+# Device drivers for IR (infra red) remote controls
+
+This repo provides a driver to receive from IR (infra red) remote controls and
+a driver for IR "blaster" apps. The device drivers are nonblocking. They do not
+require `uasyncio` but are compatible with it.
+
+# 1. IR communication
+
+IR communication uses a carrier frequency to pulse the IR source. Modulation
+takes the form of OOK (on-off keying). There are multiple protocols and at
+least two options for carrier frequency, namely 36KHz and 38KHz.
+
+The drivers support the NEC protocol and two Philips protocols, namely RC-5 and
+RC-6 mode 0. In the case of the transmitter the carrier frequency is a runtime
+parameter: any value may be specified. The receiver uses a hardware demodulator
+which must be specified for the correct frequency. The receiver device driver
+sees the demodulated signal and is hence carrier frequency agnostic.
+
+Examining waveforms from various remote controls it is evident that numerous
+protocols exist. Some are doubtless proprietary and undocumented. The supported
+protocols are those for which I managed to locate documentation. My preference
+is for the NEC version. It has conservative timing and ample scope for error
+detection. RC-5 has limited error detection, and RC-6 mode 0 has rather fast
+timing: I doubt that detection can be accomplished on targets slower than a
+Pyboard.
+
+A remote using the NEC protocol is [this one](https://www.adafruit.com/products/389).
+
+Remotes normally transmit an address and a data byte. The address denotes the
+physical device being controlled. The data is associated with the button on the
+remote. Provision exists for differentiating between a button repeatedly
+pressed and one which is held down; the mechanism is protocol dependent.
+
+# 2. Hardware Requirements
+
+The receiver is cross-platform. It requires an IR receiver chip to demodulate
+the carrier. The chip must be selected for the frequency in use by the remote.
+For 38KHz devices a receiver chip such as the Vishay TSOP4838 or the
+[adafruit one](https://www.adafruit.com/products/157) is required. This
+demodulates the 38KHz IR pulses and passes the demodulated pulse train to the
+microcontroller. The tested chip returns a 0 level on carrier detect, but the
+driver design should ensure operation regardless of sense.
+
+The pin used to connect the decoder chip to the target is arbitrary but the
+test programs assume pin X3 on the Pyboard, pin 13 on the ESP8266 and pin 23 on
+ESP32.
+
+The transmitter requires a Pyboard 1.x (not Lite) or a Pyboard D. Output is via
+an IR LED which will normally need a transistor to provide sufficient current.
+Typically these need 50-100mA of drive to achieve reasonable range and data
+integrity. A suitable LED is [this one](https://www.adafruit.com/product/387).
+
+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.
+
+# 3. Installation
+
+On import, demos print an explanation of how to run them.
+
+## 3.1 Receiver
+
+Copy the following files to the target filesystem:
+ 1. `ir_rx.py` The receiver device driver.
+ 2. `ir_rx_test.py` Demo of a receiver.
+
+There are no dependencies.
+
+The demo can be used to characterise IR remotes. It displays the codes returned
+by each button. This can aid in the design of receiver applications. When the
+demo runs, the REPL prompt reappears: this is because it sets up an ISR context
+and returns. Press `ctrl-d` to cancel it. A real application would run code
+after initialising reception so this behaviour would not occur.
+
+## 3.2 Transmitter
+
+Copy the following files to the Pyboard filesystem:
+ 1. `ir_tx.py` The transmitter device driver.
+ 2. `ir_tx_test.py` Demo of a 2-button remote controller.
+
+The device driver has no dependencies. The test program requires `uasyncio`
+from the official library and `aswitch.py` from
+[this repo](https://github.com/peterhinch/micropython-async).
+
+# 4. Receiver
+
+This implements a class for each supported protocol, namely `NEC_IR`, `RC5_IR`
+and `RC6_M0`. Applications should instantiate the appropriate class with a
+callback. The callback will run whenever an IR pulsetrain is received.
+
+Constructor:
+`NEC_IR` args: `pin`, `callback`, `extended=True`, `*args`
+`RC5_IR` and `RC6_M0`: args `pin`, `callback`, `*args`
+Args:
+ 1. `pin` is a `machine.Pin` instance configured as an input, connected to the
+ IR decoder chip.
+ 2. `callback` is the user supplied callback (see below).
+ 3. `extended` is an NEC specific boolean. Remotes using the NEC protocol can
+ send 8 or 16 bit addresses. If `True` 16 bit addresses are assumed - an 8 bit
+ address will be correctly received. Set `False` to enable extra error checking
+ for remotes that return an 8 bit address.
+ 4. `*args` Any further args will be passed to the callback.
+
+The callback takes the following args:
+ 1. `data` Integer value fom the remote. A negative value indicates an error
+ except for the value of -1 which signifies an NEC repeat code (see below).
+ 2. `addr` Address from the remote
+ 3. `ctrl` 0 in the case of NEC. Philips protocols toggle this bit on repeat
+ button presses. If the button is held down the bit is not toggled. The
+ transmitter demo implements this behaviour.
+ 4. Any args passed to the constructor.
+
+Class variable:
+ 1. `verbose=False` If `True` emits debug output.
+
+# 4.1 Errors
+
+IR reception is inevitably subject to errors, notably if the remote is operated
+near the limit of its range, if it is not pointed at the receiver or if its
+batteries are low. So applications must check for, and usually ignore, errors.
