CAN Learning - Transceiver (1)

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CAN Learning - Transceiver (1) [Copy link]

CAN Learning - Transceiver (1)

CAN means (Controller Area Network). When choosing a CAN chip, you need to pay attention to whether your chip is CAN or CAN FD. FD means Flexible Data-Rate.

The maximum rate of CAN 2.0 is 1Mbps, and the maximum rate of CAN FD is 5Mbps, which means that the transceiver of CAN2.0 may not be compatible with CAN FD, so if it is CAN FD, a chip with a suitable rate should be selected. CAN FD uses two rates, arbitration and ACK follow the previous CAN 2.0, and the data in the middle uses 1Mbps or higher rate. As shown in Figure 1 below.

Figure 1: CAN FD data format

The CAN transceiver converts data into differential signal output, as shown in Figure 2 below, referring to the TJA1050 NXP data sheet ( Vi(dif)(bus) = V CANH VCANL ).

Figure 2: Timing diagram AC characteristics

Because it is a differential output, it can have a relatively strong anti-interference ability in long-distance transmission. The maximum length and rate of the CAN bus are shown in Table 1. Figure 3 shows the meaning of the contents of Table 1.

/*English translation*/

bus length bus length

cable stub cable tap

Node distance

feet

/*English translation*/

Table 1: Relationship between CAN maximum length and rate

Bus Speed	Bus Length (L)	Cable Stub Length (l)	Node Distance (d)
1 Mbit/Sec	40 meters (131 feet)	0.3 meters (1 foot)	40 meters (131.2 feet)
500 kbits/Sec	100 meters (328 feet)	0.3 meters (1 foot)	100 meters (328 feet)
100 kbits/Sec	500 meters (1640 feet)	0.3 meters (1 foot)	500 meters (1640 feet)
50 kbits/Sec	1000 meters (3280 feet)	0.3 meters (1 foot)	1000 meters (3280 feet)

Figure 3: CAN bus distance relationship diagram

So why does differential have stronger interference capability? It is easy to introduce interference in long-distance transmission. If it is single-ended transmission, when there is a burr on the signal, the reception may be wrong. If it is differential transmission, both CAN_H and CAN_L receive this interference, but the relative voltage on CAN_H and CAN_L does not change, and there is no effect on reception. As shown in Figure 4 below.

Figure 4: Common-mode interference on the CAN bus

It is also worth noting that the terminal resistance of the CAN bus is 120ohm. Usually two 60ohm resistors are connected to CAN-H and CAN-L. The common point of the two resistors is connected to the ground with a capacitor. Usually this capacitor is 10nF or 100nF in long nodes. In different nodes of the bus, the terminal resistance of CAN-H and CAN-L is different. Refer to Figure 5

Figure 5: Terminal resistance setting

Although CAN BUS has reserved many addresses, how many nodes can a CAN transceiver support? We will talk about this in the next issue. Bye~

Reference Documentation

Converging challenges of CAN physical layer transceivers in terms of bandwidth, reliability and low power consumption

CAN FD Communication Features and Product Applications

CAN FD - The basic idea

https://www.can-cia.org/can-knowledge/can/can-fd/

NXP TJA1050 Datasheet

Maximum Cable Length For a CAN Bus

TJA1050 high speed CAN transceiver

Converging challenges of CAN physical layer transceivers in terms of bandwidth, reliability and low power consumption

LitchiCheng

CAN still encounters many problems in actual use. The most difficult ones are sampling points, and the second is electrical

xutong

LitchiCheng posted on 2024-2-20 09:39 CAN still encounters many problems in actual use. The most difficult ones are sampling points, followed by electrical problems

Haven't noticed where yet.

lugl4313820

Is Mr. Xu going to make a CAN transceiver this year? I haven't noticed that the CANFD transceiver also needs a better transceiver chip match.

xutong

lugl4313820 published on 2024-2-22 06:41 Is Mr. Xu going to make a CAN transceiver this year? I haven't noticed that the CANFD transceiver also needs a better transceiver chip match.

Your knowledge is too narrow. Learn more so you won't be eliminated.

小小莫

CAN still encounters many problems in actual use. The most difficult ones are sampling points, and the second is electrical

白猫警督

I'm working on canfd recently. The information is very good and helpful. Thank you for the information.

lkh747566933

I'm a newcomer to the automotive electronics industry. What kind of packaging is used for the 120Ω resistor in the automotive electronics CAN bus matching terminal?