A signal is generated by some abnormal event, initiated either by user at a typewriter (quit, interrupt), by a pro- gram error (bus error, etc.), or by request of another pro- gram (kill). Normally all signals cause termination of the.
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Introduction
Controller area network (CAN) offers robust communication between multiple network locations, supporting a variety of data rates and distances. Featuring>MCP2515: #TQ for 1-Mbps Operation fOSCBRP = 1BRP = 2BRP = 3BRP = 44020106.667538199.56.3334.75
Signal Stock
30157.5Signal& Bus Segments Worksheet
53.75201053.3332.51052.51.6671.25
| ADSP-BF548: #TQ for 1-Mbps Operation | ||||
| fSCLK | BRP = 5 | BRP = 6 | BRP = 7 | BRP = 8 |
| 133 | 26.6 | 22.167 | 19 | 16.625 |
| 100 | 20 | 16.667 | 14.286 | 12.5 |
| 50 | 10 | 8.3333 | 7.143 | 6.25 |
| 40 | 8 | 6.667 | 5.714 | 5 |
Step 2: Determine Bit Segment Configuration
The next step is to determine the number of TQ required for each bit segment. The most difficult case is supporting the maximum propagation delay at a 1-Mbps data rate, with a 40-m cable and isolated nodes, for example. Ideally, the bit time segments should be configured so that the sample point is as late as possible in the bit. For each integer total number of TQ in Table 1, one TQ must be allowed for the SYNC segment, and the PS2 (or TSEG2) segment must be large to accommodate the CAN-controller-information processing time (2 TQ for MCP2515, <1 TQ for ADSP-BF548 as long as BRP > 4). Also, for the MCP2515, PROP and PS1 can be 8 TQ maximum each; for the ADSP-BF548, TSEG1 (PROP + PS1) can be16 TQ maximum.
Figure 4 and Figure 5 show the possible total TQ configurations for MCP2515 and ADSP-BF548 respectively, with the latest sample point possible for the valid combinations of clock and BRP for 1-Mbps operation. The optimum total TQ for the MCP2515 is 19, requiring a hardware oscillator of 38 MHz and BRP of 1. For the ADSP-BF548, all the configurations except for a total of 5 TQ are at least 85% sample point, but the optimum setting is 10 TQ, requiring fSCLK= 50 MHz with BRP = 5.
Step 3: Match Transceiver/Isolation Delay and Bus Length to Configuration
Having achieved the optimum sample point for the CAN controller, the last step is to compare the allowable propagation delay with the CAN transceiver/isolation and bus length used. Assuming the ADSP-BF548 optimum configuration of 10 TQ (fSCLK= 50 MHz, BRP = 5), the maximum propagation delay possible is 900 ns. For the ADM3053 isolated CAN transceiver with integrated isolated power, the data-sheet maximum loop delay (TxD off to receiver inactive) is 250 ns. This must be doubled (500 ns) to include both transmit and receive delays at two nodes at the furthest ends of the bus.
Assuming a 5-ns/m cable propagation delay, a bus length of 40 meters (maximum for 1 Mbps per ISO 11898) could be accommodated with the ADSP-BF548 having a total of 10 TQ in bit time with only 1 TQ for the TSEG2 bit segment. In practice, slightly earlier sample points may suffice, as even an extreme transceiver propagation delay on one node would likely result in a simple retransmission (handled automatically by the CAN controller at the data-link layer), but the small delay between the CAN-controller I/O and the CAN transceiver makes it advisable to configure the sample point to the latest point possible.
Conclusion
Isolation adds robustness, but it will also add a propagation delay, in both transmit and receive directions. This delay must be doubled to account for two nodes in arbitration. If the allowed propagation delay in the system is fixed, then the cable length or data rate may be decreased if isolation is added. An alternative is to reconfigure CAN controllers to allow the maximum possible propagation delay to ensure that the desired data rate and bus length are possible, even with isolated nodes.
References
Isolated Controller Area Network (CAN)—Video
Marais, Hein. How to Calculate the Controller Parameters for an Isolated CAN (Controller Area Network) Network to Run at 1 Mbps
O’Brien, Maurice. Designing Robust, Isolated I2C/PMBus Data Interfaces for Industrial, Telecommunications, and Medical Applications. Analog Dialogue, Volume 48, Number 3, 2014.
Watterson, Conal. AN-1123 Applications Note. Controller Area Network (CAN) Implementation Guide. Analog Devices, Inc., 2012.