The system oscillator (SYSOSC) is an on-chip, accurate (4 or 32MHz factory-trimmed frequencies, 16 or 24MHz user-trimmed frequencies). 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 disabled 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
- A secondary output with a constant 4MHz frequency for use by MFCLK and MFPCLK
- When fSYSOSC = 32MHz, the 4MHz output is derived from SYSOSC divided digitally by 8. When fSYSOSC = 4MHz, the 4MHz output is derived from SYSOSC directly. User frequencies of 24MHz and 16MHz are supported with digital dividers of /6 and /4, respectively. SYSCTL manages the digital divider on this output to ensure a constant 4MHz output regardless of the selected SYSOSC frequency.
- Peripherals using this output through the MFCLK or MFPCLK 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 instead of at 32MHz with a divide-by-8.
The SYSOSC is active at base frequency (32MHz) by default after a brownout reset, sourcing MCLK.