Trimmed ADC_VREF

As discussed in UTE - Temperature sensor, the reference buffer ADC_VREF is a temperature compensated voltage that is trimmed and tested against specification limits during production tests. The assumption of calculating the UTE using the upper and lower limits of ADC_VREF with Δ V_{REF_ADC} of ± 50 mV could be adjusted by measuring V_{REF_ADC}. To avoid measuring out ADC_VREF in software by the user, the Final Part Test (FPT) of the netX 90 was extended to store the trimmed ADC_VREF value in the InfoPage of INTFLASH2. The FPT change is aligned with the PCN 220275. All devices with ROM Code Revision 2 enable calculating the on-chip temperature using the trimmed ADC_VREF value for V_{REF_ADC}. The trimming error of ADC_VREF is Δ V_{REF_ADC} ± 10 mV.

Access to the trimmed ADC_VREF value requires mapping the content of the InfoPage to the entry address of INTFLASH2. The code sequence, how the mapping is performed, is part of the low-level driver package within the source file system_netx.c and must be executed from internal SRAM.

The InfoPage includes two sets of data, initially supposed to be used by the end user, as a placeholder to calibrate out the temperature error. This procedure would require reference measurements at two different temperatures for each device that are often perceived as not acceptable by the user as it increases the testing time of the embedded device.

The structure of the data set is represented by a typedef in the header file system_netx.h:

typedef struct NETX_TEMP_CALIBRATION_Ttag
{
  unsigned long aulCalDate[2];
  unsigned short ulADCValue;
  unsigned char sRefTemperature;
  unsigned char saucReserved[9];
  unsigned long ulCrc32;
} NETX_TEMP_CALIBRATION_T;

The trimmed ADC_VREF value is stored in the reserved field of the data set at offset:

Offset	Size [bytes]	Name	Description
2048	8	atTempDiode[0].s.aucCalDate	Calibration date for temperature diode at temperature 0
2056	2	atTempDiode[0].s.usADCValue	ADC value equivalent to reference temperature for temperature diode at temperature 0
2058	1	atTempDiode[0].s.cRefTemperature	Reference temperature for temperature diode at temperature 0
2059	1	atTempDiode[0].s.aucReserved1	Reserved
2060	2	atTempDiode[0].s.usADCVREFValue	Trimmed ADC_VREF x 2¹⁴stored during final part test
2062	6	atTempDiode[0].s.aucReserved2	Reserved
2068	4	atTempDiode[0].ulCrc32	CRC32 value over the whole structure

The stored 16-bit integer for the variable usADCVREFValue must be divided by 2¹⁴ to get the analog value of the trimmed ADC_VREF, e.g.:

Binary : 1010011011100001
Decimal : 42721
Conversion : 42721 / 2¹⁴ = 2.6075 V

As an alternative to the calibration at two fixed temperature points, calculating the temperature using the actual trimmed ADC_VREF value for V_{REF_ADC} with Δ V_{REF_ADC} ± 10 mV, the temperature error reduces to ± 5°C.

A change request for the netX Peripheral Driver package is in progress. The version register of the netX 90 enables distinguishing in software whether the variable usADCVREFValue is supported.