Datasheet

182
32133D–11/2011
UC3D
To change the clock:
• 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 the CLOCK.CSSEL
• Wait until SR.CLKBUSY reads as zero
14.5.2 Basic Operation
14.5.2.1 Prescaler
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
PRSC
is the frequency of the internal prescaler clock CLK_AST_PRSC.
14.5.2.2 Counter operation
When enabled, the AST will increment on every 0-to-1 transition of the selected prescaler tap-
ping. When the Calender bit in the Control Register (CR.CAL) is zero, the counter operates 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 an alarm occurs (see Section 14.5.3.2 on page 184. This will
also set the OVF bit.
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.
14.5.2.3 Calendar operation
When the CAL bit in the Control Register is one, the counter operates in calendar mode. Before
this mode is enabled, the prescaler should be set up to give a pulse every second. The date and
time can then be read from or written to the Calendar Value (CALV) register.
Time is reported as seconds, minutes, and hours according to the 24-hour clock format. Date is
the numeral date of month (starting on 1). Month is the numeral month of the year (1 = January,
2 = February, etc.). Year is a 6-bit field counting the offset from a software-defined leap year
f
CNT
f
PRSC
2
PSEL 1+
-----------------------=