Datasheet

26
8024A–AVR–04/08
ATmega8HVA/16HVA
9.4 Slow RC Oscillator
The Slow RC Oscillator provides a 131 kHz clock (typical value, refer to section "Electrical Char-
acteristics" on page 164 for details). This clock can be used as a timing reference for run-time
calibration of the Fast RC Oscillator and for accurately determining the actual ULP Oscillator fre-
quency, refer to ”OSI – Oscillator Sampling Interface” on page 28 for details. The Slow RC
oscillator also provides the clock for the Coulomb Counter ADC.
The actual Slow RC Oscillator frequency depends on process variations and temperature, see
”Electrical Characteristics” on page 165.To provide a very good accuracy when used as a timing
reference, the Slow RC Oscillator has prediction bytes stored in the signature address space,
refer to section ”Reading the Signature Row from Software” on page 144 for details. The actual
clock period of the Slow RC Oscillator in μs as a function of temperature is given by:
where T is the die temperature in Kelvin and T
HOT
is the calibration temperature stored in the sig-
nature row. The parameter "Slow RC period" holds information about the actual Slow RC
oscillator period measured at Atmel production. This parameter can be read from the signature
address space. Using the formula above, the actual Slow RC frequency can be found with an
error of <1% over the temperature area from -10 °C to +70 °C.The die temperature can be found
using the Voltage ADC, refer to section ”Voltage ADC – 5-channel General Purpose 12-bit
Sigma-Delta ADC” on page 112 for details. For examples on Slow RC frequency Prediction,
please refer to application note AVR351.
9.5 Ultra Low Power RC Oscillator
The Ultra Low Power RC Oscillator (ULP Oscillator) provides a 128 kHz clock (typical value).
This oscillator provides the clock for the Watchdog Timer and Battery Protection modules. The
actual ULP Oscillator frequency depends on process variations and temperature, see ”Electrical
Characteristics” on page 165. The Oscillator is automatically enabled in all operational modes. It
is also enabled during reset. There are two alternative methods for determining the actual clock
period of the ULP Oscillator:
1. To determine the accurate clock period as a function of die temperature, if needed by the
application, the Oscillator Sampling Interface should be used. Refer to section ”OSI –
Oscillator Sampling Interface” on page 28 for details. For examples on ULP RC fre-
quency calculations, please refer to application note AVR351.
2. To determine a fixed value for the actual clock period independent of the die temperature,
for example to determine the best setting of the Battery Protection timing, use the byte
ULP_RC_FRQ stored in the signature address space, refer to section ”Reading the Sig-
nature Row from Software” on page 144 for details.
9.6 CPU, I/O, Flash, and Voltage ADC Clock
The clock source for the CPU, I/O, Flash, and Voltage ADC is the calibrated Fast RC Oscillator.
S
low RC period
Slow RC period - Slow RC temp prediction word
(
TT
HOT
)–
64
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⋅
1024
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