Microcontrollers and Sensors

Microcontrollers and Sensors

Scott Gilliland - zeroping@gmail

Microcontrollers ●

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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

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Cheap - $5

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Small

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Programmable ∘

(actual size:

)

Often in C or Assembly

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

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)

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)

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

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10-pin connector to the Atmega

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More expensive

∘

We haven't needed to buy one yet

A word on Avr-Libc[1] ●

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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

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Access to hardware registers

∘

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Many hardware modules in the Atmega – each with hardware registers

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Needed to do everything listed in the datasheet[2]

Interrupts

1.http://www.nongnu.org/avr-libc/ 2.For example: http://www.atmel.com/dyn/resources/prod_documents/doc2467.pdf

Examples

An Example: Textile Touch-button Sensing ●

Controls Input/Output pins to do Capacitive sensing ∘

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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

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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

Atmel Atmega 128

Sparkfun FT232R Breakout Board

Another Example: Gesture Watch ●

Sensing using 5 IR proximity sensors ∘

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Digital proximity sensors ●

On/Off

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0V when nothing nearby

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+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 ●

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/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

Sharp IR Proximity Sensors

Atmel Atmega 128

Sena Wireless ESD-100 Bluetooth-to-Serial module

Dlink USB Bluetooth Adapter

A Final Example: BlueSense Accelerometers ∘

∘

Designed due to lack of small, low-power Bluetooth accelerometers ●

Measure movement in 3 directions

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Also has capacitive sensing ability

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Build to be extensible

Centered around a BlueCore microcontroller ●

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A microcontroller with a Bluetooth radio built in

Uses I2C to communicate with accelerometer and capacitive sensor

A word on BlueCore Microcontrollers

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Pros: ∘

Tiny

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Integrated Bluetooth radio

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Fewer parts to put together

A word on BlueCore Microcontrollers

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Cons: ∘

Too tiny – You need to have a custom-made board to solder down

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Hard to develop for – Proprietary compiler, weird assembly

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Difficult radio design for anyone not an Electrical Engineer

I2C Communication Protocol

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Master/Slave protocol for chips on the same board

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Uses 2 wires (3 if you count ground)

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Most microcontrollers support it ∘

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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

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Often defines how to read and write to register addresses

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Ex: Accelerometers have several control registers, and registers for X, Y, and Z axes.

Overall Lessons ●

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Go with the simplest solution that works ∘

Wired communication is the easiest

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A Bluetooth module complicates the design a small amount

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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

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We have Wiimotes

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We have “serial bitwackers” - USB-controlled I/O lines