Module 04: Hardware-Accelerated Environmental Sensing

🧠 Strategic Objective

Integrating the HTS221 capacitive digital sensor to monitor temperature via the I2C2 bus, while implementing high-performance floating-point interpolation.

⚙️ Technical Specs

Peripheral: I2C2 (PB10/SCL, PB11/SDA)
Clock Source: HSI16 (16MHz) to ensure 100kHz I2C Standard Mode timing
Math: Hardware FPU enabled for linear interpolation
Clock Tree: HSI16 (16MHz) selected via RCC_CFGR for stable 115200 baud UART and 100kHz I2C timing.
FPU Activation: Manual enablement of the Coprocessor Access Control Register (CPACR) bits 20-23 in the Reset_Handler.
Protocol: I2C2 (PB10/SCL, PB11/SDA) using Open-Drain mode for physical pull-up compatibility.

🧱 Engineering Challenges & Solutions

1. The Silicon “Handbrake”: Enabling the FPU

On the Cortex-M4F, the Floating Point Unit is disabled by default. Attempting to calculate temperature calibration values without enabling the CPACR (Coprocessor Access Control Register) resulted in immediate hardware exceptions. I updated the Reset_Handler to provide full access to CP10 and CP11 coprocessors.

2. Non-Blocking I2C Driver

To prevent a “Bus Hang” from freezing the entire system, I implemented diagnostic timeouts in the I2C read/write routines. This ensures that the SysTick-based heartbeat LED continues to blink even if the sensor is physically disconnected.

3. Linear Interpolation Math

The HTS221 provides raw ADC counts that must be mapped to factory-calibrated temperature points ($T0$ and $T1$). I utilized the following linear interpolation formula in the shared driver:

\[T = \frac{(T1 - T0) \cdot (T_{out} - T0_{out})}{T1_{out} - T0_{out}} + T0\]

I implemented a denominator check to ensure $T1{out} \neq T0{out}$, preventing a Divide-by-Zero crash.

4. The Floating Point “Trap”

While the Cortex-M4F has a hardware Floating Point Unit, it is disabled by default. Attempting to perform the linear interpolation required for HTS221 calibration resulted in immediate system hangs. I resolved this by adding FPU enablement logic to the startup.c before main() is called.

5. Robust I2C State Machine

I implemented diagnostic timeouts in all I2C polling loops (TXIS, TC, RXNE). This prevents the entire firmware from freezing if the sensor is disconnected, ensuring the SysTick-driven heartbeat LED remains operational.