Radio Frequency (RF) Data Communications

1
Radio Frequency (RF) Data Communications

• By Danny Ton
• Course CSE 498
• Professor Gaetano Borriello

2
Overview

• Radio Frequency (RF) Introduction
• RF Characteristics
• Regulations on RF Products
• Virtual Wire RF Monolithics Transceivers
• RFM Features
• RFM Data radio board
• I/O interface
• Transmitter
• Receiver
• AGC/Antenna switch

3
Overview (contd)

• Packet protocol board
• I/O interface
• RS232 interface
• Protocol microcontroller
• CMOS/RS232 level converter
• RFM link-layer packet protocol
• Protocol features
• Principles of operation
• Flow control

4
Radio Frequency (RF) Intro.

• Where RF fits in the frequency spectrum

5
Radio Frequency (RF) Intro. (contd)

• Wireless communication technology
• RF is an alternating current which, if supplied
  to an antenna, will give rise to an
  electromagnetic field that propagates through
  space
• Cheap and widely used
• Over 40 millions systems manufactured each year
  utilizing low-power wireless (RF) technology for
  data links, telemetry, control and security
• Wide range of applications
• Cordless and cellular telephones, radio and
  television broadcast stations, hand-held computer
  and PDA data links, wireless bar-code readers,
  wireless keyboards for PCs, wireless security
  systems, consumer electronic remote control, etc.

6
RF Characteristics

• Low power
• Typically transmit less than 1mW of power
• Good operating range
• Operate over distances of 3 to 30 meters
• Supports data rate up to 1-2 Mbps
• Penetrates walls
• Does not require a direct transmission path (as
  opposed to IR)

7
Regulations On RF Products

• Low-power wireless (RF) systems operate on shared
  radio channels and hence are subject to
  regulation (by FCC in the US)
• Regulation general philosophy Products do not
  significantly interfere with licensed radio
  systems
• Specify limitations on fundamental power,
  harmonic and spurious emission levels,
  transmitter frequency stability, and transmission
  bandwidth
• However, once certified to comply with
  communication regulations, RF products do not
  require a license (air-time fee) for operation

8
Virtual Wire RF Monolithics Transceivers

• Communication nodes capable of transmitting and
  receiving data
• Intended for use to implement low-power wireless
  communications based on two-way half-duplex
  packet transmissions

9
RFM Features

• Serial interface (RS232)
• Power supply
• 4.5 Vdc from three 1.5 V AAA batteries
• Operating frequency 916.50 MHz
• Maximum data rate 22.5 kbps
• Operating range up to 25 meters
• Obtained in an electrically quiet outdoor
  location
• Greatly influenced by building construction
  materials and contents, other radio systems
  operating in the vicinity, and noise generated by
  nearby equipment
• Provide link-layer packet protocol

10
RFM Modules

• Radio module
• RFM data radio board
• Transmit and receive RF signals
• Protocol module
• 8-bit ATMEL AT89C2051 microcontroller on the
  protocol board
• Implement link-layer packet protocol
• RS232 module
• Maxim MAX218 Dual RS232 transceiver on the
  protocol board
• Convert to and from 4.5V CMOS and RS232 levels
• Interface to host

11
Data Radio Board
12
Data Radio Board (contd)

• Maximum data rate
• 22.5 kbps
• Frequency options
• 916.5 MHz
• Antenna
• Simple base-loaded monopole soldered to the pad
  provided for the 50 ohm RF input
• Alternatively, a 50-ohm coaxial cable can be
  soldered to the RF input pad and the adjacent
  ground, if a remotely located antenna is used

13
Data Radio Board (contd)

• AGC adjustment
• Data radio boards are adjusted at RFM for an AGC
  voltage between 1.75 and 1.80 V on the node
  between the potentiometer, R10, and resistor R11
  with no RF signal applied
• This setting doesnt affect the sensitivity level
  of the receiver
• AGC adjustment purpose
• The desired signal must be larger than the
  undesired signal for the intelligible information
  to be obtained from the receiver
• AGC circuit is not to level the desired signal
  level but, rather, to prevent the saturation and
  eventual capture of the receivers demodulator by
  interfering in-band CW or FM signals (signals
  more than 20dB above the receiver sensitivity
  level)
• Turning the potentiometer counter-clockwise
  causes AGC voltage to increase and, thus, engages
  the gain control at a lower signal level and vice
  versa

