[gen3] fixes hardfault during low level USB peripheral initialization under an atomic section

[XuGuohui]

Problem

When logging from ISR is enabled and use the Serial1 as the log interface, device runs into SOS. This is because that SD APIs require SVC interrupt to be enabled, whereas logging from ISR will prehibit the SVC interrupt and device has a chance to call into SD APIs and then hardfault.

Solution

Temporarily increase the BASEPRI value to enable SVC interrupt handling.

Example App

#include "Particle.h"

SYSTEM_MODE(MANUAL);
SYSTEM_THREAD(ENABLED);

Serial1LogHandler log(115200, LOG_LEVEL_ALL);

void setup() {
}

void loop() {
}

References

N/A

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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)