MPC7457 datasheet(53/71 Pages) FREESCALE

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System Design Information

MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8

Freescale Semiconductor

9.1.3

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 20 are observed.

It is imperative to note that the processor’s minimum and maximum SYSCLK, core, and VCO frequencies

must not be exceeded regardless of the type of clock source. Therefore, systems in which the processor is

operated at its maximum rated core or bus frequency should avoid violating the stated limits by using

down-spreading only.

1050

525

420

350

300

263

233

191

175

162

150

140

131

1100

550

440

367

314

275

244

200

183

169

157

147

138

1150

575

460

383

329

288

256

209

192

177

164

153

144

1200

600

480

400

343

300

267

218

200

185

171

160

150

1250

638

500

417

357

313

278

227

208

192

179

167

156

1300

650

520

433

371

325

289

236

217

200

186

173

163

Notes:

1. The core and L3 frequencies are for reference only. Note that maximum L3 frequency is design dependent. Some examples

may represent core or L3 frequencies which are not useful, not supported, or not tested for the MPC7457; see Section 5.2.3,

“L3 Clock AC Specifications,” for valid L3_CLK frequencies and for more information regarding the maximum L3 frequency.

2. Not all core frequencies are supported by all speed grades; see Table 8 for minimum and maximum core frequency

specifications.

Table 20. Spread Specturm Clock Source Recommendations

At recommended operating conditions. See Table 4.

Parameter

Min

Max

Unit

Notes

Frequency modulation

—

kHz

Frequency spread

—

1.0

1, 2

Notes:

1. Guaranteed by design.

2. SYSCLK frequencies resulting from frequency spreading, and the resulting core and VCO

frequencies, must meet the minimum and maximum specifications given in Table 8.

Table 19. Sample Core-to-L3 Frequencies 1 (continued)

Core

Frequency

(MHz) 2

÷2

÷2.5

÷3

÷3.5

÷4

÷4.5

÷5

÷5.5

÷6

÷6.5

÷7

÷7.5

÷8

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