Maxim Integrated MAX32660 - Page 26

MAX32660 User Guide
Maxim Integrated Page 26 of 195
4.3.1 High-Frequency Internal Oscillator
The MAX32660 includes a High-Frequency Internal Oscillator (HFIO) that is the default system oscillator after a POR, System
and Watchdog Reset.
To enter DEEPSLEEP, the HFIO must be powered down by setting register bit GCR_PM.hfiopd to 1. Failure to set this bit to 1
will inhibit the processor from entering DEEPSLEEP.
Refer to Table 4-3 and Table 4-4 for details on reset sources and the effect on the HFIO oscillator.
4.3.2 32.768kHz External Crystal or Clock
This is a very low power oscillator used for the RTC and optionally can be selected as the System Clock. This oscillator can be
driven using an external clock or a 32.768kHz crystal. The 32.768kHz clock signal is available as an output on GPIO 32KCAL.
This oscillator is the dedicated clock for the Real-Time Clock (RTC). If the RTC is enabled, the 32.768kHz external oscillator
must be enabled, independent of the selection of the System Clock.
When this oscillator is active, an RTC alarm can wake this device from SLEEP or DEEPSLEEP mode if the GCR_PM.rtcwk_en is
set to 1 and the RTC alarm is configured.
The 32kHz oscillator is disabled by hardware by a Power-On Reset. All other forms of reset do not change the 32kHz
oscillator enable bit. Refer to Table 4-3 and Table 4-4 for details on reset sources and the effect on the 32kHz oscillator.
4.3.3 8kHz Ultra-Low Power Nano-Ring Internal Oscillator
An ultra-low power internal 8kHz nano-ring oscillator is available and can be set as the System Oscillator (SYSOSC). This
oscillator is enabled at device powerup by hardware and cannot be disabled by application firmware.
4.4 System Oscillators Reset
On Power-On Reset (POR) and System Reset, all oscillator states are reset to their Power-On Reset default:
• The HFIO and the 8kHz nano-ring oscillators are enabled.
• The 32.768kHz external oscillator is disabled.
• The system clock selection and prescaler are reset.
The oscillator enable fields, system clock selection field and the system clock prescaler fields are not affected by a Soft
Reset or Peripheral Reset.
4.5 Operating Modes
The MAX32660 supports four operating modes:
• ACTIVE
• SLEEP
• DEEPSLEEP
• BACKUP
ACTIVE mode is the only mode is which code execution occurs.