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 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 ensures operation regardless of sense.
In my testing a 38KHz demodulator worked with 36KHz and 40KHz remotes, but this is obviously neither guaranteed nor optimal.
The TSOP4838 can run from 3.3V or 5V supplies. The former should be used on non-5V compliant hosts such as ESP32 and Raspberry Pi Pico and is fine on 5V compliant hosts too.
The pin used to connect the decoder chip to the target is arbitrary. The test
program acquire.py uses the following pins by default:
| Host | Pin | Notes |
|---|---|---|
| Pyboard | X3 | |
| ESP32 | 23 | |
| ESP8266 | 13 | |
| D1 Mini | D7 | WeMos name for pin 13. |
| Pico | 16 |
A remote using the NEC protocol is this one.
The receiver is a Python package. This minimises RAM usage: applications only import the device driver for the protocol in use. Clone the repository to the current directory of your PC with:
$ git clone https://github.com/peterhinch/micropython_irCopy the following to the target filesystem:
ir_rxDirectory and contents.
There are no dependencies.
The demo can be used to characterise IR remotes where the protocol is known. It displays the codes returned by each button. This can aid in the design of receiver applications. The demo prints "running" every 5 seconds and reports any data received from the remote.
from ir_rx.test import testInstructions will be displayed at the REPL.
If the protocol in use is unknown, there are two options: trial and error with the above script or run the following:
from ir_rx.acquire import test
test()This script waits for a single burst from the remote and prints the timing of
the pulses followed by its best guess at the protocol. It correctly identifies
supported protocols, but can wrongly identify unsupported protocols. The
report produced by the script exposed to an unknown protocol is unpredictable.
The test() function returns a list of the mark and space periods (in μs).
This implements a class for each supported protocol. Each class is subclassed
from a common abstract base class in __init__.py.
Applications should instantiate the appropriate class with a callback. The callback will run whenever an IR pulse train is received. Example running on a Pyboard:
import time
from machine import Pin
from pyb import LED
from ir_rx.nec import NEC_8 # NEC remote, 8 bit addresses
red = LED(1)
def callback(data, addr, ctrl):
if data < 0: # NEC protocol sends repeat codes.
print('Repeat code.')
else:
print('Data {:02x} Addr {:04x}'.format(data, addr))
ir = NEC_8(Pin('X3', Pin.IN), callback)
while True:
time.sleep_ms(500)
red.toggle()Constructor args:
pinis amachine.Pininstance configured as an input, connected to the IR decoder chip.callbackis the user supplied callback.*argsAny further args will be passed to the callback.
The user callback takes the following args:
data(int) Value from the remote. Normally in range 0-255. A value < 0 signifies an NEC repeat code.addr(int) Address from the remote.ctrl(int) The meaning of this is protocol dependent:
NEC: 0
Philips: this is toggled 1/0 on repeat button presses. If the button is held down it is not toggled. The transmitter demo implements this behaviour.
Sony: 0 unless receiving a 20-bit stream, in which case it holds the extended value.- Any args passed to the constructor.
Bound variable:
verbose=FalseIfTrueemits debug output.
Methods:
error_functionArg: a function taking a singleintarg. If specified the function will be called if an error occurs. The arg value corresponds to the error code. Typical usage might be to provide some user feedback of incorrect reception although beware of occasional triggers by external events. In my testing the TSOP4838 produces 200µs pulses on occasion for no obvious reason. See section 4.closeNo args. Shuts down the pin and timer interrupts.
A function is provided to print errors in human readable form. This may be invoked as follows:
from ir_rx.print_error import print_error # Optional print of error codes
# Assume ir is an instance of an IR receiver class
ir.error_function(print_error)Class variables:
- These are constants defining the NEC repeat code and the error codes sent to the error function. They are discussed in section 4.
NEC_8, NEC_16
Typical invocation:
from ir_rx.nec import NEC_8Remotes using the NEC protocol can send 8 or 16 bit addresses. If the NEC_16
class receives an 8 bit address it will get a 16 bit value comprising the
address in bits 0-7 and its one's complement in bits 8-15.
The NEC_8 class enables error checking for remotes that return an 8 bit
address: the complement is checked and the address returned as an 8-bit value.
A 16-bit address will result in an error.
