AN2797 Freescale Semiconductor / Motorola, AN2797 Datasheet - Page 20

• Manufacturer Part Number: AN2797
• Description: Migrating from IBM 750GX to MPC7447A
• Manufacturer: Freescale Semiconductor / Motorola

Programming Model
To illustrate the simplicity of the DFS features:
Considering the following equation:
P = (C • V2 • f) + PDS
PDS= deep sleep mode power consumptionExcluding deep sleep mode power consumption, which is a minimum
fixed power cost for an inactive core, the dynamic power consumption of the device is halved when DFS is applied.
Note that static (leakage) power is not affected by DFS, so the power consumption with DFS enabled is not exactly
50% of the full-power consumption. This provides a significant advantage in supporting dynamic processing
requirements in power sensitive applications.
Figure 7
4.3
The MPC7447A implements two registers named Memory Subsystem and Status Control Registers, MSSSR and
MSSCR that do not exist in the IBM 750GX. Some of the functions in these extra registers are held in other IBM
750GX register.
In addition to this, MSSCR stores some more configuration data. This configuration relates to features not available
in the IBM 750GX including L3 cache parameters for MPC745x devices and also defines the number of outstanding
bus transactions, MSSCR[DTQ], and intervention for MPX mode, MSSCR[EIDIS].
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1. The frequency is switched completely “on the fly”.
2. This change occurs in only one clock cycle.
3. It requires zero any idle time or operations before or during the transition.
Feature
DFS divide by two enable
PLL configuration, PLL_CFG[0-4] (Read only)
Function
Address bus parity error
Data bus parity error
Bus transfer error acknowledge
Where: P = core power consumption
Cache/Memory Subsystem Configuration
shows the bits corresponding with DFS mode.
C = effective capacitance (approx. as a constant)
V = core voltage, VDD
f = core frequency, fCORE
Figure 8
Table 8. IBM 750GX Mapping to MPC7447A MSSSR, MSSCR Registers
summarizes this relationship.
Migrating from IBM 750GX to MPC7447A, Rev. 1.0
Table 7. MPC7447A HID1/DFS Settings
IBM 750GX
SRR1[AP]
SRR1[DP]
SRR1[TEA]
MPC7447A
HID1[DFS]
HID1[PC0-PC4]
MPC7447A
MSSSR[APE]
MSSSR[DPE]
MSSSR[TEA]
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