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