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ST7LITEUS2, ST7LITEUS5
Supply, reset and clock management
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Note:
1
In I
2
C mode, the internal RC oscillator is forced as a clock source, regardless of the
selection in the option byte. Refer to note 5 in
for further details.
2
See
Section 12: Electrical characteristics
for more information on the frequency and
accuracy of the RC oscillator.
3
To improve clock stability and frequency accuracy, it is recommended to place a decoupling
capacitor, typically 100nF, between the V
DD
and V
SS
pins as close as possible to the ST7
device.
Caution:
If the voltage or temperature conditions change in the application, the frequency may need
to be recalibrated.
Refer to application note AN2326 for information on how to calibrate the RC frequency using
an external reference signal.
The ST7LITEUS2 and ST7LITEUS5 also contain an Auto-wakeup RC oscillator. This RC
oscillator should be enabled to enter Auto-wakeup from Halt mode.
The Auto-wakeup RC oscillator can also be configured as the startup clock through the
CKSEL[1:0] option bits (see
).
This is recommended for applications where very low power consumption is required.
Switching from one startup clock to another can be done in run mode as follows (see
Case 1
Switching from internal RC to AWU:
1.
Set the RC/AWU bit in the CKCNTCSR register to enable the AWU RC oscillator
2.
The RC_FLAG is cleared and the clock output is at 1.
3.
Wait 3 AWU RC cycles till the AWU_FLAG is set
4.
The switch to the AWU clock is made at the positive edge of the AWU clock signal
5.
Once the switch is made, the internal RC is stopped
Case 2
Switching from AWU RC to internal RC:
1.
Reset the RC/AWU bit to enable the internal RC oscillator
2.
Using a 4-bit counter, wait until 8 internal RC cycles have elapsed. The counter is
running on internal RC clock.
3.
Wait till the AWU_FLAG is cleared (1AWU RC cycle) and the RC_FLAG is set (2 RC
cycles)
4.
The switch to the internal RC clock is made at the positive edge of the internal RC clock
signal
5.
Once the switch is made, the AWU RC is stopped
Note:
1
When the internal RC is not selected, it is stopped so as to save power consumption.
2
When the internal RC is selected, the AWU RC is turned on by hardware when entering
Auto-wakeup from Halt mode.
3
When the external clock is selected, the AWU RC oscillator is always on.
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