If you’ve ever tried to exchange structured data between Simulink and a microcontroller, you’ve probably hit the same wall I did: Simulink wants to send and receive tidy signal vectors, while your firmware thinks in C structs and raw bytes. ASCII-based protocols like printing comma-separated values over serial are tempting, but they’re slow, fragile, and a nightmare to parse at 115200 baud when your control loop needs deterministic timing.
This post walks through the binary serial protocol I built for Mini-HVAC, an IoT Hardware-in-the-Loop platform that pairs a five-subsystem Simulink model with an ESP32-WROVER running custom FreeRTOS firmware. The protocol is simple, but getting it right required thinking carefully about struct layout, byte ordering, and how to prove correctness before plugging in a single wire.
The problem
The Mini-HVAC system needs two-way real-time communication between a PC running Simulink and an ESP32 microcontroller:
- PC to ESP32: simulated plant state (temperature, humidity, pressure) plus actuator commands (fan duty, heater duty, alarm flag).
- ESP32 to PC: real sensor readings, user-selected setpoints, operating mode (AUTO/MANUAL), and HVAC enable flag.
An ASCII protocol — something like "25.0,50.0,0.5,0.3,0.1,1\n" — would work for a demo, but it has real problems in a control loop:
- Variable-length messages make framing unreliable. A dropped byte shifts every field.
- Float-to-string-to-float round trips lose precision and burn CPU cycles on both ends.
- Parsing overhead on the ESP32 side (
sscanf,strtok, etc.) is surprisingly expensive when you’re also running FreeRTOS tasks for sensors, actuators, and an LCD.
Binary structs solve all three. Fixed-length packets, zero conversion loss, and reception is just Serial.readBytes into a struct.
The solution: matched C structs and Byte Pack/Unpack
The protocol is built around two packed C structs — one for each direction:
// PC (Simulink) → ESP32: 24 bytes
struct PcToEsp {
float T_sim; // simulated temperature
float RH_sim; // simulated humidity
float P_sim; // simulated pressure/flow
float u_fan_sim; // fan duty cycle [0, 1]
float u_heater_sim; // heater duty cycle [0, 1]
float alarm_flag; // alarm active (0.0 or 1.0)
}; // 6 × 4 = 24 bytes
// ESP32 → PC (Simulink): 32 bytes
struct EspToPc {
float T_real; // measured temperature
float RH_real; // measured humidity
float u_fan_real; // actual fan state
float u_heater_real; // actual heater state
float T_set; // temperature setpoint
float RH_set; // humidity setpoint
float mode_auto_manual; // 0.0 = MANUAL, 1.0 = AUTO
float hvac_enable; // 0.0 = OFF, 1.0 = ON
}; // 8 × 4 = 32 bytes
Every field is a float (IEEE 754 single-precision, 4 bytes). This is a deliberate choice: Simulink’s single type and C’s float use the same representation, so there’s no conversion step — just a memcpy-equivalent in both directions.
On the ESP32 side
Sending is a one-liner. The FreeRTOS TaskTX task reads sensors, updates the state struct, and writes it directly to the UART:
Serial.write((uint8_t *)&toSend, sizeof(toSend)); // 32 raw bytes
Receiving is equally simple. TaskRX waits for exactly 24 bytes, then casts them straight into the command struct:
if (Serial.available() >= (int)sizeof(PcToEsp)) {
Serial.readBytes((uint8_t *)&localCmd, sizeof(PcToEsp));
safeUpdatePcCmd(localCmd);
applyPCCommands(localCmd);
}
No parsing. No delimiters. No state machine. The struct is the wire format.
On the Simulink side
Simulink mirrors this with its Byte Pack and Byte Unpack blocks (from the Simulink Real-Time / Utility Blocks library):
-
TX_To_ESP32subsystem: takes 6singlesignals, feeds them into a Byte Pack block configured for 6 inputs of typesingle, and outputs auint8[24]vector. The signal connection order must exactly match the field order ofPcToEsp. -
RX_From_ESP32subsystem: takes auint8[32]vector from the Serial Receive block, feeds it into a Byte Unpack block configured for 8 outputs of typesingle, and produces the 8 signals that map back toEspToPcfields.
The critical configuration on the Simulink side:
- Data type:
single(notdouble— Simulink defaults todoubleinternally, so you need explicit Data Type Conversion blocks). - Byte order:
little-endian(matches the ESP32’s Xtensa architecture). - Alignment: 1 byte (no padding between fields).
The gotchas
Endianness
Both the ESP32 (Xtensa LX6) and x86/ARM PCs are little-endian, so the bytes land in the same order on both sides. If you’re targeting a big-endian platform, you’d need to byte-swap each float — but for the ESP32 + modern PC combination, this is a non-issue. Still, it’s worth verifying explicitly: the Serial Configuration block in Simulink has a byte-order setting, and it must be set to little-endian.
Struct padding
C compilers are free to insert padding bytes between struct fields for alignment. A struct with a uint8_t followed by a float might silently grow from 5 to 8 bytes. By using only float fields (all 4 bytes, naturally aligned), padding never kicks in. The struct size is always N × 4 bytes, exactly matching what Byte Pack/Unpack expects.
If your protocol needs mixed types (say, a uint8_t flag alongside floats), you’d need __attribute__((packed)) on the C side — but then you pay a performance penalty on some architectures. The all-float approach avoids this entirely.
