CY28317-2
Document #: 38-07094 Rev. *B
Page 14 of 21
How to Program 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+3)/(M+3)
“N” and “M” are the values programmed in Programmable Fre-
quency Select N-Value register and M-Value register, respec-
tively.
“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 4.
The ratio of (N+3) and (M+3) need to be greater than “1”
[(N+3)/(M+3) > 1].
The following table lists set of N and M values for different
frequency output ranges.This example uses a fixed value for
the M-Value register and selects the CPU output frequency by
changing the value of the N-Value register.
WD_TIMER[4:0]
These bits store the time-out value of the Watchdog Timer. The scale of the timer is determine by the
prescaler.
The timer can support a value of 150 ms to 4.8 sec when the prescaler is set to 150 ms. If the prescaler
is set to 2.5 sec, it can support a value from 2.5 sec to 80 sec.
When the Watchdog Timer reaches “0,” it will set the WD_TO_STATUS bit.
WD_PRE_SCALER
0 = 150 ms
1 = 2.5 sec
RST_EN_WD
This bit will enable the generation of a Reset pulse when a watchdog timer time-out occurs.
0 = Disabled
1 = Enabled
RST_EN_FC
This bit will enable the generation of a Reset pulse after a frequency change occurs.
0 = Disabled
1 = Enabled
Table 7. Register Summary (continued)
Name
Description
Table 8. Examples of N and M Value for Different CPU Frequency Range
Frequency Ranges
Gear Constants
Fixed Value for
M-Value Register
Range of N-Value Register
for Different CPU Frequency
50 MHz – 129 MHz
48.00741
93
97–255
130 MHz – 248 MHz
48.00741
45
127–245
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