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Introduction
On-Chip Clock Synthesis (OCCS), Rev. 4
Freescale Semiconductor
6-3
6.1 Introduction
The On-Chip Clock Synthesis (OCCS) module allows product design using an inexpensive
4MHz crystal or an external clock source to run the DSP56853/54/55/57/58 at any frequency
from 0 to 120MHz. The OCCS module is comprised of two major blocks: the Oscillator (OSC),
and the PLL/CGM (analog - Phase Locked Loop/digital - Clock Generation Module (CGM)).
The OSC output clocks feed the PLL/CGM block. The PLL/CGM generates a time clock for
Computer Operating Properly (COP) timer use. The PLL/CGM also generates a master clock
consumed by the System Integration Module (SIM). The SIM generates derivative clocks for
consumption by the core logic and IPBus peripherals.
The SIM divides the MSTR_CLK (typically 240MHz) by 2 to create the 56800E core clock
(typically 120MHz) and by 4 to create the IPBUS_CLK (typically 60 MHz). All peripherals on
the DSP56853/54/55/57/58 run off the IPBus clock frequency. The COP and TOD peripherals
also consumes the much lower frequency TIME_CLK (typically 31.25 KHz).
The PLL may be used to generate a high frequency clock from the low-frequency
crystal-referenced (or external clock driven) OSC circuit. The PLL provides an exact integer
multiple of the oscillator’s output reference frequency (Fref). The frequency multiplication is in
the range of 20 to 120.
The CGM controls the PLL’s output frequency. The CGM also selects between the PLL
(PLL_OUT) and OSC (Fref) as potential master clock sources and routes the selection to the
SIM. The CGM also contains circuitry to detect if the PLL is unlocked and generates an interrupt
signal for the condition.
Figure 6-1. OCCS Integration Overview
Out of reset, the CGMCR[SEL] control bit is 0, selecting the Fref path as the source of
MSTR_CLK. The core will proceed to execute code using a clock divided down from the
0
OSC
Oscillator
EXTAL
XTAL
O
PLL / CGM
Phase Locked Loop
Clock Generation Module
CGMCR[SEL]
(4 MHz)
(240 MHz)
BOLD
represents default states or typical
conditions.
* see STOP Mode Features for further details
P
Time Of Day
COP
other peripherals
IPBB
IP Bus Bridge
SIM
System
Integration
Module
divide
by 4
56800E
Core
MSTR_CLK
(120 MHz)
(60 MHz)
TIME_CLK
(31.25 KHz)
I
(
CLKOUT
(30 MHz)
Loss_Of_Lock_Interrupt
OCCS
On-Chip Clock Synthesis
PLL_OUT
4 MHz
Ftime
(31.25 KHz)
Fref
(4 MHz)