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 supports correction
with an internal resistor, depending on the device capabilities.
- 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
- A secondary output with a constant 4MHz frequency
for use by MFCLK.
- When fSYSOSC =
4MHz, the MFCLK output is derived from SYSOSC directly.
- When fSYSOSC
is not 4MHz, the MFCLK 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.