AT32UC3C2512C Automotive Atmel Corporation, AT32UC3C2512C Automotive Datasheet - Page 135

• Manufacturer Part Number: AT32UC3C2512C Automotive
• Manufacturer: Atmel Corporation

9.5.1.2
9.5.2
9.5.2.1
9.5.2.2
9166C–AVR-08/11
Basic Operation
Changing the source clock
Prescaler
Counter operation
The CLK_AST_PRSC must be disabled before switching to another source clock. The Clock
Busy bit in the Status Register (SR.CLKBUSY) indicates whether the clock is busy or not. This
bit is set when the CEN bit in the CLOCK register is changed, and cleared when the CLOCK reg-
ister can be changed.
To change the clock:
When the AST is enabled, the 32-bit prescaler will increment on the rising edge of
CLK_AST_PRSC. The prescaler value cannot be read or written, but it can be reset by writing a
one to the Prescaler Clear bit in the Control Register (CR.PCLR).
The Prescaler Select field in the Control Register (CR.PSEL) selects the prescaler bit PSEL as
source clock for the counter (CLK_AST_CNT). This results in a counter frequency of:
where f
The AST counter value can be read from or written to the Counter Value (CV) register. Note that
due to synchronization, continuous reading of the CV register with the lowest prescaler setting
will skip every third value. In addition, if CLK_AST_PRSC is as fast as, or faster than, the
CLK_AST, the prescaler value must be 3 or higher to be able to read the CV without skipping
values.
• Write a zero to CLOCK.CEN to disable the clock, without changing CLOCK.CSSEL
• Wait until SR.CLKBUSY reads as zero
• Write the selected value to CLOCK.CSSEL
• Wait until SR.CLKBUSY reads as zero
• Write a one to CLOCK.CEN to enable the clock, without changing CLOCK.CSSEL
• Wait until SR.CLKBUSY reads as zero
When enabled, the AST will increment on every 0-to-1 transition of the selected prescaler
tapping. When the Calender bit in the Control Register (CR.CAL) is zero, the counter oper-
ates in counter mode. It will increment until it reaches the top value of 0xFFFFFFFF, and
then wrap to 0x00000000. This sets the status bit Overflow in the Status Register (SR.OVF).
Optionally, the counter can also be reset when a timer alarm occurs (see
which will also set the OVF bit.
PRSC
is the frequency of the internal prescaler clock CLK_AST_PRSC.
f
CNT
=
---------------------- -
2
PSEL
f
PRSC
+
1
AT32UC3C
Section
9.5.3.2),
135