RTC Counting Time
Normally
BUSY Bit
Modify RTC
Registers Within 15
Micro Seconds
Read and Display
Time/Date Registers
1
0
RTC_SS
20.3.3.5.5 Modifying the TC Registers
To write correct data from/to the TC and TC alarm registers and read the TC alarm registers, the ARM
must first read the BUSY bit of the STATUS register until BUSY is equal to zero. Once the BUSY flag is
zero, there is a 15 µs access period in which the ARM can program the TC and TC alarm registers. Once
the 15 µs access period passes, the BUSY flag has to be read again from the STATUS register as
described previously. If the ARM accesses the TC registers outside of the access period, then the access
is not guaranteed.
The ARM can access the STATUS_REG and CTRL_REG at any time, with the exception of
CTRL_REG[5] which can only be changed when the RTC is stopped. The ARM can stop the RTC by
clearing the STOP_RTC bit of the control register. After clearing this bit, the RUN bit in the STATUS_REG
(bit 1) needs to be checked to verify the RTC has in fact stopped. Once this is confirmed, the TC values
can be updated. After the values have been updated, the RTC can be re-started by resetting the
STOP_RTC bit.
NOTE:
After writing to a TC register, the user must wait 4 OCP clock cycles before reading the
value from the register. If this wait time is not observed and the TC register is accessed, then
old data will be read from the register.
CAUTION
In order to remove any possibility of interrupting the register's read process,
thus introducing a potential risk of violating the authorized 15-microsecond
access period, it is strongly recommended that you disable all incoming
interrupts during the register read process.
Figure 20-59. Flow Control for Updating RTC Registers
20.3.3.5.5.1 General Registers
The ARM can access the STATUS_REG and the CTRL_REG at any time (except the CTRL_REG[5] bit
which must be changed only when the RTC is stopped). For the INTERRUPTS_REG, the ARM should
respect the available access period to prevent spurious interrupt.
3629
SPRUH73H – October 2011 – Revised April 2013
Timers
Copyright © 2011–2013, Texas Instruments Incorporated