Chipcon 802'15'4 Transceiver

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Chipcon 802.15.4 Transceiver

• EECS150 Fall 2006
• Lab Lecture 8
• Leo Chen

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Project Overview

• N64 Controller
• User input to your game.
• Video
• Game output to the user.
• Chipcon Transceiver the FUN Two-Week One! )
• Bidirectional communication between games.
• Game Engine
• Drives game play. Glue logic. Handles
  communication handshaking.

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Transceiver Overview (1)

• 3rd party chip mounted on expansion board.
• Uses a PCB antenna. Take a look!
• IEEE 802.15.4 standard support. Zigbee ready.
• Transmits on unlicensed 2.4 GHz spectrum. 16
  communication channels.
• Overlaps with Wi-Fi.
• 250 kbps maximum data rate.
• We will only be using a very small percentage of
  this.
• Configure, send, receive, and issue commands to
  chip over SPI to CC2420 registers.

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Transceiver Overview (2)

• 33 configuration registers.
• We change 3 of them.
• 15 command strobe registers.
• We issue 6 of them.
• These change the state of the CC2420 internal
  FSM.
• 128-byte RX FIFO 128-byte TX FIFO
• Accessed via 2 additional registers.
• Also accessible as RAM (i.e. by addressing).
  Only for debugging! Dont do this unless youre
  a masochist

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CC2420 Inputs Outputs

• Single bit status signals.
• High level transceiver operation information.
• Initialization signals.
• Drive signals once and forget about it.
• SPI interface.
• Interface to rest of chip via CC2420 registers.
• Send, receive, configuration, detailed status.

FPGA
VREG_EN RF_RESET_
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Single Bit Status Indicators

• FIFO Goes high when theres received data in RX
  FIFO.
• FIFOP Goes high when bytes received exceeds
  set threshold.
• CCA Indicates that the transmission medium
  (air) is clear. Only valid after 8 symbol
  periods in RX mode.
• SFD Goes high after SFD is transmitted low
  after packet completely sent.

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SPI Interface

• Serial interface with 4 wires
• SClk Clock signal you generate.
• CS_ Active-low chip select.
• SI Output to the CC2420.
• SO Input from the CC2420.
• Interface to the chip! Initialization,
  configuration, TX, RX, detailed status.
• Luckily for you, its provided as a black box.

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CC2420-specific SPI (1)First Byte

• First byte always has above format.
• Bit 7 Set to 0 for register access.
• Bit 6 Read/write control.
• Bits 50 Address of register. P. 60 of
  datasheet.
• Followed by data specific to register being
  accessed.

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CC2420-specific SPI (2)Writing to Configuration
Reg.

• First byte followed by 2 bytes of configuration
  data.
• Data on SO invalid here.
• Transceiver replies when first byte is sent out
  with status byte.
• True for all SPI accesses.
• Not necessary to inspect, but can be helpful for
  debugging!

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CC2420-specific SPI (3)Issuing Command Strobes

• One byte only. Nothing follows.
• Address sent indicates the command strobe being
  issued.
• Note that 0x00 is NO OP. This is useful for
  explicitly retrieving status byte.

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CC2420-specific SPI (4)Saving to TX FIFO

• After first byte, send n bytes of data to
  transmit over wireless.
• SPI session only ends when CS_ is pulled high.
• CC2420 replies with a new status byte with each
  byte thats saved to FIFO.

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CC2420-specific SPI (5)Receive from RX FIFO

• After first byte, send a n bytes of dont care
  in order to receive data.
• During first byte, CC2420 replies with status.
  Subsequent bytes are data saved in FIFO.
• Must be careful not to request data from empty
  FIFO!
• SPI session only ends when CS_ is pulled high.
• Reading from a configuration register is the
  same.

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Configuration Registers
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Command Strobe Registers
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TX/RX FIFO Registers
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Initialization
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Transmit
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Transmit(2)

• CC2420 needs 12 symbol periods to move into
  RX_SFD_SEARCH state after transmit done or SRXON.
• 1 symbol period 16 us.
• Without enforcing wait, aggressive user of your
  Transceiver module will cause CC2420 to never
  receive data from air.
• Not that big of a problem for us ? demo game
  engine transmits once every 25ms.
• You want to enforce wait, just in case you forget
  or make a mistake when doing CP4.

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CCA

• If you dont follow CCA we will dock MAJOR points
• If CCA isnt high, you must wait a RANDOM AMOUNT
  OF TIME
• CCA may also not go high if you have buffer
  overflow. FIFOP !FIFO

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Receive (1)
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Receive (2)

• Packets are only received after CC2420 has spent
  12 symbol periods in receive mode.
• There must be wait time between transmissions.
• Allows the transceiver to look for and receive
  data.

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Receive(3)

• Refers to 2 things
• CC2420 is constantly receiving and saving data
  into RX FIFO as long as its not transmitting.
  Look at CC2420 internal FSM on p. 43.
• You have to receive data from RX FIFO, filter
  it, then save wanted data into SPIFifo.

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Design Structure (1)
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Design Structure (2)

• Transceiver Highest level block. 32-bit
  input/output, channel changing, addressing.
• SPI Abstraction Takes care of details of CC2420
  SPI interface. Arbitrates between TX/RX.
• SPI (provided) Handles details of interface
  timing.
• SPIFifo (provided) Storage place for filtered,
  received data.

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Packet Format

• On transmit, only fill TX FIFO starting with
  length byte.
• Preamble SFD automatically appended.
• Transmit all zeros for CRC. CC2420 will replace.

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Channel Addresses

• There are 16 channels.
• Your group will be assigned a channel.
• You must be able to change channels without
  reset!
• Address are 8-bits wide ? 256 addresses. Zero is
  unused. 0xFF is reserved for broadcast.
• Your group has been assigned 2 addresses.

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Interference Debugging

• Roughly 2-3 groups per channel. Each group in a
  particular lab has distinct channel.
• Can also pick up data on neighboring channel.
• Very first goal is robust channel changing during
  initialization.
• Can pick up 802.11 packets sometimes.
• Your module must recover gracefully.
• Your project interferes with Wi-Fi vice versa.

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HandshakingInRequest/Invalid

• SPI uses a variation of this.
• You may want to use this internally.

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HandshakingReady/Start

• Transceiver uses this interface for input
  output.

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Debugging Tools

• Chipscope!
• We will be releasing some debugging utilities.
• Packet sniffer.
• Packet counter.

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Get Started!

• READ THE DATASHEET
• TAs will not be babysitting this checkpoint
• Obey our CCA rules
• Be smart about this, its a 2 week lab for a
  reason.

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GO PIKACHU GO!