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Document 38-08035 Rev. *K Page 67 of 83
25. Voltage Vs CPU Frequency Characteristics
Figure 25-1. Voltage vs CPU Frequency Characteristics
Running the CPU at 24 MHz requires a minimum voltage of
4.75V. This applies to any CPU speed above 12 MHz, so using
an external clock between 12 - 24 MHz must also adhere to this
requirement. Operating the CPU at 24MHz when the supply
voltage is below 4.75V can cause undesired behavior and must
be avoided.
Many enCoRe II applications use USB Vbus 5V as the power
source for the device. According to the USB specification,
voltage can be less than 4.75V on Vbus (if the USB port is a low
power port the voltage can be between 4.4V and 5.25V). Even
for externally powered 5V applications, developers must
consider that on power up and power down voltage is less than
4.75V for some time. Firmware must be implemented properly to
prevent undesired behavior.
Use of 24 MHz requires the use of the high POR trip point of
approximately 4.55 - 4.65V (Register LVDCR 0x1E3,
PORLEV[1:0] = 10b). This setting is sufficient to protect the
device from problems due to operating at low voltage with CPU
speeds above 12 MHz. This must be set before setting the CPU
speed to greater than 12 MHz. For devices with slow power
ramps, changing the POR threshold to the high level may result
in one or more resets of the device as power ramps through the
chip default POR set point of approximately 2.6V up through the
high POR set point.
If multiple resets are undesirable for slow power ramps, then
firmware must do the following:
■
Set the Low Voltage Detection circuit (Register LVDCR 0x1E3,
VM[2:0]) for one of the set points above the POR (VM[2:0] =
110b ~4.73V or 111b ~4.82V).
■
Monitor the LVD until voltage is above the trip point (Register
VLTCMP 0x1E4, bit 1 is clear).
■
Debounce the indication to ensure that voltage is above the set
point for possible noisy supplies.
■
Set the POR to the high set point.
■
Shift CPU speed to 24 MHz.
If the supply voltage dips below 4.75V and the application can
tolerate running at a CPU speed of 12 MHz, then application
firmware may also implement the following to minimize the
chance of a reset event due to a voltage transient:
■
Set the LVD for one of the desired high setting (~4.73V or
~4.82V).
■
Enable the LVD interrupt.
■
In the LVD ISR, reduce CPU speed to 12 MHz and shift the
POR to a lower threshold.
■
Firmware can monitor for VLTCMP to clear within the normal
application main loop.
■
Debounce the indication to ensure voltage is above the set
point.
■
Shift the POR to the high set point.
■
Shift the CPU to 24 MHz.
93 KHz
12 MHz
24 MHz
CPU Frequency
Vdd (volts)
4.00
4.75
5.50
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