SONY_12, SONY_15, SONY_20
Typical invocation:
from ir_rx.sony import SONY_15The SIRC protocol comes in 3 variants: 12, 15 and 20 bits. SONY_20 handles
bitstreams from all three types of remote. Choosing a class matching the remote
improves the timing reducing the likelihood of errors when handling repeats: in
20-bit mode SIRC timing when a button is held down is tight. A worst-case 20
bit block takes 39ms nominal, yet the repeat time is 45ms nominal.
A single physical remote can issue more than one type of bitstream. The Sony
remote tested issued both 12 bit and 15 bit streams.
RC5_IR, RC6_M0
Typical invocation:
from ir_rx.philips import RC5_IRThese support the RC-5 (including RC-5X) and RC-6 mode 0 protocols respectively.
MCE
Typical invocation:
from ir_rx.mce import MCEI have been unable to locate a definitive specification: the protocol was analysed by a mixture of googling and experiment. Behaviour may change if I acquire new information. The protocol is known as OrtekMCE and the remote control is sold on eBay as VRC-1100.
The remote was designed for Microsoft Media Center and is used to control Kodi on boxes such as the Raspberry Pi. With a suitable PC driver it can emulate a PC keyboard and mouse. The mouse emulation uses a different protocol: the class does not currently support it. Pressing mouse buttons and pad will cause the error function (if provided) to be called.
Args passed to the callback comprise 4 bit addr, 6 bit data and 2 bit ctrl
with the latter having the value 0 for the first message and 2 for the message
sent on key release. Intermediate messages (where the key is held down) have
value 1.
There is a 4-bit checksum which is used by default. The algorithm requires an
initial 'seed' value which my testing proved to be 4. However the only
documentation I
could find stated that the value should be 3. I implemented this as a class
variable MCE.init_cs=4. This enables it to be changed if some remotes use 3.
If the value is set to -1 the check will be skipped.
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. The user callback is not called when an error occurs.
On ESP8266 and ESP32 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 or their timing measured incorrectly. On ESP8266 some improvment may be achieved 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.
In debugging a callback can be specified for reporting errors. The value passed to the error function are represented by constants indicating the cause of the error. These are driver ABC class variables and are as follows:
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 (NEC_IR) If extended is False the 8-bit address is checked
against the check byte. This code is returned on failure.
Currently the ESP8266 suffers from this issue. Testing was therefore done without WiFi connectivity. If the application uses the WiFi regularly, or if an external process pings the board repeatedly, the crash does not occur.
Philips protocols (especially RC-6) have tight timing constraints with short pulses whose length must be determined with reasonable accuracy. The Sony 20 bit protocol also has a timing issue in that the worst case bit pattern takes 39ms nominal, yet the repeat time is 45ms nominal. These issues can lead to errors particularly on slower targets. As discussed above, errors are to be expected. It is up to the user to decide if the error rate is acceptable.
Reception was tested using Pyboard D SF2W, ESP8266 and ESP32 with signals from remote controls (where available) and from the tranmitter in this repo. Issues are listed below.
NEC: No issues.
Sony 12 and 15 bit: No issues.
Sony 20 bit: On ESP32 some errors occurred when repeats occurred.
Philips RC-5: On ESP32 with one remote control many errors occurred, but paired
with the transmitter in this repo it worked.
Philips RC-6: No issues. Only tested against the transmitter in this repo.
Protocol classes inherit from the abstract base class IR_RX. This uses a pin
interrupt to store in an array the time (in μs) of each transition of the pulse
train from the receiver chip. Arrival of the first edge starts a software timer
which runs for the expected duration of an IR block (tblock). The use of a
software timer ensures that .decode and the user callback can allocate.
When the timer times out its callback (.decode) decodes the data. .decode
is a method of the protocol specific subclass; on completion it calls the
do_callback method of the ABC. This resets the edge reception and calls
either the user callback or the error function (if provided).
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.
It is possible to capture an IR burst from a remote and to re-create it using the transmitter. This has limitations and is discussed in detail in the transmitter doc.
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.
The bitstream comprises a header (2ms mark, 1ms space) followed by 16 bits of Manchester encoded data with a bit time of 500μs. Data are encoded
ccccddddddppaaaa
Where aaaa is the address, pp is the position (toggle) field, dddddd is
data and cccc is a checksum. This is calculated by counting the ones in
ddddddppaaaa and adding 4. Data are transmitted LSB first.
The only doc I could find states that the checksum seed value is 3, but this did not match the remote I have.