14
Data Radio Board - I/O Interface

• Connector P1
• 8-pin connector interface to the protocol board
• Pin 1
• Transmitter data input with input impedance of
  18 KO
• Can be driven directly by a CMOS gate
• A high level voltage turns the transmitter
  oscillator on and a low level turns it off
• Pin 2 and Pin 5
• VCC for the transmitter and GND
• Pin 3
• PTT line that enable the transmit mode

15
Data Radio Board - I/O Interface (contd)

• When it is high (2.5 V minimum at 2.0 mA
  maximum), this line puts the transmit/receive RF
  switch in the transmit mode
• Pin 4
• Power line to the receiver AGC circuitry 2.7 -
  3.3 V
• Pin 6
• Reference voltage output (VRef) from the hybrid
  receiver used in the low battery detection
  process on the protocol board
• Pin 7
• Power line to the receiver hybrid 2.7 - 3.3 V
• Pin 8
• Data output from the comparator in the receiver
  hybrid
• CMOS compatible and capable of driving a single
  CMOS gate

16
Data Radio Board - Transmitter

• An HX surface mount hybrid device (HX2000)
• Pin 1
• Transmitter data input (connected to Pin 1 of
  connector P1)
• Pin 2
• Transmitter VCC power connection
• HX hybrids are specified to draw a maximum peak
  current of 10 - 11 mA with a VCC of 3 V. Since
  the transmitter is only turned on when the data
  line is high, the average transmitter current
  depends on the duty cycle of the incoming data
• Pin 3
• Ground

17
Data Radio Board - Transmitter (contd)

• Pin 4
• RF ouput
• The RF output power of the HX is nominally 0dBm
  with a 50O load
• The transmitter power is applied to the antenna
  port through the transmit/receive switch Q1
• When the PTT line is pulled high, Q1 is turned on
  to connect the transmitter to the antenna, Q3 is
  turned on to short the receiver input to GND and
  Q2 is turned off to disconnect the receiver input
  from the antenna during transmission

18
Data Radio Board - Receiver

• An RX hybrid receiver (RX2056)
• Pin 1
• VCC is applied to this pin from Pin 7 of
  connector P1
• Also connected to a 10 uF bypass capacitor, C5,
  which keeps the RX internal comparator switching
  noise out of the data base-band amplifier
  circuitry in the RX
• Pin 2
• Base-band data output
• Signal at Pin 2 is the demodulated filtered data
  before it is applied to the comparator input, on
  Pin 3
• Output from this Pin is DC coupled to the
  internal detector output

19
Data Radio Board - Receiver (Contd)

• Pin 3
• Comparator input
• Output from Pin 2 is connected to this pin by the
  coupling capacitor C6
• Coupling capacitor C6
• Prevent the change in DC offset of the base-band
  amplifier from false triggering the comparator
• Prevent the DC output from the detector, produced
  by an in-band CW or FM interfering signal, from
  triggering the comparator while allowing changes
  in DC level, due to the desired signal, to pass
  through to the comparator input
• The value of the coupling capacitor is determined
  by the longest pulse width to be encountered in
  the data stream

20
Data Radio Board - Receiver (Contd)

• The capacitor must be large enough to prevent the
  long data pulses from sagging at the comparator
  input
• Pin 4
• DC GND
• Pin 5
• The comparator threshold override pin
• If it is left open, the threshold voltage for
  comparator is 25mV. This voltage level is very
  desirable for the lower frequency, full
  sensitivity receivers to reduce spurious noise at
  the comparator output
• If it is grounded, the threshold voltage is zero
  volts. This is desirable for the 916.5 MHz
  receivers to obtain maximum sensitivity possible

