STPM10 datasheet(35/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

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Figure 25: Timing for writing configuration and mode bits

t1 → t2 (> 30 ns): SPI out of idle state

t2 → t3 (> 30 ns): SPI enabled for writing operation

t3: data value is placed in SDA

t4: SDA value is stable and shifted into the device

t3 → t5 (> 10 μs): writing clock period

t3 → t5: 1 bit data value

t5 → t6: 6 bit address of the destination latch

t6 → t7: 1 bit EXE command

t8: end of SPI writing

t9: SPI enters idle state

The same procedure should be applied for the mode signals, but in this case the 6-bit

address must be taken from Table 12: "Mode signal description". The LSB of the command

is also called the EXE bit because instead of the data bit value, the corresponding serial

clock pulse is used to generate the necessary latching signal. In this manner the writing

mechanism does not need the measurement clock to operate and the operation of SPI

module of STPM10 is completely independent from the rest of the device logic except for

the signal POR. Commands to change system signals should be sent during active signals

SCS and SYN, as shown in the figure above. The SYN must be put low in order to disable

the SDA output driver of the STPM10 and make the SDA an input pin. A string of

commands can be sent within one period of active SCS and SYN signals, or a command

can be followed by reading the data record. However, in this case, the SYN should be

deactivated in order to enable the SDA output driver, and SYN pulse should be applied

before activation of the SCS in order to latch the data.

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 requires 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 can happen by performing the following steps:

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