The auxiliary A/D converter channels inside RFI2, N450 are used by MCU to measure battery voltage , reference voltage output and system board temperature. This value is used to controll LCD operating voltage for optimal contrast as a function of temperature.
The A/D converters are accessed by the DSP, D152 via the ASIC, D151. The required resolution is 10 bit.
The scaling factor is created using 5% resistors and it is therefore a requirement to have an alignment procedure in the production phase. Each resistor network is supplied with a known input voltage and the measured value is used against the theoretically calculated value. As a result of this operation standard 5% resistors can be used in the voltage scaling circuitry.
The A/D converter used in RFI2, N450 for the measurement are sigma-delta type and the zero value is centered around 50 % of the supply voltage, 1.6V. This means that the A/D converter reading is negative when the input voltage to the converter is less than half of the supply voltage. In calculations the true A/D reading is got by adding 800H to the read value modulo 4096.
The MCU has 4 10 bit A/D channels which are used in parallel to the channels in N450. The MCu can measure charger voltage, battery size, battery temperature and accessory detection by using it's own converters.
MCU A/D channel 2 is used to detect accessories connected to the system connector using the XMIC/ID line. To be able to determine which accessory has been connected MCU measures the DC voltage on the XMIC/ID input. The accessory is detected in accordance with the CAP Accessory specifications. The base band has a pull-up resistor network of 32 kohm to VA. The accessory has a pull down. The A/D converter value can be calculated using the following formula:
where ACCI is the DC input impedance of the accessory device connected to the system connector.
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