STPM10 datasheet(25/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

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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 start-up procedure is complete, the device

is clocked and active. For details on mode signals refer to Section 7.18: "Mode signals", for

SPI operations refer to Section 7.19: "SPI interface".

7.12

Resetting the STPM10

The STPM10 has no reset pin. The device is automatically reset by the POR circuit when

the VCC crosses the 2.5 V value, but it can also be reset through the SPI interface by

providing a dedicated command (see Section 7.19: "SPI interface" for remote reset

command details). In case of reset caused by the POR circuit, all clocks and both of DC

buffers in the analog part are kept off for about 30 ms, as well as all blocks of the digital

part, except for the SPI interface, which is held in a reset state for about 125 ms after a

reset condition. When a reset is performed through SPI, no delayed turn-on is generated.

Resetting the STPM10 causes all the functional modules of the STPM10 to be cleared,

including the volatile memory. The reset through SPI (remote reset request) normally takes

place during production testing.

7.13

Using the STPM10 in microcontroller-based meters

The STPM10 can be used in microcontroller-based energy meters. The SPI pins (SCS,

SCL, SDA, SYN) are used for communication purposes, allowing the microcontroller to

write and read the internal STPM10 registers. The zero-crossing signal is available at the

ZCR pin (see Section 7.4: "Zero-crossing detection" for details about the ZCR signal). The

WDG pin provides the watchdog signal (DOG). The DOG signal generates a 16 ms long

positive pulse every 1.6 seconds. Generation of these pulses can be suspended if data are

read in intervals shorter than 1.6 s. The DOG signal is actually a watchdog reset signal

which can be used to control operation of an on-board microcontroller. It is set to high

whenever the VDDA voltage is below 2.5 V, but after VDDA goes above 2.5 V this signal

starts running. It is expected that an application microcontroller should access the data in

the metering device on a regular basis at least 1/s (recommended is 32/s). Every latching

of results in the metering device requested from the microcontroller also resets the

watchdog. If latching requests are not 1.6 seconds from one another, an active high pulse

on WDG is produced, because the device assumes that the microcontroller is not operating

properly. An application can use this signal either to control the reset pin of its

microcontroller, or it can be tied to an interrupt pin. The latter option is recommended for a

battery-backup application which can enter a sleep mode due to power-down conditions,

and should not be reset by a metering device as it would exit from sleep mode.

7.14

Energy-to-frequency conversion

The STPM10 provides energy-to-frequency conversion both for calibration and energy

readout purposes. In fact, one convenient way to verify the meter calibration is to provide a

pulse train signal with 50% duty cycle whose frequency signal is proportional to the active

energy under steady load conditions. In this case, the user chooses a certain number of

pulses on the LED pin that correspond to 1 kWh. This value is called P. Let us consider the

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