Software implementation

AN2679

12/16

3

Software implementation

After the first start-up following the firmware upload, the ST7 MCU performs a simple board

self-test. It reads the oscillator amplitude level and voltage across the temperature-sensing

diode D2 and checks whether these values are within a specific range (this state is indicated

by blinking diagnostic LED D3; see Table 5). The oscillator amplitude level detected during

this test is also considered as an initial oscillator level when no metal object approaches the

sensing inductor L1, and its value is recorded to the Flash memory (address 0xfc00) using

an in-application programming (IAP) method. This value is later used for amplitude

reduction comparisons caused by metal objects.

Note: The initial board self-test procedure can be performed at any time by placing a jumper

on pins 3 and 4 of the ICC connector (see Figure 7) and powering up the application.

Figure 7.

Inducing the demonstration board self-test

During normal operation, the MCU then controls the sensor output based of the information

about the oscillator amplitude and the actual temperature. The main sensor part of the

firmware is implemented in an auto-reload timer interrupt service routine. In equidistant time

intervals the oscillator amplitude is sampled and its value is compared with two system

variables (ucUpperCompThreshold and ucLowerCompThreshold). One of these defines the

ON to OFF transition, and the other defines the OFF to ON transition of the sensor state.

The distance between them determines the hysteresis. These threshold variable values are

defined as a percentage of the initial oscillator level recorded in the Flash memory and are

further modified depending on the temperature by a coefficient from a lookup table.

Table 5.

Diagnostic LED blinking modes (power up self-test)

LED status

Meaning

Blinking

Input values within limits

Constant

Error

Table 6.

Diagnostic LED blinking modes (normal operation)

Flashing style

Meaning

❋ ❋ ❋ ❋ ❋ ❋

Under-temperature

❋❋

❋❋

❋❋

❋❋

Over-temperature

❋❋❋

❋❋❋

❋❋❋

Ferrite approaching the coil