Smart Remote features a built-in protocol-agnostic infrared transmission framework, which can be used to transmit any IR symbol. The framework uses a protocol-specific driver which converts a given symbol into a series of commands sent to the PWM peripheral, which controls the IR diode. Because transmission is fully controlled by the PWM hardware (commands are fetched by the PWM peripheral using DMA), the infrared subsystem has no hard real-time requirements. It also requires only a minimum amount of processing power and memory.
The Smart Remote firmware comes with two IR protocols implemented (SIRC and NEC), however support for new ones can be easily added. Additionally, there is no technical obstacle to support several protocols simultaneously.
Implementation details
The infrared transmitter subsystem consists of the protocol-agnostic module and the protocol-specific driver. The module is responsible for integration with the Smart Remote firmware infrastructure, as well as for performing high-level power and transmission management.
The IR module provides driver with the symbol (sr3_ir_symbol_t) that should be transmitted, which is repeated in a loop until cancellation. The driver does not control the IR transmitter directly. Instead, the given symbol is translated into a series of PWM control words fetched from the memory and interpreted by the PWM controller.
Each control word specifies PWM output polarity and duty cycle and can be replayed for a predefined number of times. For example, in the SIRC protocol, a logical one bit is transmitted as a 1.2 ms pulse burst followed by 600 us of silence while the logical zero bit is represented as a 600 us pulse burst followed by 600 us of silence. To transmit the presented "10" sequence, only five control words and the following PWM setup are needed:
PWM settings:
- PWM Clock: 1 MHz
- PWM Top Value: 25
- PWM Cycle Time: 1 us * 25 = 25 us (which gives 40 kHz carrier frequency used by SIRC)
- Control word repeat count: 24 (control word time: 24 * 25 us = 600 us)
Control Words:
- Logical one: 8, 8, 0 (24 pulses with 8/25 duty cycle, followed by 24 pulses with 8/25 duty cycle, followed by 24 pulses with 0/25 duty cycle)
- Logical zero: 8, 0 (24 pulses with 8/25 duty cycle, followed by 24 pulses with 0/25 duty cycle)
With the presented approach, the complete 12-bit SIRC symbol can be encoded using just 41 PWM control words. Moreover, the whole symbol transmission is performed solely by the PWM controller hardware. The infrared subsystem does not interfere with other system activity.