MC13211_08 Datasheet, PDF (29/70 Pages) Freescale Semiconductor, Inc – ZigBee™- Compliant Platform -2.4 GHz Low Power Transceiver for the IEEE® 802.15.4 Standard plus Microcontroller

5.4 MCU Internal Clock Generator (ICG)
The ICG provides multiple options for MCU clock sources. This block along with the ability to provide
the MCU clock form the modem offers a user great flexibility when making choices between cost,
precision, current draw, and performance. As seen in Figure 17, the ICG consists of four functional blocks.
• Oscillator Block — The Oscillator Block provides means for connecting an external crystal or
resonator. Two frequency ranges are software selectable to allow optimal start-up and stability.
Alternatively, the oscillator block can be used to route an external square wave to the MCU system
clock. External sources such as the modem CLKO output can provide a low cost source or a very
precise clock source. The oscillator is capable of being configured for low power mode or high
amplitude mode as selected by HGO.
• Internal Reference Generator — The Internal Reference Generator consists of two controlled
clock sources. One is designed to be approximately 8 MHz and can be selected as a local clock for
the background debug controller. The other internal reference clock source is typically 243 kHz
and can be trimmed for finer accuracy via software when a precise timed event is input to the MCU.
This provides a highly reliable, low-cost clock source.
• Frequency-Locked Loop — A Frequency-Locked Loop (FLL) stage takes either the internal or
external clock source and multiplies it to a higher frequency. Status bits provide information when
the circuit has achieved lock and when it falls out of lock. Additionally, this block can monitor the
external reference clock and signals whether the clock is valid or not.
• Clock Select Block — The Clock Select Block provides several switch options for connecting
different clock sources to the system clock tree. ICGDCLK is the multiplied clock frequency out
of the FLL, ICGERCLK is the reference clock frequency from the crystal or external clock source,
and FFE (fixed frequency enable) is a control signal used to control the system fixed frequency
clock (XCLK). ICGLCLK is the clock source for the background debug controller (BDC).
The module is intended to be very user friendly with many of the features occurring automatically without
user intervention.
5.4.1 Features
Features of the ICG and clock distribution system:
• Several options for the MCU primary clock source allow a wide range of cost, frequency, and
precision choices:
— 32 kHz–100 kHz crystal or resonator
— 1 MHz–16 MHz crystal or resonator
— External clock supplied by modem CLKO or other source
— Internal reference generator
• Defaults to self-clocked mode to minimize startup delays
• Frequency-locked loop (FLL) generates 8 MHz to 40 MHz (for bus rates up to 20 MHz). When
using modem CLKO as external source, maximum FLL frequency is 32 MHz (16 MHz bus rate)
with CLKO = 16 MHz or maximum FLL frequency is 40 MHz (20 MHz bus rate) with CLKO =
4 MHz.
MC13211/212/213 Technical Data, Rev. 1.4
Freescale Semiconductor
29