Microcontrollers and Sensors. Scott Gilliland - zeroping@gmail

Transcription

1 Microcontrollers and Sensors Scott Gilliland - zeroping@gmail

2 Microcontrollers Think tiny computer on a chip 8 to 128 pins Runs on 3.3 to 5 Volts 8 to 20 Mhz Uses µw to mw of power ~ 4 kilobytes of flash memory ~ 256 bytes of RAM Lots of built-in hardware Analog voltage readings Communication: I2C, Serial, USB device Cheap - $5 Small Programmable (actual size: ) Often in C or Assembly

3 How Your Code Gets from Here to There (on a regular Linux computer) C compiler C code (gcc) Libc Compiled Executable (on disk) Operating System (things like printf) Other Libraries (for anything not in libc) I386 processor

4 How Your Code Gets from Here to There (on an Atmel Atmega) C code C compiler (on your desktop) avr-libc (things like sprintf) (gcc-avr) Compiled Executable (.hex file) Programmer Other Libraries (for anything not in libc) Microcontroller (in flash memory)

5 How Your Code Gets from Here to There (on an Atmel Atmega) C code C compiler (on your desktop) (gcc-avr) avr-libc (things like sprintf) Compiled Executable (.hex file) Programmer Other Libraries (for anything not in libc) One big difference: there is no OS Your code has full run of the processor There is no OS-level threading Microcontroller (in flash memory)

6 Atmega Programming USB In-System-Programmer Reads and writes flash memory 6-pin connector to the Atmega You need to wire this up yourself Provide your own power to the system We have several USB In-System-Debugger Full on-chip debugging 10-pin connector to the Atmega More expensive We haven't needed to buy one yet

7 A word on Avr-Libc[1] Includes many things you get from regular Libc stdio.h, string.h: printf family stdlib.h, math.h: Malloc, normal C math But not all No Files No Standard Out Microcontroller-specific parts Read high or low voltage levels on pins Drive pins high or low Access to hardware registers Many hardware modules in the Atmega each with hardware registers Needed to do everything listed in the datasheet[2] Interrupts For example:

8 Examples

9 An Example: Textile Touch-button Sensing Controls Input/Output pins to do Capacitive sensing Basically measures the capacitance of a line Uses the USART module of the microcontroller Acts much like an old serial port on a desktop Uses two pins TX and RX Similar to RS232 different voltage levels Uses a USB-to-serial converter One specifically made for our +5 volt signaling Shows up as a file (/dev/ttyusbn) to a Linux machine; as COMn to Windows Main program is just a while(forever) loop Senses the capacitance of 4 line Writes a few values to the computer over serial

10 Atmel Atmega 128

11 Sparkfun FT232R Breakout Board

12 Another Example: Gesture Watch Sensing using 5 IR proximity sensors Digital proximity sensors On/Off 0V when nothing nearby +5V when there is something nearby Uses a Serial-Bluetooth module Hooks up to the USART module of the Atmega Shows up as a Bluetooth serial device /dev/rfcommn under Linux Main program is initialization plus a while loop Puts the Bluetooth module into discoverable mode This is done by sending AT+BTMODE3 Senses the state of 5 lines Writes a few values to the Bluetooth module over serial

13 Sharp IR Proximity Sensors

14 Atmel Atmega 128

15 Sena Wireless ESD-100 Bluetooth-to-Serial module

16 Dlink USB Bluetooth Adapter

17 A Final Example: BlueSense Accelerometers Designed due to lack of small, low-power Bluetooth accelerometers Measure movement in 3 directions Also has capacitive sensing ability Build to be extensible Centered around a BlueCore microcontroller A microcontroller with a Bluetooth radio built in Uses I2C to communicate with accelerometer and capacitive sensor

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22 A word on BlueCore Microcontrollers Pros: Tiny Integrated Bluetooth radio Fewer parts to put together

23 A word on BlueCore Microcontrollers Cons: Too tiny You need to have a custom-made board to solder down Hard to develop for Proprietary compiler, weird assembly Difficult radio design for anyone not an Electrical Engineer

24 I2C Communication Protocol Master/Slave protocol for chips on the same board Uses 2 wires (3 if you count ground) Most microcontrollers support it Including the Atmega The I2C library from the 'wiring' project works great Protocol is left up to the device It's in the datasheet somewhere Often defines how to read and write to register addresses Ex: Accelerometers have several control registers, and registers for X, Y, and Z axes.

25 Overall Lessons Go with the simplest solution that works Wired communication is the easiest A Bluetooth module complicates the design a small amount A Bluetooth-enabled microcontroller complicates the design by an order of magnitude Sometimes, this means not building anything new We already have Bluetooth accelerometers and capacitive sensors We have Wiimotes We have serial bitwackers - USB-controlled I/O lines