Datasheet

Theory of operation STPM10
26/53 Doc ID 17728 Rev 4
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 Section 7.18, for SPI operations
refer to Section 7.19.
7.12 Resetting the STPM10
The STPM10 has no reset pin. The device is automatically reset by the POR circuit when
the V
CC
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.