STPM01 datasheet(43/60 Pages) STMICROELECTRONICS

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STPM01

Theory of operation

Doc ID 10853 Rev 8

43/60

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

t2 → t3 (> 30 ns): SPI enabled for write 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 bits 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-bits

address must be taken from the Table 17.

The LSB of command is also called EXE bit because instead of data bit value, the

corresponding serial clock pulse is used to generate the necessary latching signal. This way

the writing mechanism does not need the measurement clock in order to operate, which

makes the operation of SPI module of STPM01 completely independent from the rest of

device logic except from the signal POR.

Commands for changing system signals should be sent during active signals SCS and SYN

as it is shown in the Figure 27. The SYN must be put low in order to disable SDATD output

driver of STPM01 and make the SDATD as an input pin. A string of commands can be send

within one period of active signals SCS and SYN or command can be followed by reading

the data record but, in this case, the SYN should be deactivated in order to enable SDATD

output driver and a SYN pulse should be applied before activation of SCS in order to latch

the data.

Interfacing the standard 3-wire SPI with STPM01 SPI.

Due to the fact a 2-wire SPI is implemented in STPM01 it is clear that sending any

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

produce a proper signal on SDATD from host signals SDI, SDO and SYN. A single gate 3-

state buffer could be omitted by an emulation of SPI just to send some command. On a

microcontroller this would be done by the following steps:

disable the SPI module;

set SDI pin which is connected to SDATD to be output;

activate SYN first and then SCS;

apply new bit value to SDI and activate SCL;

deactivate SCL;

repeat the last two steps seven times to complete one byte transfer;

repeat the last three steps for any remaining byte transfer;

set SDI pin to be input;

deactivate SCS and the SYN;

10. enable the SPI module;

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

device on the SDATD pin of the STPM01 the processor needs to perform the following

steps:

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