1. Communication protocol on SPI

Transcription

1 SPI Communication Protocol This document describes the communication protocol between target application running on ADZS-BF707-BLIP2 platform and WSN master over SPI. 1. Communication protocol on SPI The WSN master is identified as the master and the application target on BLIP is identified as the slave for the communication on SPI. The target will send data to the WSN master on receiving appropriate commands from the WSN master. The communication protocol for commands and/or data from/to the WSN master is given below. The red fields indicate mandatory fields. The yellow fields indicate optional fields: Field Length (in bytes) Description CMD 4 Identifies the command/data packet SIZE 1 The size of the data in a data packet including 3 bytes for FR EN, FR NO and CHK fields FR EN 1 Identifies if the data associated with the packet is fragmented FR NO 1 The fragment number of the packet if the data is fragmented CHKSUM 1 The checksum on the packet from the 4th byte onwards excluding this byte. Checksum is computed as a sum of the associated bytes. DATA Variable The actual data associated with the packet 1.1 Command packets Command packets can only be sent from the master to the slave. Command packets have only one field as shown below. The field CMD has the following values :

2 CMD Field Value 0x (START_MONITOR) 0x (SEND_OCCU_CMD) 0x (SEND_USAGE_CMD) 0x (SEND_BLOBINFO_CMD) 0x (SEND_DATA_CMD) 0x F (STOP_MONITOR) Description Command from WSN master to target to start sending occupancy data. After receiving this, slave will monitor occupancy and send occupancy data when there is change in occupancy status. Command from WSN master to target to send occupancy metadata. The master will send this command when it wants slave to send current occupancy status. On receiving this, slave will not wait for change in occupancy and immediately send current occupancy status in data packets. Command from WSN master to target to send target CPU usage data. Command from WSN master to target to send current Blob information data. Command from WSN master to target to send a response data packet. Command from WSN master to target to stop sending occupancy data. 1.2 Data Packets These packets can be sent from slave to master and vice-versa. The packets have the following fields The field CODE indicates the packet type. The field CODE has the following values CODE Field Value 0x (DATA PKT OCCUPANCY) 0x (DATA PKT TARGET USAGE) 0x (DATA PKT BLOB INFO) Description Indicates that the response sent from target to WSN master is occupancy Indicates that the response sent from target to WSN master is Target Usage Indicates that the response sent from target to WSN master is Blob information Note: The data bytes associated with the data packet is currently identified as only occupancy information and can be extended for future revisions of the protocol DATA PKT OCCUPANCY

3 The DATA PKT OCCUPANCY packet will have the following values in the fields. (The values are in Hexadecimal) The Data field will have a value of 0x0 - No occupancy 0x1 - Occupancy 0x2 - Out of Reach/ No metadata to give (This can happen when application is not in Ready state) DATA PKT TARGET USAGE In Revision 1.0, the DATA PKT TARGET USAGE packet will have only the following values in the fields. (The values are in Hexadecimal) The Data field will have a value of (Target Usage percentage) (VAR indicates variable value in the field) DATA PKT BLOB INFO The DATA PKT BLOB INFO packet will have only the following values in the fields. (The values are in Hexadecimal) The Data field will have following information 1st Byte contains current occupancy status 0x0 - No occupancy 0x1 - Occupancy 0x2 - Out of Reach/ No metadata to give (This can happen when application is not in Ready state) 2nd Byte contains NUM_BLOBS(number of blobs). Subsequent bytes have data containing blob information for each of NUM_BLOBS one after the other.

