AN1769 Freescale Semiconductor / Motorola, AN1769 Datasheet - Page 32

• Manufacturer Part Number: AN1769
• Description: Designing a Minimal PowerPC System
• Manufacturer: Freescale Semiconductor / Motorola

Part 4 Clock
Unlike some systems, the clock circuitry for a minimal system is quite simple. The processor, memory
controller and two SRAM memories all need a separate bus clock (anywhere from 1 Hz to 100 MHz
have a 250 ps point-to-point skew allowance. The simplest way to do this is to connect a crystal oscillator
device to all four loads as shown in . This is generally achievable with most clock oscillators.
The clock generator must have a very low output impedance in order to drive four loads from one output,
and it may be unacceptable unless the clock traces can be kept very short (on the order of 3 cm or so).
If this is not possible, an alternative is to employ an inexpensive low-skew clock generator such as the
Motorola MPC904 as shown in Figure 19. Using a crystal or an oscillator with this device, each component
can have a dedicated clock signal. This can make the board routing much easier, and other devices in the
Motorola MPC9xx family can provide other clocks that may be needed along with the primary system
needs, increasing integration.
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respective hardware reference datasheets for details.
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Note: MPC604-class devices are not fully static and have minimum clock frequencies. MPC603- and MPC750-class devices are fully static. Refer to the
SRAM
SRAM
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Freescale Semiconductor, Inc.
SRAM
For More Information On This Product,
Figure 18. Simplest Clock Connection
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Figure 19. MPC904 Clock Connection
Minimal PowerPC System Design
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SRAM
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Go to: www.freescale.com
SRAM
MPC904
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10 to 83 MHz
Oscillator
Controller
Memory
FPGA
3.3V
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Controller
Memory
FPGA
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PowerPC
CPU
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PowerPC
equal-length
traces to each
device
CPU
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equal-length
traces to each
device
MOTOROLA
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