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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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  • 0

    I'm getting more familiar with the mbed and its libraries by taking baby steps. I'll post whenever I have something to report, and summarise my results in a detailed review later.

     

    The mbed site has several useful sections. The handbook documents the core libraries; the cookbook documents contributed code which the authors feel is ready for sharing.

     

    Each user can also create notebooks in which to record work in progress. You can make a notebook visible if you want, but you don't have to.

     

    Yesterday I made a start by hooking up a Text LCD to the mbed and using some cookbook code to drive it. I modified the sample code slightly to use different mbed pins.

     

    Here's all you need to drive the LCD:

     

    #include "mbed.h"
    #include "TextLCD.h"

     

    TextLCD lcd(p10, p12, p15, p16, p17, p18); // rs, e, d0-d3

     

    int main() {
        lcd.printf("Hello World!\n");
        lcd.printf("What fun!");
    }

     

    I'm currently working on a different driver which will use I2C and a PCF8574 port expander to drive the LCD.

Reply
  • 0

    I'm getting more familiar with the mbed and its libraries by taking baby steps. I'll post whenever I have something to report, and summarise my results in a detailed review later.

     

    The mbed site has several useful sections. The handbook documents the core libraries; the cookbook documents contributed code which the authors feel is ready for sharing.

     

    Each user can also create notebooks in which to record work in progress. You can make a notebook visible if you want, but you don't have to.

     

    Yesterday I made a start by hooking up a Text LCD to the mbed and using some cookbook code to drive it. I modified the sample code slightly to use different mbed pins.

     

    Here's all you need to drive the LCD:

     

    #include "mbed.h"
    #include "TextLCD.h"

     

    TextLCD lcd(p10, p12, p15, p16, p17, p18); // rs, e, d0-d3

     

    int main() {
        lcd.printf("Hello World!\n");
        lcd.printf("What fun!");
    }

     

    I'm currently working on a different driver which will use I2C and a PCF8574 port expander to drive the LCD.

Children
  • 0 in reply to romilly

    My mbed road test activity for today has been the development of some I2C code. The mbed I2C library is really easy to use.

     

    It's gone well. I started by getting a simple program to drive a PCF8574 port expander. You can see it running on the right.image

     

    My next program was an I2C-based LCD driver. It's working, but I need to clean up the code before I publish it. I hit a couple of minor problems on the way. Both issues are covered in the documentation, but they are easy to forget if you are coming from an arduino background.

     

    The first is that the mbed I2C interface now uses 8-bit addressing, since that's what is used by most I2C component datasheets. The arduino Wire library uses 7-bit addressing. If you're talking to a PCF8574 port expander, you would address it at 0x20 in an arduino script but as 0x40 in an mbed program.

     

    This is clearly stated on the I2C page of the mbed on-line handbook, but the PCF8574 example in the cookbook used the 7-bit address; as a result the code example did not work. I've now fixed the cookbook page.

     

    The second problem is also covered in the handbook. You must have pull-up resistors on the SDA and SCL lines of an I2C design. That's mandated by the standard, but many of us arduino developers have got out of the habit; the adruino I2C library turns on internal pull-ups, so you don't need external ones. With the mbed you do.

     

    While debugging my code I discovered one very useful tool - Simon Ford's I2CU bus walker. This neat utility tries to read from each possible I2C device address and reports on each one it finds. If you forget the pull-up resitors, it won't find anything. Once you've added them, it will help you to find the correct 8-bit addresses for your I2C slaves.

  • 0 in reply to romilly

    Great review so far, Romily.

     

    Keep up the good work!

     

     

    Elias

  • 0 in reply to Former Member

    Thanks, Elias. I'm glad you found the review interesting.

     

    I've now published the LCD I2C code on the mbed website.

     

    Next I'll be experimenting with the mbed Serial libraries.