Magnetic Sensor

Service manuals quotations, for a 2002 manual-tranny Subaru WRX

• TRANSMISSION_SECTION\Manual_Transmission_and_Differential_MT\007__Vehicle_Speed_Sensor.pdf (page MT-39):
  describes how to remove the speed sensor from the transmission gearbox
• BODY_SECTION/Instrumentation/Driver_Info_IDI/002__Combination_Meter_System.pdf (page IDI-6):
  Check Vehicle Speed Sensor: Measuring voltage of the Vss output while going faster than 12mph should read between 1 volt and 4 volts.

  The power supply going into the speed sensor should be >10v (eg. 12 volts)

• MECHANISM_AND_FUNCTION_SECTION/INSTRUMENTATION/DRIVER_INFO/001__Combination_Meter.pdf (page IDI-8):
  Needle points to 60 km/h (37 .3 miles) when 2,548 pulses are input per minute (which means that a pulse is sent every 15 inches driven)

  The vehicle speed sensor uses a Hall IC pick-up to generate speed signals. (MT model)

  This sensor is installed on the transmission case and detects rotating speed of the transmission output gear.

  The sensor generates 4 pulses per rotation of the speed sensor driven shaft and send them to the speedometer.

  The speed sensor mainly consists of a Hall IC, magnet ring, driven shaft and spring.

Here is a basic description of hall sensors in general. From what I can read in various postings, it really looks like this is a 0-5v square wave.

Microcontroller

• VSS signal will be processed by a RS232 line-receiver, as it will accept anything between -30v and +30v, and convert down to TTL levels. Hysteresis and other noise protection is also included.
• Total number of pulses seen (aka pulse counter) is stored and transmitted as an unsigned 32bit number. At maximum speed, it shouldn't wrap around for the first 331 days of continuous driving at 125mph.
• Pulse wavelength is transmited as an unsigned 16-bit number. Units not decided yet.
• Data sent in little-endian form (since I only work on windows and linux)

Excel RTD Server

• COM-port configuration read from registry. One thread reads from serial, one thread responds to Excel.
• Although the current time-of-day is available via Excel functions, it's also provided by the RTD server (as a double), so that the spreadsheet's time and distance will both be updated at the same instant, to minimize the inconsistencies presented on the spreadsheet.

Excel

• Excel uses doubles to represent dates. They take up 64 bits total, of which 52 bits are used for the fraction. The number of days used is either since 1900 or since 1904. Either way, 104 years = 37985 days, which should use up about 15 bits of the 52. So there's ~37 bits left for the time (expressed as fractional days). Which provides for about 500 nanoseconds between each possible time (that's 0.0006 milliseconds resolution, which should be far more than enough)