4 Each Blob Information is packed in following manner MinX - X Co-Ordinate of top left corner of the rectangular bounding box (2 Bytes or Half word) MinY - Y Co-Ordinate of top left corner of the rectangular bounding box (2 Bytes or Half word) MaxX - X Co-Ordinate of bottom right corner of the rectangular bounding box (2 Bytes or Half word) MaxY - Y Co-Ordinate of bottom right corner of the rectangular bounding box (2 Bytes or Half word) (VAR indicates variable value in the field) 1.3 Sequence Diagrams Request Occupancy Metadata WSN master session sends START_MONITOR to target session. Target session on receiving the packet shall send an interrupt to the WSN master. On receiving the interrupt, WSN master sends SEND_DATA_CMD to target. The target sends DATA PKT OCCUPANCY. After this whenever there is a change in occupancy, the target will send an interrupt to the WSN master. On receiving the interrupt WSN master sends SEND_DATA_CMD to target. The target sends DATA PKT OCCUPANCY. WSN master session sends STOP_MONITOR and on receiving this command target session will stop sending DATA PKT OCCUPANCY packets anymore.

5 1.3.2 Request Blobinfo WSN master session sends SEND BLOB INFO command. On receiving this command, target will send an interrupt to the WSN master. On receiving the interrupt, WSN master sends SEND_DATA_CMD to target. The target then sends DATA PKT BLOB INFO.

6 1.4 Error scenarios WSN master sends a START_MONITOR but no response from target. WSN master session has to handle the situation WSN master sends a STOP_MONITOR but target still keeps sending data packets. WSN master session has to handle the situation. 1.5 SPI Protocol The SPI communication is always initiated by the Master. The clock settings and polarity have to be fixed between the master and the slave before communication is initiated. The communication is handled by the master in the following way:

7 SPI Write The master sets the data in the TX buffer and drives the clock to transmit and sends the data via MOSI line. This is received in the RX buffer and based on polling or interrupt, the slave knows about new data and receives it. SPI Read

8 When the master needs to receive data from the slave, it sends a predefined set of clock cycles and the slave will put the data in the MISO line.if the data is of 2 bytes, then the master has to transmit the clock for 2 bytes. This can be full duplex, ie when the Master is transmitting the next request, it can read the data over MISO from the previous request. 1.6 Design Design of Slave The reading of the receive buffer by the slave will be interrupt based. When there is data in the MOSI line, the application target session reads the command. In the ISR, the target session will do the following. a. Copy the received command into the command buffer. b. Set a flag so that the target can send an interrupt to the master on the WSN_INT pin outside the ISR. On an average, the target polls this flag every occupancy detection on frame. c. Submit the next buffer to receive the command from master. In the read function, the target does the following a. Read the command and create the data packet to be send.

9 b. Submit the Transmit Buffer for the size of the packet. c. Send an interrupt on the WSN_INT. NOTE : The slave should ensure that once the SEND_DATA_CMD is received the SPI interrupt has the highest priority to get serviced, since it will transmit the data bytes Design of Master To Send Commands a. The master sends commands to the target (4 bytes). To Receive Data Packets The master on receiving interrupt on WSN_INT does the following: a.send SEND_DATA_CMD to the target while receiving 4 bytes response. (This is the command response from the master if valid data is received) b.send 5 clock cycles (4+1, 4 for the CODE field and 1 for the size field) to the slave if data received previously is all zeros else send 1 clock cycle to read data size. c.read the size and send X clock cycles to the slave. d.read X bytes NOTES: 1. The master should ensure that when the interrupt comes the SEND_DATA_CMD has the highest priority to go out among all other commands it needs to send because the slave is waiting to be serviced. 2. The master should ensure that the minimum time interval between commands exceeds the detection rate of the application target on BLIP. For example, if the target is detecting at 30 fps, then the time interval between commands to the target should exceed 33 milliseconds, preferably 40 milliseconds Interrupt Driven The transfer from slave to master will be initiated by an interrupt on the WSN_INT pin. The interrupt will be edge triggered. The signal on interrupt pin is held high for 48 clock cycles before driving it low.

10 2. Terminology Term WSN VOS SPI IOS BLIP Description Wireless Sensor Node Video Occupancy Sensor Serial Peripheral Interface Indoors Occupancy Sensor Blackfin Low Power Imaging Platform