24
8052B–AVR–09/08
ATmega4HVD/8HVD
8.4 Slow RC Oscillator
The Slow RC Oscillator provides a 131 kHz clock (typical value, refer to section "Electrical
Characteristics" 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 Oscil-
lator frequency, refer to ”OSI – Oscillator Sampling Interface” on page 27 for details.
To provide good accuracy when used as a timing reference, the Slow RC Oscillator has cali-
bration bytes stored in the signature address space, refer to section ”Reading the Signature
Row from Software” on page 124 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
signature row. The die temperature can be found using the ADC, refer to ”ADC - Analog-to-
Digital Converter” on page 90 for details.
8.5 Ultra Low Power RC Oscillator
The Ultra Low Power RC Oscillator (ULP Oscillator) provides a 128 kHz clock (typical value,
refer to section ”Electrical Characteristics” on page 142). There are two alternative methods
for determining the actual clock period of the ULP Oscillator:
1. To determine the actual clock period as a function of die temperature, the Oscillator
Sampling Interface should be used. Refer to section ”OSI – Oscillator Sampling Inter-
face” on page 27 for details.
2. To determine a fixed value for the actual clock period independent of the die tempera-
ture, for example to determine the best setting of the Battery Protection timing, use the
calibration byte ULP_RC_FRQ stored in the signature address space, refer to section
”Reading the Signature Row from Software” on page 124 for details.
8.6 CPU, I/O, Flash, and ADC Clock
The clock source for the CPU, I/O, Flash, and ADC is the calibrated Fast RC Oscillator.
8.7 Watchdog Timer and Battery Protection
The clock source for the Watchdog Timer and Battery Protection is the Ultra Low Power RC
Oscillator. The Oscillator is automatically enabled in all operational modes. It is also enabled
during reset.
8.8 Clock Startup Sequence
When the CPU wakes up from Power-save, the CPU clock source is used to time the start-up,
ensuring a stable clock before instruction execution starts. When the CPU starts from reset,
there is an additional delay allowing the voltage regulator to reach a stable level before com-
mencing normal operation. The Ultra Low Power RC Oscillator is used for timing this real-time
part of the start-up time. Start-up times are determined by the SUT Fuses as shown in Table
Clock period
Slow RC Word
1024
---------------------------------------
(1-Slow RC Temp Prediction TT
HOT
–())⋅=
(13 pages)
(23 pages)
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