Robust Low-Power MSP430 Networking with a Buffer Insertion Ring

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Robust Low-Power MSP430 Networking with
aBuffer Insertion Ring

• Glen Worstell
• Sugle Consulting
• worstell_at_copper.net
• More info
• www.worstell.com

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Agenda

• Application areas for small networks
• Common small network solutions
• The Buffer Insertion Ring network
• Protocol
• MSP430 Hardware
• MSP430 Software
• Comparison of BIR with other solutions
• Description of BIR demonstration code
• QA

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Small network applications

• Auto and other vehicle data control
• LIN commonly used
• Industrial process control
• Lab data collection
• Home automation
• Medical data collection
• HVAC

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Common Micro Networks

• CAN
• Multi-Master
• 1Mbps
• 8 Byte Messages
• LIN
• Master-Slave (sort of)
• 20K bps
• 8 Byte Messages
• BIR
• Multi-Master
• 50K bps
• 8 Byte Messages
• Home-grown RS-485

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Why BIR?

• Extremely low idle power
• Isolated (with isolated power supply)
• Lowest cost, esp. in small quantities
• Uses available hardware (UART)
• Low EMI generation
• Low EMI susceptibility
• Multi-master
• Can use with other micros (but not low power)

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Why not BIR?

• Cant disable interrupts for very long
• But could use 2nd MSP430
• Requires Xtal
• But could use LIN-like scheme
• 1 Byte latency per node
• About 1 Msec per node at 10K baud

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BIR Concept

• Unique node numbers
• Node sends message (to all other nodes) whenever
  necessary
• Asynchronous serial node transceivers
• Built-in UART
• Bit-Bang (F1121)
• 4-Wire Cable (RJ-11?)
• Power, Ground
• Balanced pair BIR data
• 2 to 32 nodes
• Optical Isolators

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BIR message format
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Transceiver Hardware
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9-bits/byte Message Format

• Address Byte
• Node Number of Sender (5 bits)
• Number of Data Bytes (2 bits)
• Parity (1 bit)
• Data Byte (8 bits)
• Extra Data Bytes (8 bits each)
• Checksum (8 bits, like LIN or CRC-8)

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Number of Data Bytes Coding

• 0 One Data Byte
• 1 Two Data Bytes
• 2 Four Data Bytes
• 3 Eight Data Bytes

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Insertion Buffer Idea

• Each node has a buffer
• Incoming data goes in buffer
• To Send
• If buffer is empty, put message into buffer
• Otherwise, wait until it is empty
• Messages from other nodes are passed on
• Messages from this node are not passed
• A note about baud rate divisors
• If small n, n should be odd

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BIR software flowchart
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The Three-node Demo

• Each node sends to a random other node
• Sending occurs at random intervals
• Random message turn on LED or turn off LED
• Stop blinking for one minute if any error is
  detected
• Source code on web
• Uses simple RTOS and Random number generator

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Demo photo

• BIR demo - 2 HW UART, 1 Timer-A UART

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An RTOS for limited RAM

• Single Stack
• Cooperative
• Standard C
• See Salvo for similar scheme
• Source code on web
• www.worstell.com

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Basic RTOS idea

• Suspend only from Main()
• Nothing is on the stack
• Need to save only the return address
• See ProtoThreads, Adam Dunkels,
  www.sics.se/adam/pt/

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A good PRNG without hardware multiply

• Poor PRNGs are very common
• Feedback shift registers
• arithmetic modulo a power-of-2
• Random Number Generators - Good Ones are Hard to
  Find
• Park ,SK KW Miller, CACM, Oct. 1998
• X(n1) (AX(n)B) mod M
• Multiplicative, Congruential Random-Number
  Generators with Multiplier - 2K1 - 2K2 and
  Modulus 2P - 1, Pei Chi Wu, ACM Trans. On
  Mathematical Software, June 1977

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QA

• Email worstell_at_copper.net
• More info, source code www.worstell.com