MAX32665-MAX32668 User Guide
Maxim Integrated Page 50 of 457
4. System, Power, Clocks, Reset
There are several clocks used by different peripherals and subsystems. These clocks are highly configurable by firmware,
allowing developers to select the combination of application performance and power savings required for the target
systems.
The selected System Oscillator (SYS_OSC) is the clock source for most internal blocks. Select SYS_OSC from the following
clock sources:
• 96MHz Internal High-Frequency Oscillator
• 60MHz Low-Power Internal Oscillator
• 7.3728MHz Internal Oscillator
Selectable for UART baud rate generation
• 8kHz Internal Ultra-Low Power Nano-Ring Oscillator
• 32.768kHz External Crystal Oscillator
Clock source for the Real-Time Clock (RTC)
• 32MHz External Crystal Oscillator
Clock source for the Bluetooth 5 radio
The selected SYS_OSC is the input to the system oscillator prescaler to generate the System Clock (SYS_CLK). The system
oscillator prescaler divides SYS_OSC by a prescaler using the GCR_CLK_CTRL.sysclk_prescale field as shown in Equation 4-1.
Equation 4-1: System Clock Scaling
GCR_CLK_CTRL.sysclk_prescale is selectable from 0 to 7, resulting in divisors of 1, 2, 4, 8, 16, 32, 64 or 128.
SYS_CLK drives the Arm Cortex-M4 with FPU cores and is used to generate the following internal clocks as shown below:
• Advanced High-Performance Bus (AHB) Clock
HCLK = SYS_CLK
• Advanced Peripheral Bus (APB) Clock,
• Always On Domain (AOD) Clock,
• GCR_PCLK_DIV.aondiv is selectable from 0 to 3 for divisors of 1, 2, 4 or 8
There are additional internal clocks that are generated. These clocks are independent of SYS_OSC and SYS_CLK as follows:
• The USB PHY uses the 96MHz oscillator
• The SDHC/SDIO controller uses the high speed 96MHz oscillator divided by 2
• The Audio interface uses the high speed 96MHz oscillator
• The RTC uses the 32.768kHz oscillator
• (MAX32666/MAX32668 only) The Trust Protection Unit (TPU) uses the 60MHz Low-Power Internal Oscillator
All oscillators are reset to default at Power-On Reset (POR) and System Reset. Oscillator status is not reset by a Soft Reset or
Peripheral Reset.
4.1 Oscillator Sources and Clock Switching
On Power-On Reset (POR) and System Reset, all oscillator states are reset to the default: The 60MHz and 8kHz oscillators
are enabled, while the 96MHz, 32MHz 32.768kHz and 7.3728MHz oscillators are disabled. Oscillators are not reset on Soft
Reset or Peripheral Reset.
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