21
Data Radio Board - Receiver (Contd)

• To avoid spurious noise on the comparator output
  of the 916.5 MHz receivers, use a 10 MO resistor,
  R4, from Pin 3 to GND. This resistor effectively
  reduces to DC offset on the comparator output,
  which is equivalent to using a very low threshold
  level
• Pin 6
• The reference voltage output of the power supply
  included in the custom IC used in the RX
• This pin must be bypassed by a 1 uF capacitor,
  C4, to avoid comparator switching noise in the
  base-band amplifier.
• Pin 7
• The comparator output or data output
• The comparator is capable of driving a single
  CMOS gate input

22
Data Radio Board - Receiver (Contd)

• Pins 8 and 9
• RF grounds
• Pin 10
• RF input port of the RX device
• This port is driven from a 50 O source

23
Data Radio Board - AGC/Antenna Switch

• Issue
• The out-of-band interfering signal rejection of
  the amplifier-sequenced receiver architecture is
  excellent and allows the receiver to perform in
  the presence of large interfering signals without
  range degradation
• However, this does not take care of in-band
  interference. The majority of in-band
  interference encountered is CW and primarily
  comes from unintentional radiators such as clock
  harmonics from computers or local oscillators
  from superheterodyne receivers
• An AGC circuit primarily intended for CW or FM
  in-band interfering signals. These signals are
  of particular concern in an office environment

24
Data Radio Board - AGC/Antenna Switch (contd)

• The RX receiver has capacitive coupling between
  the base-band amplifier output and the comparator
  input
• Hence, the DC level generated in the detector and
  base-band amplifier by either an FM or a CW
  signal is blocked from the comparator input and
  only desired signal passes
• However, the DC level at which the detector and
  its associated base-band amplifier saturate is
  limited (approximately -80 dBm for 433.92 MHz,
  and -50 dBm for 916.5 MHz)

25
Data Radio Board - AGC/Antenna Switch (contd)

• The AGC circuit used on the data radio board is
  to prevent saturation of the detector and
  base-band amplifier by keeping such in-band
  interfering signals below the saturation level at
  the receiver input
• An RF attenuator (particularly transistors Q2 and
  Q3) is placed between the antenna and the
  receiver input, effectively extends the range
  over which the receiver can operate w/o
  saturation by 40 dB (20 dB for each transistor)

26
Data Radio Board - AGC/Antenna Switch (contd)

• The RF attenuators Q1, Q2, and Q3 also serve as
  the transmit/receive RF switch for the radio
  board
• In transmit mode, the PTT line is pulled high,
  overriding the AGC circuit by directly biasing
  the bases of Q1 and Q3 on and turning the base of
  Q2 off through R15 and U1B. This connects the
  transmitter to the antenna port and disconnects
  the receiver from the antenna port
• In receive mode, PTT is low, allowing the
  receiver to be connected to the antenna port with
  its input level controlled by the AGC circuit only

27
Packet Protocol Board
28
Packet Protocol Board (contd)

• Why not connect the data radio board directly to
  a computer serial port using an RS232 to CMOS
  level converter?
• Error detection limited to byte parity checking
  many errors go undetected
• Greatly reduce the data radios range due to very
  poor DC balance in the data
• The protocol microcontroller provides data-link
  protocol
• error dectection
• automatic message retransmission
• message routing
• link alarms and DC-balanced packet coding

29
Packet Protocol Board (contd)

• Node address programming
• Maximum of 15 nodes addresses, set by placing
  jumpers on the double row of pins located between
  the two ICs

30
Packet Protocol Board (contd)

• Power supply options
• 4.5 Vdc nominal from three 1.5 V AAA batteries
• RS232 interface
• Level conversion from 4.5V CMOS to RS232 levels
  is provided by the MAX 218 IC.
• It is possible to remove this IC and jumper
  socket Pin 7 to 14 and Pin 9 to 12 for direct
  CMOS operation
• LED functions Three LED indicators are provided
  on the protocol board
• RXI indicates RF signals are being received
  (Diode D5)
• RF RCV indicates a valid RF packet has been
  received (DiodeD4)
• PC RCV indicates a message has been received from
  PC (Diode D3)

