AN1849 Motorola / Freescale Semiconductor, AN1849 Datasheet - Page 8

• Manufacturer Part Number: AN1849
• Description: MPC107 Design Guide
• Manufacturer: Motorola / Freescale Semiconductor

Using the information and equations listed above, we can see what sort of designs are possible, as shown in
Table 4.
As you might expect, the key to achieving a high-speed memory design is to minimize the capacitive loading
and trace length; registered DIMMs have become invaluable for just this reason. Once the data have been
determined, we can examine the timing of the memory system.
As expected, the capacitive load on case “C” was far too high to achieve 100 MHz; even 83 MHz operation
was well beyond its capabilities. For cases where 100 MHz is possible, Figure 5 shows an example of timing
factors as information flows between the MPC107 and the SDRAM.
8
DIMM type
SDRAM type
PCB Trace
Co’
Trace delay
TOF
Prognosis
0.91 pF/cm +
(2 DIMMs * 7.5 pF)
/ 8.9 cm
sqrt(3930 pH/cm * 2.60
pF/cm)
Clock-to-output
Max Bus Speed
2 Registered DIMMs
Time-of-flight
Input Setup
Cycle Time
Input Hold
Factor
Margin
Jitter
2.60 pF/cm
101 ps/cm
registered
Case “A”
Excellent
8.9 cm
899 ps
any
Table 4. Example SDRAM Designs
Table 5. Memory Timing Analysis
Case “A” Value
MPC107 Design Guide
10000 ps
- 5500 ps
- 2000 ps
- 1000 ps
100 MHz
- 150 ps
- 899 ps
451 ps
0.91 pF/cm +
(1 SODIMMs * 30 pF)
/ 6.4 cm
sqrt( 3930 pH/cm * 5.60 pF/cm)
Single SODIMM module
5.60 pF/cm
unbuffered
148 ps/cm
Case “B”
Case “B” Value
6.4 cm
947 ps
16-bit
Good
- 5500 ps
- 2000 ps
- 1000 ps
10000 ps
100 MHz
- 150 ps
- 947 ps
403 ps
Case “C” Value
- 5500 ps
- 3814 ps
- 2000 ps
- 1000 ps
- 2464 ps
10000 ps
0.91 pF/cm +
(4 DIMMs * 70 pF)
/ 12.7 cm
sqrt(3930 pH/cm * 22.96 pF)
- 150 ps
78 MHz
3 Unbuffered DIMMs
22.96 pF/in
unbuffered
Case “C”
12.7 cm
3814 ps
300 ps
4 bits
Poor