EmCan Protocol Specification

• EmCan Master index

The 48 bits are divided into three fields, the sign, exponent, and mantissa. 

The sign field is in bit 47 (the most significant bit) and is 1 bit wide.  A value of 0 means the overall number is zero or positive, and a value of 1 means it is negative. 

The exponent field is in bits 46:32 and is 15 bits wide.  It is a unsigned integer value indicating the power of 2 to be applied to the mantissa value, plus 16384.  For example, a value of 16387 means the mantissa is to be scaled by 2³ = 8, and 16380 that the scale factor is 2^-4 = 1/16.  The special value of 0 is reserved for when the overall number value is zero.  For non-zero values, the exponent field range is therefore 1-32767, which is a scaling range of 2^-16383 to 2¹⁶³⁸³. 

The mantissa field is in bits 31:0 and is 32 bits wide.  It is the most significant 32 fraction bits of a number with the units bit 1 and all higher bits 0.  Put another way, there is a implied integer one and binary point immediately to the left of the mantissa value.  The mantissa field therefore represents a value from 1 up to but not including 2. 

The special case of all 48 bits 0 indicates the value zero. 

To determine the overall value of the 48 bit floating point number, perform the following steps:

1. Check for all bits 0.  If so, the overall all value is 0 and the remaining steps are skipped. 

2. Determine the value represented by the mantissa field. This is always greater than or equal to 1 and less than 2. 

3. Multiply the value from step 2 by the scale factor represented by the exponent field. 

4. If the sign bit is 0, don't alter the number. If the sign bit is 1, negate the number.