31
Packet Protocol Board - I/O Interface

• Connector J1
• The I/O interface between the protocol and data
  radio boards
• 8 pins
• Pin 1
• Carry the transmit data stream from U2-Pin 7 to
  the RTX input on the data radio board
• Pin 2
• Provide power to the transmitter hybrid on the
  radio board
• Pin 3
• Provide the transmit enable signal (PTT) from PNP
  transitor Q2
• The data radio board requires 2 mA at 2.5 V on
  the PTT input to enable the transmit mode

32
Packet Protocol Board - I/O Interface (contd)

• Pin 4
• Provide power to the receiver AGC circuitry
• Pin 5 - GND
• Pin 6
• The reference voltage input (VREF) from the
  hybrid receiver to the protocol board, used in
  the low battery detection process
• Pin 7
• Provide power to the receiver hybrid
• Pin 8
• Receiver output signal (RRX) from the data radio
  board
• FET Q1 provides the required high input impedance
  buffer between this signal and the input to U2

33
Packet Protocol Board - RS232 Interface

• Connector J2
• 9-pin female connector configured to appear as a
  DCE (modem)
• The protocol board implements software flow
  control, so only Pins 2 and 3 carry active signal
• Pin 2 (RD or PTX)
• Send data to the host computer
• Pin 3 (TD or PRX)
• Receive data from the host computer
• Pins 4 and 6 (DTR DSR) are connected Pins 1,
  7, and 8 are also connected
• Pin 5 - GND

34
Packet Protocol BoardProtocol Microcontroller

• Implements the link-layer protocol
• An 8-bit ATMEL AT89C2051 Microcontroller (U2)
• Operates from an 22.118 MHz quartz crystal
• 2 Kbytes of flash PEROM memory and 128 bytes of
  RAM
• Two 16-bit timers
• A hardware serial port
• The timers and hardware serial port makes it
  especially suitable as a link-layer packet
  controller
• The timers, serial port and input interrupts
  remain active while the processor is in the
  power-saving idle mode, allowing the link-layer
  protocol to be implemented on a low average
  current budget

35
Packet Protocol BoardProtocol Microcontroller
(contd)

• Inputs to the microcontroller
• Node programming pins ID0 - ID3 (Pins 14, 15, 16,
  and 17)
• The buffered receive data (RRX) on Pin 6
• The CMOS-level input from the host computer (Pin
  2)
• The reference voltage (VREF) input on Pin 13
• Outputs from the microcontroller
• The transmit data on Pin 7
• The data output to the host computer on Pin 3
• The transmit enable signal on Pin 19
• The RS232-transceiver control on Pin 18
• The LED outputs on Pins 8 (RXI), 9 (RF RCV) and
  11 (PC RCV)
• Diode D2 and capacitor C7 form the power-up reset
  circuit for the microcontroller

36
Packet Protocol BoardCMOS/RS232 Level Converter

• Conversion to and from RS232 and 4.5V CMOS logic
  levels is done by a Maxim MAX218 Dual RS232
  Transceiver (U1)
• The operation of MAX218 is controlled by the
  microcontroller (U2) to minimize average current
  consumption
• L1, D1, and C5 operate in conjunction with the
  ICs switch-mode power supply to generate /-6.5
  V for the transmitter and receiver conversions
• Pin 3 on the MAX 218
• Controls the switched-mode supply via U2 Pin 18

37
Packet Protocol BoardCMOS/RS232 Level Converter
(contd)

• The RS232 serial input signal from J2-Pin 3 is
  input on U1-Pin 12 and is converted to a 3V CMOS
  level (note inversion) and output on U1-Pin 9
• The CMOS serial output signal from U2-Pin 2 is
  input on U1-Pin 9 and converted to an RS232
  output (note inversion) on U1-Pin 12. This
  signal is found on J2-Pin 3
• Bypass RS232 conversion for direct CMOS operation
  by removing U1 from its socket and placing one
  jumper in socket Pins 7 and 14 and a second
  jumper in socket Pins 9 and 12

38
RFM Link-Layer Packet Protocol

• Firmware running on the protocol board
• Provide automatic, verified, error-free
  transmission of messages between Virtual Wire
  radio nodes
• Radio packet format
• Provide link-layer interface between a Virtual
  Wire transceiver and its host processor via
  serial connection
• RS232 packet format

