The system oscillator (SYSOSC) is an on-chip,
accurate high frequency clock source (up to 32MHz). The SYSOSC provides a
flexible high-speed clock source to the system in cases where the high frequency
crystal oscillator (HFXT) is either not present or not used, or where fast
wake-up from a low-power mode is required.
Key features of the SYSOSC include:
- High accuracy when using optional frequency correction loop (FCL) and reference resistor
- The frequency correction loop may support correction via an external resistor (ROSC) or an internal resistor, depending on the device capabilities. Refer to the device-specific data sheet to determine if a device supports the FCL with an internal or external resistor, or both
- Fast start-up time from a low power state
- Capable of switching from base frequency to low
frequency, or low frequency to base frequency, in one clock cycle with no
functional interruption (gear shift)
- Phase-aligned transition
to minimize disturbance to peripherals
- Fast settling to
specified accuracy
- SYSCTL can initiate
seamless gear shift frequency switch in STOP mode to reduce SYSOSC
current
- Refer to the device-specific data sheet to determine if multiple
frequency settings for SYSOSC are supported
- The SYSOSC clock source provides a user trimmable frequency start at 16MHz or
24MHz.
- In-system trimming is supported by the usage of the FCC (frequency clock
counter)
- A secondary output with a constant 4MHz frequency
for use by MFCLK and MFPCLK
- When fSYSOSC = 4MHz, the MFCLK/MFPCLK output is derived from
SYSOSC directly.
- When fSYSOSC is not 4MHz, the MFCLK/MFPCLK output is derived
by a digital divider sourcing from SYSOSC with the appropriate ratio
- SYSCTL manages the digital divider on this output to ensure a constant
4MHz output regardless of the user selected SYSOSC frequency.
- Peripherals using this
output see a continuous 4MHz functional clock in RUN, SLEEP, and STOP
modes, with reduced current in STOP mode when gear
shift is enabled due to SYSOSC running natively at 4MHz.
The SYSOSC is active at the device's base
frequency by default after a brownout reset, sourcing MCLK.