+These are flagged by data values < `REPEAT` (-1).
+
+On the ESP8266 there is a further source of errors. This results from the large
+and variable interrupt latency of the device which can exceed the pulse
+duration. This causes pulses to be missed. This tendency is slightly reduced by
+running the chip at 160MHz.
+
+In general applications should provide user feedback of correct reception.
+Users tend to press the key again if the expected action is absent.
+
+Data values passed to the callback are normally positive. Negative values
+indicate a repeat code or an error.
+
+`REPEAT` A repeat code was received.
+
+Any data value < `REPEAT` denotes an error. In general applications do not
+need to decode these, but they may be of use in debugging. For completeness
+they are listed below.
+
+`BADSTART` A short (<= 4ms) start pulse was received. May occur due to IR
+interference, e.g. from fluorescent lights. The TSOP4838 is prone to producing
+200µs pulses on occasion, especially when using the ESP8266.
+`BADBLOCK` A normal data block: too few edges received. Occurs on the ESP8266
+owing to high interrupt latency.
+`BADREP` A repeat block: an incorrect number of edges were received.
+`OVERRUN` A normal data block: too many edges received.
+`BADDATA` Data did not match check byte.
+`BADADDR` Where `extended` is `False` the 8-bit address is checked
+against the check byte. This code is returned on failure.
+
+# 4.2 Receiver platforms
+
+The NEC protocol has been tested against Pyboard, ESP8266 and ESP32 targets.
+The Philips protocols - especially RC-6 - have tighter timing constraints. I
+have not yet tested these, but I anticipate problems.
+
+# 4.3 Principle of operation
+
+Protocol classes inherit from the abstract base class `IR_RX`. This uses a pin
+interrupt to store in an array the start and end times of pulses (in μs).
+Arrival of the first pulse triggers a software timer which runs for the
+expected duration of an IR block (`tblock`). When it times out its callback
+(`.decode`) decodes the data and calls the user callback. The use of a software
+timer ensures that `.decode` and the user callback can allocate.
+
+The size of the array and the duration of the timer are protocol dependent and
+are set by the subclasses. The `.decode` method is provided in the subclass.
+
+CPU times used by `.decode` (not including the user callback) were measured on
+a Pyboard D SF2W at stock frequency. They were NEC 1ms for normal data, 100μs
+for a repeat code. Philips codes: RC-5 900μs, RC-6 mode 0 5.5ms.
+
+# 5 Transmitter
+
+This is specific to Pyboard D and Pyboard 1.x (not Lite).
+
+It implements a class for each supported protocol, namely `NEC`, `RC5` and
+`RC6_M0`. The application instantiates the appropriate class and calls the
+`transmit` method to send data.
+
+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,
+ and for 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 0 or 1.
+ In the case of NEC, if an address < 256 is passed, normal mode is assumed and
+ the complementary value is appended. 16-bit values are transmitted as extended
+ addresses.
+ In the case of NEC the `toggle` value is ignored. For Philips protocols it
+ should be toggled each time a button is pressed, and retained if the button is
+ held down. The test program illustrates a way to do this.
+
+The `transmit` method is synchronous with rapid return. Actual transmission
+occurs as a background process, 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.
+
+# 5.1 Wiring
+
+I use the following circuit which delivers just under 40mA to the diode. R2 may
+be reduced for higher current.
+
+
+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.
+
+
+The transistor type is not critical.
+
+These circuits assume 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.py` should be changed to 100.
+
+# 5.2 Principle of operation
+
+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 in the subclass which populates this
+array. On completion `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 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.
+
+The `IR.append` enables times to be added to the array, keeping track of the
+notional carrier on/off state for biphase generation. The `IR.add` method
+facilitates lengthening a pulse as required in the biphase sequences used in
+Philips protocols.
+
+# 6. References
+
+The NEC protocol is described in these references:
+[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)
+
+The Philips protocols may be found in these refs:
+[RC5](https://en.wikipedia.org/wiki/RC-5)
+[RC6](https://www.sbprojects.net/knowledge/ir/rc6.php)
+
+# Appendix 1 NEC Protocol description
+
+A normal burst comprises exactly 68 edges, the exception being a repeat code
+which has 4. An incorrect number of edges is treated as an error. All bursts
+begin with a 9ms pulse. In a normal code this is followed by a 4.5ms space; a
+repeat code is identified by a 2.25ms space. A data burst lasts for 67.5ms.
+
+Data bits comprise a 562.5µs mark followed by a space whose length determines
+the bit value. 562.5µs denotes 0 and 1.6875ms denotes 1.
+
+In 8 bit address mode the complement of the address and data values is sent to
+provide error checking. This also ensures that the number of 1's and 0's in a
+burst is constant, giving a constant burst length of 67.5ms. In extended
+address mode this constancy is lost. The burst length can (by my calculations)
+run to 76.5ms.
+
+A pin interrupt records the time of every state change (in µs). The first
+interrupt in a burst sets an event, passing the time of the state change. A
+coroutine waits on the event, yields for the duration of a data burst, then
+decodes the stored data before calling the user-specified callback.
+
+Passing the time to the `Event` instance enables the coro to compensate for
+any asyncio latency when setting its delay period.
+
+The algorithm promotes interrupt handler speed over RAM use: the 276 bytes used
+for the data array could be reduced to 69 bytes by computing and saving deltas
+in the interrupt service routine.
projects / micorpython_ir.git / blobdiff