CY28378 Cypress Semiconductor, CY28378 Datasheet - Page 11

• Manufacturer Part Number: CY28378
• Description: FTG for Pentium 4 and Intel 845 Series Chipset
• Manufacturer: Cypress Semiconductor

Document #: 38-07519 Rev. **
Program the CPU output frequency
When the programmable output frequency feature is enabled
(Pro_Freq_EN bit is set), the CPU output frequency is deter-
mined by the following equation:
Fcpu = G * N/M.
“N” and “M” are the values programmed in Programmable
Frequency Select N-Value Register and M-Value Register,
respectively.
Table 6. Examples of N and M Value for Different CPU Frequency Range
Frequency Ranges
128 – 203
66 – 127
SET SOFTW ARE FSEL
Set SW Freq_Sel = 1
Set FS(0:4)
Gear Constants
Frequency Revert Bit =
Set Frequency to
47.99750
63.99667
FS_HW_Latched
0
INITIALIZE W ATCHDOG TIMER
Set WD(0:3) = (# of Sec ) x 2
RESET W ATCHDOG TIMER
Set SRESET# = 0 for 6 msec
Set Frequency Revert Bit
Figure 1. Watchdog Flowchart
Clock Output to Ramp to
W ATCHDOG TIMEOUT
Fixed Value for M-Value Register
Set WD(0:3) Bits = 0
Wait for 6msec For
Target Frequency
Y
Hang?
Reset
48
40
Set Frequency to FS_SW
Frequency Revert Bit =
“G” stands for the PLL Gear Constant, which is determined by
the programmed value of FS[4:0] or SEL[4:0]. The value is
listed in Table 2.
The ratio of N and M need to be greater than “1” [N/M> 1].
The following table lists set of N and M values for different
frequency output ranges. This example use a fixed value for
the M-Value Register and select the CPU output frequency by
changing the value of the N-Value Register.
N
1
Set WD(0:3) Bits = 0
SET DIAL-A-FREQUENCY
CLEAR W D
Load M and N Registers
Set Pro_Freq_EN = 1
Range of N-Value Register for Different
CPU Frequency
66 – 127
80 – 127
Exit
CY28378
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