ATmega16HVB datasheet(124/273 Pages) ATMEL

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124

8042A–AVR–07/09

ATmega16HVB/32HVB

21.3

Operation

When the device is in power-off state, the Voltage Reference will be switched off. After a Power-

on reset condition the Voltage Reference will automatically be enabled.

By default the Bandgap Buffer will be enabled as the buffered reference voltage is used as refer-

ence for the Battery Protection, the internal Cell Balancing, the V-ADC and the CC-ADC. If any

of these modules are enabled the Bandgap Buffer must be enabled, otherwise it is recom-

mended to disable the buffer by writing to the BGD bit in BGCSR to save power. Note that the

Bandgap Buffer needs settling time before the voltage is stable. For details on settling time, see

”Bandgap Buffer Settling Time” on page 125.

To ensure that the battery protection has safe operation condition, the Voltage Reference

includes a Short-circuit Detector. The Bandgap Short Detector continuously monitors the internal

1.100V reference voltage against the VREF pin voltage to detect potentially external short condi-

tions to VCC or GND. If an external short condition is detected, the Bandgap Short Circuit

Detection is capable of interrupting and waking up the CPU from any sleep mode. If a Bandgap

Short-circuit condition occurs software should immediately disable the C-FET and D-FET. If no

external protection is provided to detect such a condition it is recommended to always enable

this feature, by setting the BGSCDE bit in the Bandgap Control and Status Register.

The Temperature Sensor generates a voltage Proportional-To-Absolute-Temperature, VPTAT.

This voltage is connected to the multiplexer at the V-ADC input and it can be used for runtime

compensation of temperature drift in both the voltage reference and the On-chip Oscillator. To

get the absolute temperature in degrees Kelvin, the measured VPTAT voltage must be scaled

with the VPTAT factory calibration value stored in the signature row. See ”Reading the Signature

Row from Software” on page 199 for details. See”Electrical Characteristics” on page 228 for

details on temperature accuracy.

21.4

Bandgap Calibration

To guarantee ultra low temperature drift the Voltage Reference includes two calibration registers

that could be changed run-time by software. Changing values to the BGCCR IO register will

change the nominal value of the Bandgap Reference Voltage, while changing values to the

BGCRR IO register trims the temperature gradient of the bandgap reference.

When the calibration registers are changed it will affect both the Voltage Regulator output and

BOD-level. The BOD will react quickly to new detection levels, while the regulator will adjust the

voltage more slowly, depending on the size of the external decoupling capacitor. To avoid that a

BOD-reset is issued when calibration is done, it is recommended to change the values of the

BGCC and BGCR bits stepwise, with a step size of 1, and with a hold-off time between each

step. See ”Electrical Characteristics” on page 228 for details on hold-off time.

Changing VREF will influence the conversion results for the V-ADC and CC-ADC. It is therefore

not recommended to do V-ADC and CC-ADC conversions while calibrating the bandgap.

To guarantee ultra low temperature drift it is recommended to perform factory calibration using a

two-step calibration algorithm. By default, Atmel factory calibration is performed at hot tempera-

ture, and the result is stored in the signature row. See ”Reading the Signature Row from

Software” on page 199 for details. The customer can easily implement the second calibration

step in their test flow.

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