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64 lines
3.0 KiB
ReStructuredText
64 lines
3.0 KiB
ReStructuredText
DATA STRUCTS
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============
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The struct serves as the data payload for sending control signals from the transmitting device to the receiver using ESP-NOW.
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In addition, it may contain additional data such as telemetry, battery status, etc. The *sensors_data_t* struct encapsulates all control commands and sensor states
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relevant to the vehicle's operation. It's intended to be sent from a transmitting device (like a remote control) to a receiver
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(such as a microcontroller on board of the vehicle).
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.. code-block:: c
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typedef struct {
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int x_axis; // Joystick x-position
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int y_axis; // Joystick y-position
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bool nav_bttn; // Joystick push button
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bool led; // LED ON/OFF state
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uint8_t motor1_rpm_pwm; // PWMs for 4 DC motors
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uint8_t motor2_rpm_pwm;
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uint8_t motor3_rpm_pwm;
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uint8_t motor4_rpm_pwm;
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} __attribute__((packed)) sensors_data_t;
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.. code-block:: c
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struct motors_rpm {
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int motor1_rpm_pwm;
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int motor2_rpm_pwm;
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int motor3_rpm_pwm;
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int motor4_rpm_pwm;
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};
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When used with communication protocols like ESP-NOW, this struct is **encoded** into a byte stream, then
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**transmitted** at regular intervals or in response to user input, and finally
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**decoded** on the receiving end to control hardware.
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.. admonition:: What is struct?
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In C programming, a struct (short for structure) is a user-defined data type that lets you group multiple variables of different types together under a
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single name. It's like a container that holds related information — perfect for organizing data that logically belongs together. Structs are especially
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powerful in systems programming, embedded projects, and when dealing with raw binary data — like parsing sensor input or transmitting control packets over
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ESP-NOW.
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Data Payload
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------------
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*x_axis* and *y_axis* fields capture analog input from a joystick, determining direction and speed.
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*nav_bttn* represents a joystick push-button.
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*led* allows the transmitter to toggle an onboard LED and is used for status indication (e.g. pairing, battery warning, etc).
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*motor1_rpm_pwm* to *motor4_rpm_pwm* provide individual PWM signals to four DC motors.
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This enables fine-grained speed control, supports differential drive configurations, and even allows for maneuvering in multi-directional platforms like omni-wheel robots.
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Why use __attribute((packed))?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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ESP-NOW uses fixed-size data packets (up to 250 bytes). The *__attribute__((packed))* removes compiler-added padding for precise byte alignment.
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As *packed* attribute tells the compiler not to add any padding between fields in memory, this makes the struct:
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- Compact
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- Predictable for serialization over protocols like UART or ESP-NOW
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- Ideal for low-latency transmission in embedded systems
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This ensures the receiver interprets the exact byte layout you expect, minimizing bandwidth and maximizing compatibility across platforms. |