UM0470 |
Single wire interface module (SWIM) |
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3.4SWIM 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)
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Command |
Data1 |
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... |
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0 |
b2 |
b1 |
b0 |
pb |
ack |
0 |
b7 |
b6 |
b5 |
b4 |
b3 |
b2 |
b1 |
b0 |
pb |
ack |
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Italic: Bit sent by the Host |
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Bold: Bit sent by the device |
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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.
●optionally several data packets (in case of WOTF) made of:
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Header: 1 bit at ‘0’ |
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b7-b0: |
8-bit data |
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pb: |
parity bit sent after data. XOR between all b(i) |
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ack: |
acknowledge |
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Figure 9. Data format (Target -> Host) |
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data n-1 |
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data n |
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data n+1 ... |
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b5 |
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1 |
b7 |
b6 |
b4 |
b3 |
b2 |
b1 |
b0 |
pb |
ack |
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Italic: Bit sent by the Host |
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Bold: Bit sent by the device |
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Each data frame is made of: |
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Header: 1 bit at ‘1’ |
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b7-b0: |
8-bit data |
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pb: |
parity bit sent after data |
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ack: |
acknowledge |
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Doc ID 14024 Rev 3 |
13/37 |