Arduino Zero System Clock and I2C SCL frequency

Projects Networking, Protocols, and Devices
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I am trying to speed up the I2C SCL clock speed on my Arduino Zero which is communicating with an Analog Devices ADXL372 digital accelerometer. Using Wire.setClock, if I set my SCL to 100kHz I read a speed of 85kHz on my scope, and if I set my SCL to 400kHz, I read a speed of 315kHz. Digging deeper into the libraries I found that the I2C baud rate is defined as follows:

sercom->I2CM.BAUD.bit.BAUD = SystemCoreClock / ( 2 * baudrate) - 5 - (((SystemCoreClock / 1000000) * WIRE_RISE_TIME_NANOSECONDS) / (2 * 1000));

Now the Arduino Zero documentation says the CPU clock is 48MHz, but if you look in system_samd21.c, the macro __SYSTEM_CLOCK which is assigned to the variable SystemCoreClock is 1000000. Why is there this discrepancy?

If I plug in 1000000 to the calculation above I get negative numbers. What does this mean for the I2C SCL?

Furthermore, the datasheet for the SAMD21 says that it supports I2C speeds up to 3.4MHz.

Here's how I am setting up the I2C:

Wire.begin(i2c_addr);
Wire.setClock(400000);

Does anyone have any insight?

Edit: on further investigation, I find that SystemCoreClock is set equal to the macro VARIANT_MCK which is defined as 48000000ul in startup.c, but if I plug this value into the calculation above, for a 400kHz SCL I get the result 52. I have no idea what this means.

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The description of the formula and how it's derived is described in section 28.6.2.4. of the datasheet (page 543).

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