STPM10 datasheet(23/51 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

Doc ID 17728 Rev 3

23/51

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 k

Ω. The oscillator frequency can be compensated using the CRC configuration bit.

Quartz: If RC = 0, the oscillator works with an external crystal. The recommended

circuit is depicted in Figure 20 (b).

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 Table 11) 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;

In this way the RC oscillator is started. If the registers are read again, it can be seen that RC

bit is set and BANK is cleared. Once the RC startup procedure is complete, the device is

clocked and active. For details on mode signals refer to Chapter 7.18, for SPI operations

refer to Chapter 7.19.

Figure 20.

Different oscillator circuits with (a) quartz, (b) internal oscillator, (c) external source

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