/*
Milan, Italy
Originally published: 10/04/2010
latest revision: 17/06/2018
PJDL (Padded Jittering Data Link) v2.0 specification
Invented by Giovanni Blu Mitolo,
released into the public domain

Related implementation: /src/strategies/SoftwareBitBang/
Compliant versions: PJON 9.0 and following

Changelog:
- Added frame separation
- Added communication modes specification
*/

PJDL v2.0

PJDL (Padded Jittering Data Link) is a simplex or half-duplex data link layer, that can be easily software emulated, enabling one or many to many communication over a single conductive medium or bus, connected to device's input-output ports. It has been engineered to have limited minimum requirements and to be efficiently implemented on limited microcontrollers with low clock accuracy. No additional hardware is required to apply PJDL and, being implemented in less than 350 lines of code, it is easily portable to many different architectures. Bus maximum length is limited by its electric resistance; it has been tested with up to 50m long insulated wires and results demonstrate the same high performance achieved with shorter lengths.

Basic concepts

  • Define a synchronization pad to identify a byte
  • Use synchronization pad's falling edge to achieve byte level synchronization
  • Use 3 consequent synchronization pads to identify a frame
  • Detect interference or absence of communication at byte level
  • Support collision avoidance and detection
  • Support error detection
  • Support 1 byte synchronous response to frame transmission
PJDL SINGLE WIRE BUS                            ______
 ______    ______    ______    ______          |      |
|      |  |      |  |      |  |      |         |DEVICE|
|DEVICE|  |DEVICE|  |DEVICE|  |DEVICE|         |______|
|______|  |______|  |______|  |______|             |
___|__________|________|___________|_______/\/\/\__| IO PIN
 ___|__    __|___    ___|__    ___|__   |  110-180 Ω
|      |  |      |  |      |  |      |  |
|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 reduce externally induced interference. Pins can be optionally protected against overload adding a current limiting resistor to each connected pin. The resistor value can be obtained solving the following equation R = (operating voltage / pin max current drain), for example to obtain the current limiting resistor value for an Arduino Uno simply substitute its characteristics: R = (5v / 0.030A) = 166.66Ω.

Byte transmission

PJDL byte transmission is composed by 10 bits and occurs LSB-first. The 2 most significant bits are a longer than standard logic 1 followed by a standard logic 0, they are called synchronization pad and they are used to obtain binary sampling synchronization. The 8 least significant bits contain information. The reception technique is based on finding a logic 1 which duration is equal or acceptably shorter than the expected synchronization pad's duration, synchronizing to its falling edge and ensuring that it is followed by a standard logic 0. If this pattern is detected, reception starts, if not, interference, synchronization loss or simply absence of communication is detected at byte level.

 __________ ___________________________
| SyncPad  | Byte                      |
|______    |___       ___     _____    |
|  |   |   |   |     |   |   |     |   |
|  | 1 | 0 | 1 | 0 0 | 1 | 0 | 1 1 | 0 |
|__|___|___|___|_____|___|___|_____|___|
   |
Minimum acceptable HIGH padding bit duration

The synchronization pad adds a certain overhead, although, including synchronization along with the data eliminates the need of accurate timing or of a dedicated clock line. The minimum acceptable synchronization pad's duration is the timeframe in which a receiver initiating polling can correctly receive a byte. If the length of the first padding bit is less than the minimum acceptable duration, the received signal is considered interference. The minimum acceptable duration of the first padding bit must be shorter than a padding bit duration; a large minimum acceptable duration reduces the chances of false positive's occurrences, a small minimum acceptable duration instead mitigates timing inaccuracies, for this reason it is suggested to evaluate its setting depending on requirements and available resources. The presence of synchronization pads with their logic 1 between each byte also ensures that a frame composed of a series of bytes with decimal value 0 can be transmitted safely without risk of third-party collision.

Frame transmission

Before a frame transmission, the communication medium is analysed, if logic 1 is present ongoing communication is detected and collision is avoided, if logic 0 is detected for a duration longer than a byte transmission plus a small random timeframe, frame transmission starts with a symbol composed by 3 consequent synchronization pads followed by data bytes. The synchronization pad is used for frame initialization and byte initialization to reduce the amount of resources required to implement those features.

 ________ _________________ __________________________________
|ANALYSIS|   FRAME INIT    | DATA 1-65535 bytes               |
|________|_____ _____ _____|________________ _________________|
|        |Sync |Sync |Sync |Sync | Byte     |Sync | Byte      |
|        |___  |___  |___  |___  |     __   |___  |      _   _|
|        |   | |   | |   | |   | |    |  |  |   | |     | | | |
|00000000| 1 |0| 1 |0| 1 |0| 1 |0|0000|11|00| 1 |0|00000|1|0|1|
|________|___|_|___|_|___|_|___|_|____|__|__|___|_|_____|_|_|_|

When a frame is received a low performance microcontroller with an inaccurate clock can correctly synchronize with transmitter during the frame initializer and consequently each byte is received. On receiver's side a frame reception starts if 3 synchronization pads are detected and if their duration is equal or higher than:

frame initializer expected duration - (sync pad bit 1 duration - sync pad bit 1 minimum acceptable duration)

To ensure 100% reliability the synchronization pad's minimum acceptable duration must be higher than 1 standard bit duration. Selecting a correct sync pad bit 1 / standard bit ratio, called pad-bit ratio, frame initialization is 100% reliable, false positives cannot occur if not because of externally induced interference. synchronization pad's first bit duration must not be an exact multiple of a standard bit duration, for this reason pad-bit ratio of 2.0, 3.0 or 4.0 must be avoided because consecutive bits may be interpreted as a frame initializer.

Synchronous response

A frame transmission can be optionally followed by a synchronous response by its recipient.

Transmission                                    Response
 ______  ______  ______  ______                   _____
| INIT || BYTE || BYTE || BYTE | CRC COMPUTATION | ACK |
|------||------||------||------|-----------------|-----|
|      ||      ||      ||      | LATENCY         |  6  |
|______||______||______||______|                 |_____|

Between frame transmission and a synchronous response there is a variable timeframe which duration is influenced by latency and CRC computation time. In order to avoid other devices to consider the medium free and disrupt an ongoing exchange, sender cyclically transmits a shorter than one bit logic 1 (which exact length depends on practical requirements) and consequently attempts to receive a response. On the other side receiver can synchronize its response transmission after the last incoming high bit, detect if acknowledgement was lost by transmitter and try again if necessary.

Transmission                                      Response
 ______  ______  ______  ______   _   _   _   _   _ _____
| INIT || BYTE || BYTE || BYTE | | | | | | | | | | | ACK |
|------||------||------||------| | | | | | | | | | |-----|
|      ||      ||      ||      | | | | | | | | | | |  6  |
|______||______||______||______|_| |_| |_| |_| |_| |_____|

The maximum time dedicated to potential acknowledgement reception and consequent medium jittering is estimated adding the maximum frame length CRC computation time to the expected latency. Thanks to the presence of the jittering wave many differently configured devices can coexist on the same medium with no risk of collision.

Communication modes

The proposed communication modes are the result of years of testing and optimization and have been selected to be easily supported by limited microcontrollers.

MODE Bit timing Sync bit timing Pad-bit ratio Speed
1 40 112 2.8 2.11kB/s - 16944Bd
2 36 88 2.444 2.42kB/s - 19417Bd

Binary timing durations are expressed in microseconds.


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