[hal] i2c: bus is driven low if building with JTAG/SWD enabled.

[XuGuohui]

Problem

I2C bus is driven LOW after Wire.begin() when building with JTAG/SWD enabled on Electron.

Solution

Initialize I2C pins to be float input.

Example App

#include "Particle.h"

SYSTEM_THREAD(ENABLED);
SYSTEM_MODE(SEMI_AUTOMATIC);

void setup() {
    Wire.begin();
}

void loop() {
}

Steps to test

1. Build modular firmware with JTAG/SWD enabled:
   make PLATFORM=electron DEBUG_BUILD=y USE_SWD_JTAG=y

2. Flash system firmware and the above application.

3. Use logic analyser to monitor the SDA(D0) and SCL(D1) and see if both pins go to high level or not.

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)