STPM10 datasheet(24/51 Pages) STMICROELECTRONICS

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Theory of operation

STPM10

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Doc ID 17728 Rev 3

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 for remote reset command details).

In case of reset caused by the POR circuit, all clocks and both 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 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 case in which the LED pin is configured to be driven from internal signal

AW (active energy) whose frequency is proportional to the active energy. The signal AW is

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