EECS 498 Lecture 3

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EECS 498Lecture 3

• ATmega8 Hardware features

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Status?

• Labs
• Lab 1 due today.
• May have been too short
• Lab 2 posted
• Prelab due Friday, inlab/postlab on Tuesday.
• Use a board weve designed and modify part of our
  software.
• Project
• Well discuss project ideas, groups etc. on
  Thursday.
• Draft proposal due the following week.
• Space
• Stuff is a mess at the moment. Hope to have
  really ready over weekend (as much as furniture
  status allows).

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AVR stuff

• Timers
• SPI
• USART
• Much taken from Pascal Stangs class at Stanford.
  
• http//www.stanford.edu/class/ee281/

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Timers Review

• Provide accurately timed delays or actions
  independent of code execution time
• How are Timers used?
• Accurate delay
• Read the timer, store value as K. Loop until
  timer reaches K100.
• Schedule important events
• Setup an Output Compare to trigger an interrupt
  at a precise time
• Measure time between events
• When event1 happens, store timer value as K
• When event2 happens, read timer value and
  subtract K
• he difference is the time elapsed between the two
  events

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Timer 0

• 8 Bit Up Counter
• counts from 0 to 255 (0xFF), then loops to 0
• Internal or External Clock source
• Prescaler
• Interrupt on Overflow
• Transition from 255 to 0 can trigger interrupt if
  desired

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• Up counter (only)
• ClkT0 can be generated from an external or
  internal clock source, selected by the clock
  select bits (CS020).
• TOVn goes high (and may generate an interrupt)
  when going from 0xFF to 0x00

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Timer 0
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Timer 1

• 16 Bit Counter
• Counts from 0 to 65535 (0xFFFF), then loops
• Internal clock source with prescaler or External
  Clock
• Dual Comparators
• Interrupts possible on
• Overflow
• Compare A/B
• Input Capture of external event on ICP pin
• Can also act as an 8, 9 or 10 bit PWM Up-Down
  Counter

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Quick note 16 bit operations

• 16-bit operations are tricky
• Stores
• When writing the 16-bit locations, you must
  always write the high byte first.
• The write goes to a temp register and is ignored
  until the low byte is written
• You need to disable interrupts so the temp
  register doesnt get trashed
• Loads
• When you read the low the high value goes to the
  (same?) temp.
• When you read the high you get the temp.
• Notes
• Our lab2 code doesnt do this! (oops)
• Some 16-bit accesses dont do this (reading
  OCR1A/B) is the only one I know of

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Timer 1 continued

• The timer1 on the ATmega8 has two output compares
  (OCR1A/B)
• OCR1A/B are 16-bit registers
• When the value of OCR1x matches that of Timer1
• A user-defined action can take place on the OC1x
  pin (set/clear/inv)
• An interrupt can be triggered
• Timer1 can be cleared to zero
• Once set up, output compares operate continuously
  without software intervention
• Can drive pins OC1A and OC1B
• Great for
• Precise recurring timing
• Frequency/Tone generation (maybe sound effects)
• All kinds of digital signal generation
• Infrared communications
• Software-driven serial ports

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Timer 1 modes

• Modes of operation (chosen by bits WGM130)
• Normal mode
• Just an up-counter, interrupt can be generated on
  overflow.
• Can be used to time external events
• Can be used to trigger interrupts
• Either compare or overflow
• CTC (Clear Timer on Compare)
• Has a number of features, but lets you modify the
  period by having the counter reset when it hits a
  certain value.
• Other modes for PWM exist
• They are fairly complex and full featured.

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Timer 2

• 8-bit
• Has external pin it controls
• Can do CTC
• Has PWM features
• Can have own external clock (32Khz)

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SPI (Serial Peripheral Interface)

• Allows transfer of information, 8 bits at a time,
  between microcontroller and any number of
  peripherals
• Read and Write operations happen simultaneously
• Using chip selects allows lots of peripherals to
  be connected to a single SPI bus at the same time
• Has no start/stop bit overhead
• High data rates 250Kbit to 2Mbit/sec
• Common uses
• Inter-Processor Network
• Sending MP3 data to MP3 decoders
• Interfacing to external serial RAM/EEPROM/FLASH
• Interfacing to serial graphic LCDs
• Compatible with thousands of chips with SPI,
  Microwire, I2S, and other serial interfaces

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SPI pins and registers

• SPI Pins
• MOSI (master out, slave in)
• MISO (master in, slave out)
• SCK (serial clock)
• SS (slave select, optional)
• SPI Registers
• SPDR (transferred data read/write register)
• SPCR (control register)
• SPSR (status register)
• SPI Transfer Complete interrupt

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USART

• The USART, or Universal Synchronous Asynchronous
  Receiver Transmitter, provides hardware support
  for a serial port on AVR processors
• Signaling is compatible with PC/Mac/Unix serial
  (RS-232C)
• Provides
• Parallel-to-Serial and Serial-to-Parallel
  conversion
• Start and Stop Bit framing
• Parity Generation
• Baud-Rate Generation (2400-115.2kbps at 3.686 or
  7.37MHz)
• Interrupts
• Transmit Complete
• Transmit Data Register Empty
• Receive Complete

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Projects

• Some thoughts about projects
• I want you to use the AVR platform if possible.
• Overlap with work not allowed unless line is well
  defined.
• If you are paid, its work.
• Solving a problem rather than competing in the
  mass-market probably best
• Next week you will have formal requirements

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Some ideas

• Have 3 potential customers
• Speaker tracking
• Has
• Human Powered Submarine. adjust weight to
  stabilize their sub
• Card-reader
• Read the order of a deck of cards.
• No really.
• Sensor gathering
• ?