[micorpython_ir.git] / TRANSMITTER.md

# 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 needs a simple circuit 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 the demo uses pins 21 and 23 for IR output and pins 18 and 19 for
pushbuttons. These pins may be changed.

## 1.1 Pyboard 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.

## 1.2 ESP32 Wiring

The ESP32 RMT device does not currently support the carrier option. A simple
hardware gate is required to turn the IR LED on when both the carrier pin and
the RMT pin are high. A suitable circuit is below.  
![Image](images/gate.png)

The transistor type is not critical. A gate could be built with two similarly
connected N-channel MOSFETS. The 1KΩ resistors would not be required. The
MOSFETS would require a low RDSon at Vgs == 3.3V. A ZVN4210A seems suitable
but I haven't tried it.

# 2. Dependencies and installation

## 2.1 Dependencies

The device driver has no dependencies.

On ESP32 a firmware version >= V1.12 is required. The Loboris port is not
supported owing to the need for the RMT device.

The demo program requires `uasyncio` from the official library and `aswitch.py`
from [this repo](https://github.com/peterhinch/micropython-async).

## 2.2 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 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.

#### Common to all classes

Constructor args:  
 1. `pin` A Pin instance instantiated as an output. On a Pyboard this is a
 `pyb.Pin` instance supporting Timer 2 channel 1: `X1` is employed by the test
 script. On ESP32 any `machine.Pin` may be used. 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 (a lot of) 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 values < 256.

A value passed in `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. Principle of operation

## 4.1 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 ) 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.

## 4.2 ESP32

The carrier is output continuously at the specified duty ratio. A pulse train
generated by the RMT instance drives a hardware gate such that the IR LED is
lit only when both carrier and RMT are high.

The carrier is generated by PWM instance `.pwm` running continuously. The ABC
constructor converts the 0-100 duty ratio specified by the subclass to the
0-1023 range used by ESP32.

# 5. 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)