SoftwareBitBang

Medium: Wire | Pins used: 1 / 2


SoftwareBitBang is based on micros and delayMicroseconds, needs 1 or optionally 2 wires and no additional hardware to handle one or many to many communication on a single channel or bus. It can be run on cheap and low performance microcontrollers. Bus maximum length is mostly limited by the resistance of the common conductive element used. It has been tested with up to 50m long insulated wires, results demonstrate the same high performance achieved with shorter lengths. It complies with PJDL v1.1 data link layer specification. Take a look at the video introduction for a brief showcase of its features.

 ______     ______      ______      ______      ______
|      |   |      |    |      |    |      |    |      |
|DEVICE|   |DEVICE|    |DEVICE|    |DEVICE|    |DEVICE|
|______|   |______|    |______|    |______|    |______|
___|___________|___________|___________|___________|___
 ___|__     ___|__    ___|__     ___|__  | SINGLE WIRE BUS
|      |   |      |  |      |   |      | |
|DEVICE|   |DEVICE|  |DEVICE|   |DEVICE| |___/\/\/\___ GND
|______|   |______|  |______|   |______|     1-5 MΩ

It is suggested to add 1-5 MΩ pull-down resistor as shown in the graph above to protect MCU pins and to reduce interference.

Compatibility

  • ATmega88/168/328 16Mhz (Diecimila, Duemilanove, Uno, Nano, Mini, Lillypad)
  • ATmega2560 16Mhz (Arduino Mega)
  • ATmega16u4/32u4 16Mhz (Arduino Leonardo)
  • ATtiny45/85 16Mhz external oscillator, see ATtiny interfacing
  • SAMD (Arduino Zero)
  • ESP8266 v.1-7 80Mhz "AI-THINKER AT" firmware, see Arduino
  • ESP8266 NodeMCU v0.9-1.0 80Mhz, see Arduino
  • MK20DX256 96Mhz (Teensy 3.1)

Performance

SWBB_MODE can be configured in 3 different modes, SWBB_STANDARD, SWBB_FAST and SWBB_OVERDRIVE:

  • SWBB_STANDARD (1) runs at 16944Bd or 2.12kB/s cross-architecture, promiscuous clock/architecture compatible.
  • SWBB_FAST (2) runs at 21504Bd or 2.68kB/s cross-architecture, promiscuous clock/architecture compatible.
  • SWBB_OVERDRIVE (3) runs a specific architecture at its maximum limits (non cross-architecture compatible). Every architecture has its own limits, Arduino Duemilanove for example runs at 31250Bd or 3.906kB/s, Arduino Zero can reach 48000Bd or 6kB/s.

When including and using SoftwareBitBang, as data link layer of a PJON bus, you have the complete access to the microntroller ready to be used, as usual, untouched. This happens because SoftwareBitBang is completely software emulated strategy with a non blocking implementation, transforming a painful walk to the hill in a nice flight.

Single wire simplicity let you to experiment quickly and with creativity. The first suggested test, at the tester's risk, is to let two arduino boards communicate through a living body touching with the left hand the digital port of the first board (5v 40ma, harmless) and with the right the port of the other one. It is stunning to see highly accurate digital communication running inside a living biological body. This opens the mind to possible creative solutions.

How to use SoftwareBitBang

Pass the SoftwareBitBang type as PJON template parameter to instantiate a PJON object ready to communicate in this Strategy. All the other necessary information is present in the general Documentation.

  /* The default SoftwareBitBang mode is SWBB_STANDARD
     (Transfer speed: 16.944kBb or 2.12kB/s) */

  /* Set SoftwareBitBang mode to SWBB_FAST before PJON.h inclusion
     (Transfer speed: 21.504kBd or 3.15kB/s)
     When used with a group of different devices a pull-down resistor is
     suggested to have optimal channel reliability */
  #define SWBB_MODE 2

  /* Set SoftwareBitBang mode to SWBB_OVERDRIVE before PJON.h inclusion
     (Architecture / Toolchain dependant) */
  #define SWBB_MODE 3

  /* Synchronous acknowledgement response timeout (1.5 milliseconds by default)
     If (latency + CRC computation) > SWBB_RESPONSE_TIMEOUT synchronous
     acknowledgement reliability could be affected or disrupted higher
     SWBB_RESPONSE_TIMEOUT if necessary. */
  #define SWBB_RESPONSE_TIMEOUT 1500

  /* Set the back-off exponential degree (default 4) */
  #define SWBB_BACK_OFF_DEGREE     4

  /* Set the maximum sending attempts (default 20) */
  #define SWBB_MAX_ATTEMPTS       20

  #include <PJON.h>

  PJON<SoftwareBitBang> bus;

  void setup() {
    bus.strategy.set_pin(12);       // Set the pin 12 as the communication pin
                                    // or
    bus.strategy.set_pins(11, 12);  // Set pin 11 as input pin and pin 12 as output pin
  }

After the PJON object is defined with its strategy it is possible to set the communication pin accessing to the strategy present in the PJON instance.

Why not interrupts?

The use of libraries is really extensive in the Arduino environment and often the end user is not able to go over collisions. Very often a library is using hardware resources of the microcontroller as timers or interrupts, colliding or interrupting other libraries. This happens because in general Arduino boards have limited hardware resources. To have a universal and reliable communication medium in this sort of environment, software emulated bit-banging, is a good, stable and reliable solution that leads to "more predictable" results than interrupt driven systems coexisting on small microcontrollers without the original developer and the end user knowing about it.

SoftwareBitBang video introduction

Known issues

  • A 1-5 MΩ pull down resistor could be necessary to reduce interference, see deal with interference.
  • Consider that this is not an interrupt driven system, during the time passed in delay or executing other tasks a certain amount of packets could be potentially lost, PJON does its job scheduling the packet to be sent again in future but a certain amount of bandwidth can be wasted. Structure intelligently your loop cycle to avoid huge blind timeframes.
  • SoftwareBitBang strategy can have compatibility issues with codebases that are using interrupts, reliability or bandwidth loss can be experienced because of the cyclical interruptions made by third party software to the PJON procedure.

This document is automatically generated from the github repository. If you have noticed an error or an inconsistency, please report it opening an issue here