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Another mbed review

ratsept

Hello everyone,

 

I would like to point out that this is my first review ever and all suggestions for the future would be welcome.

 

I was very surprised to even be selected for a road test as I have only been a member here for a very short time. So you can imagine how it felt to actually get a package from Farnell on Friday. I was just about to go to a folk music festival when I discovered it. I was so anxious to start playing with it that I took it with me. Well I got to test some examples with onboard leds and virtual serial port and everything was really simple but I just couldn't wait to get back home to start serious testing.

 

So today at work I plugged it into the breadboard for the first time and got it connected to some testing hardware.

 

The documentation is really easy to understand and seems to cover everything needed to get your system up and running but since everything is so easy and all the examples actually work as described you soon run out of things to do. For beginners it is very nice and really gives you confidence to dive into experimenting on your own. That is where it gets difficult. Although there are a lot of libraries for different things to interface there are some that should be there but for some reason were overlooked. For example standard SPI or I2C memory library would have been nice. This is not difficult to implement yourself and I think the reason it is missing might be because the platform is quite new. The updates on the page seem to be regular (judging from history) and each update brings new features.

 

In my oppinion the biggest problem right now is the USB interface. It works as a virtual serial port and also shows up as a flash disc but there seems to be very little information about how to program the mbed to register as a USB device. For example the hardware in this controller would be perfect for some USB HID testing but I couldn't get it to show up as one yet. Granted I have only spent a few hours actually using the device.

 

One nice thing for me is the fact that you can use your mbed as a bridge between a laptop and pretty much any piece of electronic hardware you can think of. There are examples for using the virtual com port to control pretty much any interface on the mbed. This means that I can use my laptop USB to control CAN, Ethernet, I2C, SPI and with analog IO plus plenty of DIO there are few things I can't experiment with.

 

One thing I noticed was that the controller gets pretty hot when driving all the onboard leds (there are five, all blue). Also the reset button is actually a simple tactile button not like they show on the pictures.

 

Right now I am using my mbed to test a few radio transceivers and it has really saved me a lot of time as I usually build my test devices with AVR controllers and they take considerably more effort to work as bridging devices.

 

More to follow as I continue to test this wonderful piece of modern engineering.

  • Part 2 of my review.

     

    My biggest problem with mbed was that I couldn't get it to show up as a USB device of my choosing. Well after spending lots of time surfing around on mbed.org I finally found what I was looking for. There is actually a library to make mbed act like a USB mouse (http://mbed.org/users/simon/programs/USBMouse/latest/docs/ by Simon Ford). This code acted as a basis for a project that I just finished. I used mbed as a HID keyboard and sent keystrokes using CC1101 wireless transceiver chips. One end was my mbed connected to my laptop through USB and to the radio through SPI. The other end was a serial-to-radio module by AMBER WIRELESS that also used CC1101 (or CC1100 - the older version). This combination was used to test the reliability of a radio link in difficult conditions. We actually put one of the radios inside a jar and then put this jar under water to test for effects of moisture on range. The mbed logged all received data into a text file (like writing on a keyboard). All this testing was for a project that must work in an underground mine but we will not use something as powerful as an mbed in the end product. This is, in my opinion, exactly what the mbed was designed for - rapid experimentation and proof of concept.

     

    A problem I found this time was that the mbed only has a regulator for 3.3 volts so to work as a USB host (it has the hardware and code examples) you need to have either an external 5V supply or use the Vusb pin but that only has power when connected to a computer.

     

    Next I hope to use the mbed in a robot and compete with this robot in a sumo competition. Again it will be a waste of controller resources but as I only have 2 months to do this and I also start have to work and attend some classes this semester it will save me time. Right now the plan is to use my mbed with two L293D motor controllers. L293D is a dual bridge but for bigger motors and more power i will use two chips and parallel the outputs. There seems to be a library for DC motors on mbed.org and that can probably work with L293D just fine. For sensors I plan to use Sharp digital distance sensors as they are a lot faster than analog and also less affected by noise. I will also use 2 or 4 IR reflective sensors for ring edge detection. It should be a simple enough project to pull off. I will try to post pictures as I go along because for my last project I couldn't (it was for my work).

     

    In the future I want to test I2S on the mbed as it is supported in harware but not yet by the software. This would be used to sende data from the mbed to a high quality audio DAC.

     

    There is a holiday 3 day weekend coming up and I hope to get some time to try a few things done during that time.

     

    Tiit