STPM10 datasheet(36/51 Pages) STMICROELECTRONICS

36 / 51 page

Theory of operation

STPM10

36/51

Doc ID 17728 Rev 3

7.22.1

Interfacing the standard 3-wire SPI with the STPM10 SPI

Due to the fact that a 2-wire SPI is implemented in the STPM10, it is clear that sending any

command from a standard 3-wire SPI would require a 3-wire to 2-wire interface, which

should produce a proper signal on SDA from host signals SDI, SDO and SYN. The need for

a single-gate 3-state buffer could be avoided through an emulation of SPI just to send some

commands. On a microcontroller this would be done by performing the following steps:

1. Disable the SPI module

2. Set the SDI pin, which is connected to SDA as an output

3. Activate SYN first, and then SCS

4. Apply a new bit value to SDI, and activate SCL

5. Deactivate SCL

6. Repeat the previous two steps seven times to complete a one byte transfer

7. Repeat the previous three steps for any remaining byte transfers

8. Set the SDI pin as an input

9. Deactivate SCS and SYN

10. Enable the SPI module

In case of a precharge command (0xFF), the emulation above is not necessary. Due to the

pull-up device on the SDA pin of the STPM10, the processor needs to perform the following

steps:

1. Activate SYN first in order to latch the results

2. After at least 1 µs, activate SCS

3. Write one byte to the transmitter of SPI (this produces 8 pulses on SCL with SDI=1)

4. Deactivate SYN

5. Optionally read the data records (the sequence of reading is altered)

6. Deactivate SCS

7.23

Energy calculation algorithm

Within the STPM10, the computing section of the measured active power uses a completely

new patented signal processing approach. This approach allows the device to reach high a

level of performance in terms of accuracy.

The signals, coming from the sensors, for the instantaneous voltage are calculated as

follows:

Equation 5

v(t) = V•sin

ωt

where V is the peak voltage and

ω is related to the line frequency.

The instantaneous current is calculated using:

Equation 6

i(t) = I • sin (

ωt + ϕ)

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