9S08AC16 Temperature Sensor Voltage

[airbusboy]

Hi All

I've just migrated up from an 08QG8 to an 08AC16 part and was wondering why i was getting wildly inaccurate temperature readings from the internal temperature sensor using imported 08QG8 code.

Anyway, i found out the reason - there's been a subtle change in the temperature sensor reference voltage @25°C from 0.7012 to 1.41mV.

However, the footnote for this value states "Typical values assume VDDAD = 5.0V, Temp = 25C, fADCK=1.0 MHz unless otherwise stated. Typical values are for reference only and are not tested in production".

So my question is this, has this value been definitively determined yet for various values of VCC  etc. I based my original 08QG8 code around AN3031, unfortunately, this now appears obsolete for 08AC16 use.

Regards

Gary

[bigmac]

Hello Gary,

The temperature sensor for the 'QG8 consists of a single diode, presumably fed by a constant current source.  The voltage drop across the diode is measured.

For the 'AC16 device, it would be reasonable to assume that the sensor now consists of two diodes connected in series, to effectively double the resolution of the temperature measurement.

The calibration processes described in AN3031 should still be applicable to the later device.  Note that the variation of the readings for different Vdd voltages, as graphed in the application note, is primarily because of the similar variation of Vref for the ADC module (Vref = Vdd).

Regards,

Mac