UM0470 datasheet(10/37 Pages) STMICROELECTRONICS

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Single wire interface module (SWIM)

UM0470

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3.4

SWIM communication protocol

When in ACTIVE mode, communication can be initiated by host or device. Each byte or

command is preceded by a 1-bit header in order to arbitrate if both host and device initiate

the communication at the same time.

The host header is ‘0’ in order to have the priority over the device in case of arbitration, due

to open-drain capability. The host can start the transfer only if there is no transfer on-going.

Figure 8.

.Command Format (Host -> Target)

Each command sent by the host is made of:

●

1 command (ROTF, WOTF or SWRST) made of

Header: 1 bit at ‘0’

b2-b0: 3-bit command

pb: parity bit: XOR between all b(i)

ack:acknowledge (1 bit at ‘1’). The receiver must send the not-acknowledge

value if it has detected a parity error (NACK: not acknowledge = 1 bit at ‘0’), or

it is not yet ready.

●

and optionally several data packets (in case of WOTF) made of:

Header: 1 bit at ‘0’

b7-b0:

8-bit data

pb:

parity bit sent after data. XOR between all b(i)

ack:

acknowledge

Figure 9.

Data Format (Target -> Host)

Each data frame is made of:

Header: 1 bit at ‘1’

b7-b0:

8-bit data

pb:

parity bit sent after data

ack:

acknowledge

ack

Command

Data1

pb ack

...

Italic: Bit sent by the Host

Bold: Bit sent by the device

pb ack

data n

data n+1

...

data n-1

Italic: Bit sent by the Host

Bold: Bit sent by the device

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