X-Git-Url: https://vault307.fbx.one/gitweb/micorpython_ir.git/blobdiff_plain/22acf8e1d02787907c7eacba1756b5acd1c4495f..36140aa568d4a538adafb03e575b26de8a0784ef:/TRANSMITTER.md?ds=inline diff --git a/TRANSMITTER.md b/TRANSMITTER.md new file mode 100644 index 0000000..3478ba8 --- /dev/null +++ b/TRANSMITTER.md @@ -0,0 +1,193 @@ +# IR Transmitter + +##### [Main README](./README.md#1-ir-communication) + +# 1. Hardware Requirements + +The transmitter requires a Pyboard 1.x (not Lite), a Pyboard D or an ESP32. +Output is via an IR LED which will need a transistor to provide sufficient +current. Typically these need 50-100mA of drive to achieve reasonable range and +data integrity. A suitable 940nm LED is [this one](https://www.adafruit.com/product/387). + +On the Pyboard the transmitter test script assumes pin X1 for IR output. It can +be changed, but it must support Timer 2 channel 1. Pins for pushbutton inputs +are arbitrary: X3 and X4 are used. The driver uses timers 2 and 5. + +On ESP32 pin 23 is used for IR output and pins 18 and 19 for pushbuttons. The +ESP32 solution has limitations discussed in [section 5.2](./TRANSMITTER.md#52-esp32). + +## 1.1 Wiring + +I use the following circuit which delivers just under 40mA to the diode. R2 may +be reduced for higher current. +![Image](images/circuit.png) + +This alternative delivers a constant current of about 53mA if a higher voltage +than 5V is available. R4 determines the current value and may be reduced to +increase power. +![Image](images/circuit2.png) + +The transistor type is not critical. + +The driver assumes circuits as shown. Here the carrier "off" state is 0V, +which is the driver default. If using a circuit where "off" is required to be +3.3V, the constant `_SPACE` in `ir_tx.__init__.py` should be changed to 100. + +# 2. Installation + +The transmitter is a Python package. This minimises RAM usage: applications +only import the device driver for the protocol in use. + +Copy the following to the target filesystem: + 1. `ir_tx` Directory and contents. + +The device driver has no dependencies. + +The demo program requires `uasyncio` from the official library and `aswitch.py` +from [this repo](https://github.com/peterhinch/micropython-async). The demo is +of a 2-button remote controller with auto-repeat. It may be run by issuing: + +```python +from ir_tx.test import test +``` +Instructions will be displayed at the REPL. + +# 3. The driver + +This is specific to Pyboard D, Pyboard 1.x (not Lite) and ESP32. + +It implements a class for each supported protocol, namely `NEC`, `SONY_12`, +`SONY_15`, `SONY_20`, `RC5` and `RC6_M0`. Each class is subclassed from a +common abstract base class in `__init__.py`. The application instantiates the +appropriate class and calls the `transmit` method to send data. + +The ESP32 platform is marginal in this application because of imprecision in +its timing. The Philips protocols are unsupported as they require unachievable +levels of precision. Test results are discussed [here](./TRANSMITTER.md#52-esp32). + +Constructor +All constructors take the following args: + 1. `pin` An initialised `pyb.Pin` instance supporting Timer 2 channel 1: `X1` + is employed by the test script. Must be connected to the IR diode as described + below. + 2. `freq=default` The carrier frequency in Hz. The default for NEC is 38000, + Sony is 40000 and Philips is 36000. + 3. `verbose=False` If `True` emits debug output. + +Method: + 1. `transmit(addr, data, toggle=0)` Integer args. `addr` and `data` are + normally 8-bit values and `toggle` is normally 0 or 1; details are protocol + dependent and are described below. + +The `transmit` method is synchronous with rapid return. Actual transmission +occurs as a background process, on the Pyboard controlled by timers 2 and 5. +Execution times on a Pyboard 1.1 were 3.3ms for NEC, 1.5ms for RC5 and 2ms +for RC6. + +#### NEC class + +This has an additional method `.repeat` (no args). This causes a repeat code to +be transmitted. Should be called every 108ms if a button is held down. + +The NEC protocol accepts 8 or 16 bit addresses. In the former case, a 16 bit +value is transmitted comprising the 8 bit address and its one's complement, +enabling the receiver to perform a simple error check. The `NEC` class supports +these modes by checking the value of `addr` passed to `.transmit` and sending +the complement for 8 bit values. + +`toggle` is ignored. + +#### Sony classes + +The SIRC protocol supports three sizes, supported by the following classes: + 1. 