39
RFM Link-Layer Packet Protocol (contd)

• Radio packet format
• To/From
• To (higher 4 bits) receiver node address
• From (lower 4 bits) sender node address
• 0x00 broadcast packet
• 15 different node addresses available
• Packet number byte 1-7
• Data size byte
• Number of actual data bytes
• Message data
• ASCII or binary, up to 32 bytes
• 16-bit FCS (Frame Check Sequence)
• 16-bit ISO 3309 error detection calculation to
  test message integrity
• The calculation is based on all bits in the
  message following the start symbol

40
RFM Link-Layer Packet Protocol (contd)

• RS232-side packet format
• Radio packet format without the Start symbol and
  the 16-bit FCS
• Example of a RS232-side packet from node 3 to
  node 2, with one is the packet number,
  containing 3 bytes of data
• 23 01 03 02 1C 03

41
RFM Link-Layer Packet Protocol (contd)

• Automatic packet retransmission until
  acknowledgment is received 8 retries with
  semi-random back-off delays (0, 120, 240, or 360
  ms)
• ACK and NAK alarm messages to host
• Operation on both the RS232 side and the radio
  side is half-duplex
• The protocol software services one input line at
  a time (radio or RS232 receive line)
• Since the protocol does not support hardware flow
  control, host software will have to do some
  timekeeping to interface to the protocol software
  (avoid sending data if RFM is busy)

42
Theory of Operation

• Operation of RS232 serial connections
• 19.2 kbps
• Eight data bits (byte), one stop bit, and no
  parity bit
• Radio operation
• Transmission rate of 22.5 kbps, using 12-bit
  dc-balanced symbol representing the data byte
• Radio receiver is slightly squelched when not
  receiving data, and will output occasional random
  positive noise spikes
• Messages are sent and received from the RS232
  interface in standard asynchronous format via PTX
  and PRX

43
Theory of Operation (contd)

• I/O lines on the protocol microcontroller
• RRX - radio receive line (J1-8)
• RTX - radio transmit line (J1-1)
• PTT - radio transmit/receive control line, high
  on transmit (J1-3)
• PRX - RS232 receive line (J2-3)
• PTX - RS232 transmit line (J2-2)
• RXI, RF RCV, PC RCV - three LED control lines

44
Theory of Operation (contd)

• The protocol software continually tests the RRX
  and the PRX lines searching for a start bit
• When the start bit is detected on one of the
  input lines (radio or RS232), the software will
  attempt to receive a message on that input line
• If error is detected, the message will be
  discarded and the software will resume testing
  the input lines
• If a valid message is received on the PRX input
  line, the software will format a radio packet
  from the message and queue the packet for xmission

45
Theory of Operation (contd)

• Each byte xmitted by the radio is converted into
  a 12 bit, dc-balanced symbol for best noise
  immunity
• The queued packet is xmitted (RTX line with PTT
  high), and the software then looks for a packet
  received ACK (on the RRX line)
• Radio ACK 0x55 RS232 ACK FCS
• On acknowledgement of the queued packet, an ACK
  (less Start symbol and FCS) is sent to host on
  PTX line, and the queued packet is discarded.
  The software then resumes testing the input lines

46
Theory of Operation (contd)

• If an acknowledgement packet is not received in
  120ms, the packet is then resent after a randomly
  selected delay of 0, 120, 240 or 360ms.
• If the packed is not acknowledged after a total
  of eight tries, the software will send a NAK
  message to host on the PTX line, discard the
  queued packet, and resume testing the input lines

47
Theory of Operation (contd)

• When a start symbol is detected on the RRX line,
  the software will attempt to receive and verify a
  message by checking for a correct TO/FROM
  address, a valid packet sequence number, and a
  valid number of data bytes (or ACK character),
  and a correct FCS calculation
• If the message is not valid, it is discarded and
  testing the input lines is resumed

48
Theory of Operation (contd)

• If the packet is verified and the TO nibble
  matches, the TO/FROM address, packet sequence
  number, number of data bytes and the data bytes
  of the message (i.e a RS232-side packet) are sent
  out on the PTX line, and a radio ACK is
  transmitted back on the RTX line.
• If an acknowledged packet is received a second
  time (based on the current value of the message
  sequence counter), it is reacked on RTX but not
  retransmitted on PTX