Compile-time size checks with static_assert
Even with an all-float layout, a typo (an extra field, a wrong type) could silently change the struct size and break the protocol. The firmware guards against this with compile-time assertions:
static_assert(sizeof(PcToEsp) == 24, "PcToEsp must be 24 bytes");
static_assert(sizeof(EspToPc) == 32, "EspToPc must be 32 bytes");
If someone adds a field and forgets to update the Simulink side, the build fails immediately instead of producing a subtle runtime bug where fields shift by 4 bytes and the fan duty cycle ends up in the temperature slot.
Field ordering
The most insidious bug in binary protocols is a field-order mismatch. The bytes are correct — the same floats, the same endianness, the same total size — but field 3 on one side is field 4 on the other. Nothing will catch this at compile time.
The defense is a single source of truth. In this project, the C struct definition in the firmware is the spec. The Simulink subsystem wires its signals in the same order, and the MATLAB validation scripts (next section) verify byte-for-byte equivalence against that order.
The validation: proving correctness without hardware
Here’s the part I found most valuable in practice. Before the ESP32 was even connected, two MATLAB scripts validated that Simulink and the firmware would agree on every byte.
TEST_TX_To_ESP32_bytes.m — validating the transmit path
This script builds the expected uint8[24] byte vector for a known set of PcToEsp values:
% Define test values matching the C struct field order
T_sim = 25.0; RH_sim = 50.0; P_sim = 0.5;
u_fan_sim = 0.3; u_heater_sim = 0.1; alarm_flag = 1.0;
% Pack as single-precision floats, then reinterpret as bytes
vals = single([T_sim, RH_sim, P_sim, u_fan_sim, u_heater_sim, alarm_flag]);
expected_bytes = typecast(vals, 'uint8'); % uint8[24]
After running the Simulink model, the tx_bytes output from the TX_To_ESP32 subsystem is compared against this reference:
isequal(expected_bytes, out.tx_bytes) % must return 1 (true)
If there’s a mismatch, the script reports which byte differs — making it trivial to trace back to a wiring or type-conversion error in the Simulink model.
TEST_RX_From_ESP32_bytes.m — validating the receive path
The reverse direction: generate the 32 bytes that an ESP32 would send, inject them into the RX_From_ESP32 subsystem via a Constant block, and verify that the unpacked signals match the original values.
% Build the EspToPc byte vector
vals = single([T_real, RH_real, u_fan_real, u_heater_real, ...
T_set, RH_set, mode_auto_manual, hvac_enable]);
esp_bytes = typecast(vals, 'uint8'); % uint8[32]
% Verify round-trip: bytes → singles → check against originals
vals_recovered = typecast(esp_bytes, 'single');
Both scripts also perform a round-trip check: bytes back to floats, compared against the originals. This catches subtle issues like accidentally using double instead of single somewhere in the chain (which would produce 48 or 64 bytes instead of 24 or 32).
Why this matters
This validation strategy means that by the time the ESP32 is physically connected, the only things that can go wrong are hardware-level issues: a loose wire, a wrong baud rate, a task-scheduling conflict. The data packing and unpacking layer has already been proven correct. In practice, this saved significant debugging time — the first hardware test worked on the first try.
Putting it all together
The full data flow looks like this:
┌─────────────────────────────────────────────────────┐
│ PC (Simulink) │
│ │
│ MiniHVAC_Zone ──► TX_To_ESP32 ──► Serial Send │
│ (plant model) (Byte Pack) (uint8[24]) │
│ │ │
│ Scopes/Control ◄── RX_From_ESP32 ◄── Serial Recv │
│ (Byte Unpack) (uint8[32]) │
│ │ │
└────────────────────────────────────────┼─────────────┘
USB Serial
115200 baud
┌────────────────────────────────────────┼─────────────┐
│ ESP32-WROVER │ │
│ │ │
│ TaskRX ──────► applyPCCommands() │ │
│ (24 bytes in) (fan PWM, heater, │ │
│ alarm LED) │ │
│ │ │
│ TaskTX ◄────── updateSensorsAndInputs() │
│ (32 bytes out) (DHT22, pots, buttons) │
│ │
│ TaskLCD ──────► 16×2 I²C display │
│ │
└──────────────────────────────────────────────────────┘
Three FreeRTOS tasks share the PcToEsp and EspToPc structs through mutex-protected accessors (safeUpdatePcCmd, safeReadEspState, etc.), keeping synchronization simple and deterministic.
Key takeaways
-
Use all-float structs to avoid padding issues. If you need boolean flags, encode them as
0.0/1.0floats — it’s 3 extra bytes per flag, but it eliminates an entire class of alignment bugs. -
static_assertthe struct sizes in firmware. It’s one line of code that catches protocol-breaking changes at compile time. -
Validate byte-for-byte in MATLAB before touching hardware. MATLAB’s
typecast(single(...), 'uint8')gives you the exact wire representation. Compare it against Simulink’s output and the firmware’s expected input. -
Set Simulink’s byte order explicitly to
little-endian. Don’t rely on defaults — the Serial Configuration block’s byte-order setting is easy to miss. -
Keep one source of truth for field ordering. The C struct definition is the spec; everything else (Simulink wiring, MATLAB test vectors) derives from it.
The full project — firmware, Simulink models, MATLAB validation scripts, schematics, and 11 chapters of documentation — is on GitHub: dcamacho16/sdp-minihvac.
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