SPI Data Exchange

The communication with the irj45 module uses the well-known Serial Peripheral Interface (SPI). A transfer must always be triggered by the application core (AC) and must at least be once during the heartbeat timeout. The implementation in the irj45 updates the SPI data after each transfer to make sure it doesn't interrupt a running transfer.

Clock domains and communication cycle

Figure 1 ctc clock domains

Operation of the device includes two clock domains, which run independently of each other. First clock domain is the fieldbus side. Commonly the PLC interacts with the device in one clock domain, where the PLC controls the timing of the output data. Second clock domain is drived by the AC through initiation of the SPI cycle, which reads the output data from the device and updates input data for the next fieldbus cycle. Therefore, a specific timing of the process data is set up as shown in Figure 2 Communication Cycle.

Following data transports occur:

1. New output data from PLC

2. Processing of output data in CC module and preloading of SPI transfer buffer

3. A finished SPI transfer initiated by the AC executes data exchange between AC and CC

4. Preloading of new input data for the next SPI transfer

5. A finished sequential SPI transfer executes data exchange between AC and CC, thus providing new input data for the next fieldbus transfer

Figure 2 Communication Cycle

Technical data

• Transfer length:

  • Cyclic only: 72 Bytes

  • Cyclic + RPC data: 128 Bytes

• Baud-rate: max. 10 Mbps, usually 2 Mbps.

• Delay between SPI transfers: 0.5 ms ... Heartbeat Timeout (1 second)

• Minimal round-trip time: 4 ... 6 ms (depending on the used protocol and setup)

SPI Timing

Following simplified diagram shows basic SPI timing which must be considered by the application controller.

Figure 3 Basic SPI timing

SPI Speed

The communication module supports SPI speed in the range of 29.3 kHz and 10MHz. Default SPI speed is 2MHz.

SPI Setup Time

In Figure 3 Basic SPI timing, the communication scheme between AC and CC is shown. Following time needs to be considered by the Application Controller during communication: the SPI Setup Time - the time between activation of the module using the Slave Select signal and first data - must at least be 10ns, before the module accepts data over SPI.

SPI Cycle Time

This time is the distance between two consecutive SPI transfers. Between two transfers a minimum time of 250 us must be kept to secure proper processing in the module.

SPI Frame Structure

The SPI frame must contain the structure from Table 18: SPI Frame Structure to get accepted by the irj45 module.

Table 18 SPI Frame Structure

The same layout is send back by the device containing its local sequence counter. The sequence counter is tracked and if it doesn't change during the heartbeat timeout the communication gets stopped until the sequence is updated again.

Fletcher-16 Checksum (16 Bit)

To calculate the Fletcher-16 checksum this Wikipedia entry can be used. Start index is byte 4 and end is at 127. After the calculation the value 0x0007 needs to be added to not have false positives if the whole area is set to zeros. In the frame the value has a width of 16 bit and needs to have the little endian encoding.

Sequence Counter (8 Bit)

The sequence counter must be incremented on each sent out frame to be recognized as new data. It can start at any 8 bit value.

Data Length (8 Bit)

If only cyclic data is transferred the data length can be set from 0 to 73 bytes. When using RPC over SPI the data length needs to have a fixed value of 124.

Remote Procedure Call

The RPC protocol used by the Micro Core-To-Core (MCTC) implementation of the irj45 module is transferred in byte 77 – 127. RPC transfers are always acknowledged to minimize data loss on erroneous transfers as good as possible. Also the RPC calls can be larger than the available 50 bytes as the irj45 module internally stores each received RPC frame in a ring-buffer and waits until a partitioned transfer is completed before proceeding with the request.

RPC Frame Structure

The RPC frame must contain the structure from Table 19: RPC Frame Structure to be accepted by the irj45 module.

Table 19 RPC Frame Structure

Each time a new local sequence is sent the irj45 will respond with the corresponding acknowledge in the second transferred frame. If no acknowledge was received the AC can retransmit its frame to re-request the acknowledge.

Fletcher-16 Checksum (16 Bit)

To calculate the Fletcher-16 checksum this Wikipedia entry can be used. Start index is byte 6 and end is at the included data length. After the calculation the value 0x0007 needs to be added to not have false positives if the whole area is set to zeros. In the frame the value has a width of 16 bit and needs to have the little endian encoding.

