Raw sensor collection: avoid String heap churn by writing directly into SD buffer

[awickert]

Background

As part of ongoing standardization of the Northern Widget sensor API, a generic raw-reading collection function is planned for the Margay library (using C++ templates so it works with any NW sensor library without a runtime dependency):

template<typename SensorType>
void collectRawReadings(SensorType& sensor, uint16_t nReadings,
                        uint8_t component, File& sdFile);

Each sensor library will implement a standard takeRawReading() method that returns one reading's worth of data.

Issue

If takeRawReading() returns an Arduino String, the collection loop will make N heap allocations in rapid succession — one per reading. On the ATMega1284p (16 KB SRAM), this risks heap fragmentation over a long-running deployment, which can cause silent failures.

Proposed enhancement

Rather than returning a String, takeRawReading() should write directly into a caller-provided char buffer at a given offset:

uint16_t takeRawReading(char* buf, uint16_t offset);
// writes CSV data into buf starting at offset, returns new offset

The Margay collection function maintains a 512-byte char buffer (matching the SD card sector size) and flushes to SD at sector boundaries. This:

• Eliminates all heap allocation during the collection loop
• Batches SD writes at the natural sector size (minimizing write operations and power consumption)
• Keeps memory usage fixed and predictable

Context

• SD card sector size = 512 bytes → natural buffer size
• ATMega1284p: 16 KB SRAM, currently ~1940 bytes used by globals in a typical Margay sketch
• This design is part of the broader NW sensor API standardization effort across Apis, Haar, MaxBotix, T9602, and Walrus libraries