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mbed road test - first impressions/what next?

romilly

I was lucky enough to be selected for an mbed road test, and the mbed package arrived yesterday.

 

I'd read Riccardo Tinivella's excellent mbed review. Like him I found the product very easy to get going. The website suggests that you can get started in 60 seconds, and that's exactly how long it took me.

 

I use two PCs - a laptop running Windows 7 64-bit on an AMD processor, and a Linux workstation. Both systems drive the mbed happily. The combination of web-based development and filesystem-based programming is a stroke of genius. It completely bypasses the USB driver problems I've hit with other development products on my Windows machine.

 

The first project I had in mind for the mbed was a poor man's oscilloscope/logic analyser to help me debug the I2C boards I am working on at the moment. I needed something which would let me visualise the digital signals on the I2C bus, and if possible plot some analogue data as well.

 

It turned out to be really easy to implement. There's a lot of sample code on the mbed website. It's well organised, and it's easy to search. After a few minutes I was able to write formatted data to the mbed file system, including  digital and analogue inputs. I wrote the data out in csv format, and then displayed the results on my PC.

 

This is a (very simple) example of the sort of prototyping for which the mbed was created. I'm seriously impressed by mbed's ease of use and range of available libraries. I expect to use the mbed a lot for future projects.

 

For my main review, I'm proposing to port some code I wrote for the arduino using the I2c bus. It's a homebrew version of the microwriter keyboard, and drives an LCD display and an EPROM memory. From what I've seen so far, it looks as if the porting will be very straightforward.

 

Riccardo has already covered many of the questions people might have about the mbed. Is there anything you would like me to investigate for my review?

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    My latest experiments with the mbed have been focused on SPI.image

     

    The mbed has two SPI interfaces available. Each can be configured to handle normal or inverted polarity, and to cope with data on rising or falling clocks. The size of the SPI frame can vary; it defaults to 8 bits, but 16 bits are possible. I think smaller values may also be supported, but the documentation is contradictory.

     

    SPI is (normally) faster than I2C, and the LPC1768FBD100,551LPC1768FBD100,551 processor can handle SPI clock rates of up to 1/8th the CPU clock speed. Since the mbed is clocked at 96MHz, you can drive SPI peripherals at a blazing 12MHz!

     

    That's really useful for the chip I investigated. I hooked up a Microchip 23K256. It's a 32K byte static ram with an SPI interface. It can be clocked at up to 20Mh, so the mbed's 12MHz is comfortably within spec.

    This gives a useful, low-cost 32k byte memory extension. It's not as fast as directly driven external ram, but it only needs four pins to drive it.

     

    The API for the 23K256 is tricky, so I decided to write, test and publish a library on the mbed website. It's taken a couple of days to get there, but I'm pleased with the result, and the chip looks as if it's going to be really useful. I'm thinking of driving a battery of 16 of them using a 74HC154 to select individual chips. That would give me 512K of deep memory for logic analysis, and handle data bus speeds of up to 2MHz.

     

    I continue to be excited by the mbed's potential. Thanks again to Farnell/Element14 and to the mbed team for a chance to learn about this fantastic product.

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  • 0

    My latest experiments with the mbed have been focused on SPI.image

     

    The mbed has two SPI interfaces available. Each can be configured to handle normal or inverted polarity, and to cope with data on rising or falling clocks. The size of the SPI frame can vary; it defaults to 8 bits, but 16 bits are possible. I think smaller values may also be supported, but the documentation is contradictory.

     

    SPI is (normally) faster than I2C, and the LPC1768FBD100,551LPC1768FBD100,551 processor can handle SPI clock rates of up to 1/8th the CPU clock speed. Since the mbed is clocked at 96MHz, you can drive SPI peripherals at a blazing 12MHz!

     

    That's really useful for the chip I investigated. I hooked up a Microchip 23K256. It's a 32K byte static ram with an SPI interface. It can be clocked at up to 20Mh, so the mbed's 12MHz is comfortably within spec.

    This gives a useful, low-cost 32k byte memory extension. It's not as fast as directly driven external ram, but it only needs four pins to drive it.

     

    The API for the 23K256 is tricky, so I decided to write, test and publish a library on the mbed website. It's taken a couple of days to get there, but I'm pleased with the result, and the chip looks as if it's going to be really useful. I'm thinking of driving a battery of 16 of them using a 74HC154 to select individual chips. That would give me 512K of deep memory for logic analysis, and handle data bus speeds of up to 2MHz.

     

    I continue to be excited by the mbed's potential. Thanks again to Farnell/Element14 and to the mbed team for a chance to learn about this fantastic product.

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