Datasheet

MPC7447A RISC Microprocessor Hardware Specifications, Rev. 5
36 Freescale Semiconductor
System Design Information
9.1.2 System Bus Clock (SYSCLK) and Spread Spectrum Sources
Spread spectrum clock sources are an increasingly popular way to control electromagnetic interference
emissions (EMI) by spreading the emitted noise to a wider spectrum and reducing the peak noise
magnitude in order to meet industry and government requirements. These clock sources intentionally add
long-term jitter in order to diffuse the EMI spectral content. The jitter specification given in Table 8
considers short-term (cycle-to-cycle) jitter only and the clock generator’s cycle-to-cycle output jitter
should meet the MPC7457 input cycle-to-cycle jitter requirement. Frequency modulation and spread are
separate concerns, and the MPC7457 is compatible with spread spectrum sources if the recommendations
listed in Table 14 are observed.
01101 24x 2x 792
(1600)
1200
(2400)
11101 28x 2x 924
(1866)
1400
(2800)
00110 PLL bypass PLL off, SYSCLK clocks core circuitry directly
11110 PLL off PLL off, no core clocking occurs
Notes:
1. Ratios below 5:1 require an AACK delay See
MPC7450 RISC Microprocessor Family Reference Manual , Section
9.3.3, “MPX Bus Address Tenure Termination.”
2. The sample bus-to-core frequencies shown are for reference only. Some PLL configurations may select bus, core,
or VCO frequencies that are not useful, not supported, or not tested for by the MPC7455; see Section 5.2.1, “Clock
AC Specifications,” for valid SYSCLK, core, and VCO frequencies.
3. In PLL-bypass mode, the SYSCLK input signal clocks the internal processor directly and the PLL is disabled.
However, the bus interface unit requires a 2x clock to function. Therefore, an additional signal, EXT_QUAL, must be
driven at one-half the frequency of SYSCLK and offset in phase to meet the required input setup t
IVKH
and hold time
t
IXKH
(see Table 9). The result will be that the processor bus frequency will be one-half SYSCLK while the internal
processor is clocked at SYSCLK frequency. This mode is intended for factory use and emulator tool use only.
Note: The AC timing specifications given in this document do not apply in PLL-bypass mode.
4. In PLL-off mode, no clocking occurs inside the MPC7447A regardless of the SYSCLK input.
Table 13. MPC7447A Microprocessor PLL Configuration Example for 1420-MHz Parts (continued)
PLL_
CFG[0:4]
Example Bus-to-Core Frequency in MHz (VCO Frequency in MHz)
Bus-to-
Core
Multiplier
Core-to-
VCO
Multiplier
Bus (SYSCLK) Frequency
33
MHz
50
MHz
67
MHz
75
MHz
83
MHz
100
MHz
133
MHz
167
MHz