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8024A–AVR–04/08
ATmega8HVA/16HVA
The ripple counter that implements the prescaler runs at the frequency of the undivided clock,
may be faster than the CPU's clock frequency. It is not possible to determine the state of the
prescaler, and the exact time it takes to switch from one clock division to the other cannot be
exactly predicted. From the time the CLKPS values are written, it takes between T1 + T2 and T1
+ 2*T2 before the new clock frequency is active. In this interval, two active clock edges are pro-
duced. Here, T1 is the previous clock period, and T2 is the period corresponding to the new
prescaler setting.
To avoid unintentional changes of clock frequency, a special write procedure must be followed
to change the CLKPS bits:
1.
Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in
CLKPR to zero.
2.
Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is
not interrupted.
9.11
VADC Clock Prescaler
The VADC clock will be automatically prescaled relative to the System Clock Prescaler settings,
see ”System Clock Prescaler” on page 27. Depending on the Clock Prescale Select bits,
CLKPS1..0, the VADC clock, clk
VADC, will be prescaled by 4, 2 or 1 as shown in Table 9-4 on
page 32.
9.12
OSI – Oscillator Sampling Interface
9.12.1
Features
Runtime selectable oscillator input (Slow RC or ULP RC Oscillator)
7 bit prescaling of the selected oscillator
Software read access to the phase of the prescaled clock
Input capture trigger source for Timer/Counter0
9.12.2
Overview
The Oscillator Sampling Interface (OSI) enables sampling of the Slow RC and Ultra Low Power
RC (ULP) oscillators in ATmega8HVA/16HVA. OSI can be used to calibrate the Fast RC Oscilla-
tor runtime with high accuracy. OSI can also provide an accurate reference for compensating
the ULP Oscillator frequency drift.
The prescaled oscillator phase can be continuously read by the CPU through the OSICSR regis-
ter. In addition, the input capture function of Timer/Counter0 can be set up to trigger on the rising
edge of the prescaled clock. This enables accurate measurements of the oscillator frequencies
relative to the Fast RC Oscillator.
A simplified block diagram of the Oscillator Sampling Interface is shown in Figure 9-2 on page
29.
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