49
Theory of Operation (contd)

• The software will accept message packets and
  acknowledgement packets in any sesquence
• Broadcasting
• The TO/FROM address of 0x00 is treated as a
  broadcast packet. In this case, a received
  packet is sent out on the PTX line if the number
  of data bytes are in a valid range and the FCS
  calculation matches.
• A broadcast packet is not acknowledged by the
  receiving node(s)
• In the broadcast mode, the packet is transmitted
  eight times to enhance probability or reception.

50
Flow Control

• The protocol software does not support flow
  control
• If a start bit is detected on either RRX or PRX,
  the software receives and acts on the information
  on that input line and doesnt service the other
  input line until it has received and acted on the
  data of the first input line
• Host application will have to do some timekeeping
  to make sure that the RFM is not busy. This is
  done by sending just the To/From address byte to
  the RFM

51
Flow Control (contd)

• If this byte is echoed back within 50ms, host
  application has control of the PRX interrupt
  process and can send the rest of the packet in
  the following 200ms
• Else, it can assume that the RFM is busy on an
  RRX interrupt either receiving a packet or
  tripped by receiver output noise. The host
  program should hold off about 100ms and retry.
• An inbound packet can occur at any time, so any
  character with the high nibble equal to the local
  node address or any 0x00 byte should be processed
  to test for a valid message

52
Sample Codes

• RFSend(Byte ToFrom, Char Data)
• FIRST
• SerSend(ToFrom, 1)
• SECOND
• begin TimGetTicks()
• do
• end TimGetTicks()
• SerReceiveCheck(numBytes)
• while(numBytes lt 0 (end-begin/100) lt .05)
• THIRD
• if(numBytes gt 0)
• SerReceive(rcvQueue, numBytes)
• if(rcvQueue0 ToFrom)
• SerSend(pktNum, 1)
• SerSend(StrLen(data), 1)
• SerSend(data, StrLen(data))

• FIRST
• Send ToFrom byte to RFM
• SECOND
• Check to see if anything is echoed back in 50ms
• THIRD
• Check to see if this byte is the ToFrom byte
• If it is, then RFM is ready for the rest of the
  packet packet number, data size, and actual
  data else, can assume that RFM is busy.
• Note assume data size is less than 32 bytes,
  which is the maximum number of bytes that a
  packet can take

53
Sample Codes (contd)

• FOURTH
• SerReceive(rcvQueue, 3)
• switch(rcvQueue2)
• case 0xE1
• case 0xE2
• case 0xE3
• case 0xE4
• case 0xE5
• case 0xE6
• case 0xE7
• case 0xE8 if(pktNum gt 8) pktNum 1
• return true
• case 0xDD return false
• return false

• FOURTH
• Receive the echo-back packet (3 bytes in size)
• If the last byte is 0xEn where n 1 - 8 (the
  number of retries), then the packet is the ACK,
  and data is successfully sent and received Else,
  if the last byte is 0xDD, it is the NAK,
  signaling a link failure

54
Sample Codes (contd)

• RFReceive(Byte localAddr, Byte From, Byte
  PktNum, Char retData)
• FIRST
• SerReceiveCheck(numBytes)
• if (numBytes gt 3)
• SerReceive(rcvQueue, 1)
• SECOND
• if ((rcvQueue0 gtgt 4) localAddr)
• SerReceive(rcvQueue1, 2)
• From rcvQueue0 0x0F // Get FROM nibble
• PktNum rcvQueue1
• SerReceive(rcvQueue3, rcvQueue2)
• StrCopy(retData, rcvQueue3)
• return true
• return false

• FIRST
• Check the serial receive queue to see whether at
  least 3 bytes have been received (To/From, packet
  number, packet size)
• SECOND
• Get the first byte to see if it is equal to the
  local node address
• If it is, get the next 2 bytes packet number
  and packet size, and then get the rest of the
  packet based on the packet size received
• Else, do nothing