Clock Module And Low-Power Modes; Overview; Cpu Core And Peripherals Clock Source - Motorola M-CORE MMC2001 Series Reference Manual

CLOCK MODULE AND LOW-POWER MODES

8.1 Overview

The two clock sources to the MMC2001 are a 16.38-MHz clock input pin
(HI_REFCLK) and a 32.768-kHz crystal interface. A low-power crystal oscillator cir-
cuit (OSC) generates the constant crystal frequency clock LOW_REFCLK from the
external 32.768 kHz crystal.
The CPU core is clocked by HI_REFCLK.
Peripheral modules can use the following as reference frequencies:
• The 32.768-kHz LOW_REFCLK
• The 16.38-MHz HI_REFCLK
• A prescaled version of LOW_REFCLK
• A combination of several of the above references.
The use of different clocks allows some of the peripherals to maintain a constant
operational frequency even if the 16.38-MHz input signal is shut down. Changing the
peripheral frequency or allowing different parts of the same peripheral to use different
frequencies imply rigid constraints on clock frequencies and synchronization issues
which must be addressed carefully. Refer to each peripheral definition for further
details.
In addition, the CLKOUT pin can be driven by one of the internal clock sources
(LOW_REFCLK or HI_REFCLK) under software control.
The following table describes the clock source for each system block.
Table 8-1 CPU Core and Peripherals Clock Source
External Interface Module
Interrupt Controller
Watchdog Timer
Time-of-Day Timer
Interval Timer (PIT)
GPIO/Keypad
NOTES:
1. When enabled and not low-power disabled
MMC2001
REFERENCE MANUAL
Freescale Semiconductor, Inc.
SECTION 8
Peripheral
CPU Core
UART
ISPI
PWM
JTAG/OnCE
CLOCK MODULE AND LOW-POWER MODES
For More Information On This Product,
Go to: www.freescale.com
Peripheral Clock Source
HI_REFCLK
HI_REFCLK
HI_REFCLK
HI_REFCLK
HI_REFCLK
HI_REFCLK
LOW_REFCLK/16384 (2 Hz)
LOW_REFCLK/128 (256 Hz)
LOW_REFCLK/4 (8.192 kHz)
LOW_REFCLK/128 (256 Hz)
for interrupt debouncer
HI_REFCLK, TCK
1
MOTOROLA
8-1