Local Sequence (8 Bit)

The sequence counter must be incremented for each RPC frame with changed data. For each unseen incremental frame the irj45 module will put the transferred data into its internal ring-buffer and after the length matches it will process the RPC call.

Remote Sequence Acknowledge (8 Bit)

For each processed received RPC frame the AC needs to send out an acknowledge. If the received frame doesn't match the expected sequence the AC must leave the acknowledge on the previous sequence to trigger a resend from the irj45 module. If the resend fails the AC can also perform a re-sync.

Data Length (8 Bit)

Contains the length of the RPC data and must be between 0 (acknowledge-only frame) and 44.

Flags (8 Bit)

Table 20 RPC Header Flags

Sync Request

If set to 1 the irj45 module enters the RPC synchronization.

Sync Acknowledge

During the Sync Request both sides need to set the Sync Acknowledge flag, see RPC Synchronization for details.

Request Acknowledge

Forces an acknowledge from the partner device.

RPC Synchronization

Before the irj45 module is ready to operate and after a synchronization is lost the RPC must be synchronized. This is triggered by setting the Sync Request flag to 1.

Table 21 RPC Synchronization

After the synchronization has finished the irj45 both devices must initiate the generic RPC call SetupStateGet to verify if the partner device is in the same state. If one partner is already configured but the other is not, than the configured parter must reboot and the synchronization starts again.

Normal Operation

If the AC successfully passed the synchronization stage and configured the irj45 module it must call the RPC API SetupDone. At this point the irj45 module is ready for normal operation.

RPC Protocol

The GOAL RPC protocol uses a virtual push/pop stack to call remote API functions with arguments. Each call must have a return value that is usually set to GOAL_OK when the call succeeded.

RPC Request/Response Structure

An RPC request/response consists of the parts shown in Table 21 and Table 22.

Table 22 RPC Request Structure

Table 23 RPC Response Structure

Static Identifier

The 2 byte static identifier is used on the irj45 module to detect the start of a new RPC request or response. If the identifier is also contained in the data bytes the Fletcher-16 checksum will make sure that it don't gets threaded like an RPC request/response.

Data Length

The data length contains the count of bytes starting with the "Function Id" and ending with the last data byte.

RPC Id

The RPC id is the module or group id. For example GOAL PROFINET uses the RPC id GOAL_ID_PNIO to register its calls.

Function Id

The function id is used as a sub id to map the function calls in the specific module to their handlers.

CTC Id

The CTC id is used to match requests and responses to their specific MCTC internal handle. A response must use the same CTC id as the request.

Flags

The flags define the type of the request, see Table 5: RPC Request/Response Flags for details.

Table 24 RPC Request/Response Flags

Data

The data part contains the virtual stack of the RPC request/response.

Fletcher-16 Checksum (16 Bit)

To calculate the Fletcher-16 checksum the Wikipedia entry can be used. Start index is byte 16 and end is at the included data length. After the calculation the value 0x0007 needs to be added to not have false positives if the whole area is set to zeros. In the frame the value has a width of 16 bit and needs to have the little endian encoding.

GOAL PROFINET Data Mapper API

The GOAL PROFINET Data Mapper API allows to map PROFINET subslots, producer & consumer states and the connection information to the cyclic data stream of the Data Mapper that is used for example in MCTC transfers.

Map Subslot Data – goal_pnioDmSubslotAdd

The API goal_pnioDmSubslotAdd maps input and output data of the given subslot to the also given instances of the DM.

Table 25 goal_pnioDmSubslotAdd API Description

Map Subslot IOCS/IOPS - goal_pnioDmSubslotIoxsAdd

The API goal_pnioDmSubslotIoxsAdd maps IOCS and IOPS states of the given subslot to the given instances of the DM.

Table 26 : goal_pnioDmSubslotIoxsAdd API Description

Map APDU Status – goal_pnioDmApduAdd

The API goal_pnioDmApduAdd maps the APDU status to the given instance of the DM.

Table 27 goal_pnioDmApduAdd API Description

Map Data Provider Status – goal_pnioDmDpAdd

The API goal_pnioDmDpAdd maps the Data Provider status to the given instance of the DM.

Table 28 goal_pnioDmApduAdd API Description