Low Power Oscillator and Clock Detect (LPOCLKDET)
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SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
14.4.5 LPOCLKDET Disable
14.4.5.1 Disable Clock Detect
It is possible to disable the clock detect circuitry. For protection, this clock detect disable employs a 2-bit
key:
•
RANGE DET ENA SSET (CLKTEST.24) must be set to 1
•
RANGE DET CTRL (CLKTEST.25) must be cleared to 0
In this case, the LPO HF and LF clocks are still active but the clock detect circuitry is disabled. The clock
detect unconditionally switches GCM_CLK_SRC(0) back to the oscillator so care should be taken to
insure that the oscillator is good before disabling the clock detect circuitry.
14.4.5.2 Disable LPO HF and LF Clocks
The LPO may be disabled by holding the relaxation oscillator clocks (HF and LF) in reset. The clock
detect must be disabled, and any clock domains using either HF or LF clocks must be switched to a
different clock source. The LPO HF clock is reset by setting CSDIS.5; CSDISSET.5 is an easy way to set
specific bits without disturbing the rest of the register. The HF LPO clock disables several HF LPO cycles
after CSDIS is set.
Similarly, the LPO LF clock is reset by setting CSDIS.4, and in a similar way CSDISSET.4 can set the
specific CSDIS register bit without using a read-modify-write construction. The LF LPO disables several
LF LPO cycles after CSDIS is set.
Restarting the LPO clocks from this condition is fast and is known as a warm re-start. The CSDISCLR
register allows the user to clear CSDIS bits without using a read-modify-write code-construct.
14.4.5.3 Disable LPO Current Bias
The LPO current source may be disabled after the clock detect is disabled and HF and LF clock sources
are disabled. Turning off this current source places the LPOCLKDET into its lowest power configuration.
The bias may be disabled by clearing the BIAS ENABLE bit (LPOMONCTL.24).
Restarting the LPO when the bias current has been disabled requires the current source to initialize first
and is, therefore slower than a warm re-start; re-enabling the LPO from this condition is known as a warm
re-start (similar to what happens during nPORRST active).
14.4.6 Trimming the HF LPO Oscillator
The HF LPO range varies considerably around 9.6MHz from device to device. In order to provide tighter
monitoring of the crystal/resonator, it is useful to trim the oscillator. During device test, a trim value is
written into the one-time programmable section of the flash memory (OTP), address 0xF008_01B4. Bits
31:16 of this OTP word contain a 16 bit value that may be programmed into LPOMONCTL(15:0) in order
to initialize the trim for both HF LPO and LF LPO.
When trimming the HF LPO, it is recommended to step the trim value so as not to make a large change to
any TRIM setting.
After the initial trim, further trimming may be done in LPOMONCTL, using the dual clock compare module
(DCC, please see Dual Clock Compare User’s Guide) in order to determine the resultant frequency. This
module allows for comparison of two clock frequencies. Once the HF LPO is determined to be in-range
with the initial HFTRIM setting from the OTP, the crystal oscillator may be used as a reference against
which the HF LPO and LF LPO may be further adjusted.