Datasheet
STPM10 Theory of operation
Doc ID 17728 Rev 4 25/53
7.11 Clock generator
All the internal timing of the STPM10 is based on the CLKOUT signal. This signal can be
generated in three different ways:
1. RC: this oscillator mode can be selected using the RC configuration bit. If RC = 1, the
STPM10 runs using the RC oscillator. A resistor connected between CLKIN and
ground sets the RC current. For 4 MHz operation, the recommended settling resistor is
12 kΩ. The oscillator frequency can be compensated using the CRC configuration bit.
2. Quartz: If RC = 0, the oscillator works with an external crystal. The recommended
circuit is depicted in Figure 20 (b).
3. External clock: by keeping RC=0, it is also possible to feed the CLKOUT pin with an
external oscillator signal.
The clock generator is powered from an analog supply and is responsible for two tasks. The
first is to retard the turn-on of some function blocks after POR in order to help smooth the
start of the external power supply circuitry by keeping off all major loads. The second task of
the clock generator is to provide all necessary clocks for the analog and digital parts. During
this task, the MDIV configuration bit is used to inform the device about the nominal
frequency value of CLKOUT. Two nominal frequency ranges are expected to be from 4.000
MHz to 4.194 MHz (MDIV = 0) or from 8.000 MHz to 8.192 MHz (MDIV = 1).
7.11.1 RC startup procedure
To use the device with RC oscillator the configuration bit RC (see Ta bl e 1 1 ) must be set.
Since the default configuration is for a crystal oscillator, when a RC oscillator is used instead
and the device is supplied for the very first time it is not internally clocked and consequently
the DSP is inactive. In this condition it is not possible to set RC or any other configuration bit.
The following SPI procedure can be run in order to set the RC bit and provide the clock to
the device:
● Set the mode signal BANK;
● Perform a software reset;
● Read the registers: BANK mode signal should be checked and the records should
show something (not 000000F0);
● Clear the mode signal BANK;
● DO NOT perform a reading, and write configuration bit RC;
Figure 20. Different oscillator circuits with (a) quartz, (b) internal oscillator, (c) external source