12 bit (7 data, 5 address) `SONY_12` + 2. 15 bit (7 data, 8 address) `SONY_15` + 3. 20 bit (7 data, 5 addresss, 8 extended) `SONY_20` + +The `.transmit` method masks `addr` and `data` values to the widths listed +above. `toggle` is ignored except by `SONY_20` which treats it as the extended +value. + +#### Philips classes + +The RC-5 protocol supports a 5 bit address and 6 or 7 bit (RC5X) data. The +driver uses the appropriate mode depending on the `data` value provided. + +The RC-6 protocol accepts 8 bit address and data values. + +Both send a `toggle` bit which remains constant if a button is held down, but +changes when the button is released. The application should implement this +behaviour, setting the `toggle` arg of `.transmit` to 0 or 1 as required. + +# 4. Test results + +# 5. Principle of operation + +## 5.1 Pyboard + +The classes inherit from the abstract base class `IR`. This has an array `.arr` +to contain the duration (in μs) of each carrier on or off period. The +`transmit` method calls a `tx` method of the subclass which populates this +array. This is done by two methods of the base class, `.append` and `.add`. The +former takes a list of times (in μs) and appends them to the array. A bound +variable `.carrier` keeps track of the notional on/off state of the carrier: +this is required for bi-phase (manchester) codings. + +The `.add` method takes a single μs time value and adds it to the last value +in the array: this pulse lengthening is used in bi-phase encodings. + +On completion of the subclass `.tx`, `.transmit` appends a special `STOP` value +and initiates physical transmission which occurs in an interrupt context. + +This is performed by two hardware timers initiated in the constructor. Timer 2, +channel 1 is used to configure the output pin as a PWM channel. Its frequency +is set in the constructor. The OOK is performed by dynamically changing the +duty ratio using the timer channel's `pulse_width_percent` method: this varies +the pulse width from 0 to a duty ratio passed to the constructor. The NEC +protocol defaults to 50%, the Sony and Philips ones to 30%. + +The duty ratio is changed by the Timer 5 callback `._cb`. This retrieves the +next duration from the array. If it is not `STOP` it toggles the duty cycle +and re-initialises T5 for the new duration. + +## 5.2 ESP32 + +This is something of a hack because my drivers work with standard firmware. + +A much better solution will be possible when the `esp32.RMT` class supports the +`carrier` option. A fork supporting this is +[here](https://github.com/mattytrentini/micropython). You may want to adapt the +base class to use this fork: it should be easy and would produce a solution +capable of handling all protocols. + +A consequence of this hack is that timing is imprecise. In testing NEC +protocols were reliable. Sony delivered some erroneous bitsreams but may be +usable. Philips protocols require timing precision which is unachievable; these +are unsupported and an exception will be thrown on an attempt to instantiate a +Philips class on an ESP32. + +The ABC stores durations in Hz rather than in μs. This is because the `period` +arg of `Timer.init` expects an integer number of ms. Passing a `freq` value +enables slightly higher resolution timing. In practice timing lacks precision +with the code having a hack which subtracts a nominal amount from each value to +compensate for the typical level of overrun. + +The carrier is generated by PWM instance `.pwm` with its duty cycle controlled +by software timer `._tim` in a similar way to the Pyboard Timer 5 described +above. The ESP32 duty value is in range 0-1023 as against 0-100 on the Pyboard. + +# 6. References + +[General information about IR](https://www.sbprojects.net/knowledge/ir/) + +The NEC protocol: +[altium](http://techdocs.altium.com/display/FPGA/NEC+Infrared+Transmission+Protocol) +[circuitvalley](http://www.circuitvalley.com/2013/09/nec-protocol-ir-infrared-remote-control.html) + +Philips protocols: +[RC5](https://en.wikipedia.org/wiki/RC-5) +[RC5](https://www.sbprojects.net/knowledge/ir/rc5.php) +[RC6](https://www.sbprojects.net/knowledge/ir/rc6.php) + +Sony protocol: +[SIRC](https://www.sbprojects.net/knowledge/ir/sirc.php)