not going into any routers, it's not meant for that in any way.  The reason routers are cheap is because they are mass quantity consumer items.  512KB/64KB is actually the top of the line LPC17xx chip and I can't imagine a lot of the embedded ethernet applications that this chip is meant for will be using the 512KB/64KB part.  Most likely the 128KB or even the 32KB part will be more in demand.  It's really supposed to be an end point device.
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The mbed does not need a JTAG connector since the USB interface chip does the JTAGing. That obviously won't work if you want to program a standalone LPC17xx chip, but maybe you can use a mbed to program blank LPC17xx chips just like an Arduino can be used to program blank AVRs by using the existing processor to emulate a JTAG. Or even use a mbed or Arduino as a JTAG interface for a totally different platform... (Repair a bricked PC motherboard with an Arduino?)

NiHaoMike:
http://mbed.org/forum/mbed/topic/1138/

--- Quote ---Good question you pose. First up, no, there's (not that I know of) any parts of the Mbed library that's going to connect the GPDMA controller to a GPIO pin. Probably with good reason as you read on...

The first problem you face is finding an 8bit wide port on the Mbed Dip pins. The only contiguous pins of any one port are P0.4 through P0.11 and as you can see straight away, there's an alignment problem you have to overcome. Next problem you have is providing a "write strobe" pulse. The GPDMA doesn't connect to any GPIO pin to provide such an output so the only way I can see to provide it is with a timer-out or pwm output clocking on the same clock speed the DMA is moving the data. However you'd then need to provide a mechanism to sync the strobing pin to the DMA transfers, not easy.
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I think that's a pretty major design oversight. One of the reasons to use a faster processor is faster I/O. Indeed, if it weren't for that problem, the mbed would have made a nice low end oscilloscope with the addition of an ADC and analog front end.

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