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
- 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.