WEBVTT NOTE This file was exported by MacCaption version 7.0.13 to comply with the WebVTT specification dated March 27, 2017. 00:00:00.367 --> 00:00:03.437 align:center line:-1 position:50% size:41% Let's take a look at CAN timing, 00:00:03.437 --> 00:00:12.312 align:center line:-1 position:50% size:63% and why did we see those bits at the beginning of the frame jumping up and down 00:00:12.312 --> 00:00:17.251 align:center line:-1 position:50% size:37% and that one bit at the end had a lower dominant level? 00:00:17.251 --> 00:00:21.822 align:center line:-1 position:50% size:63% What those are, the ones that were intermittent at the beginning of the frame, 00:00:21.822 --> 00:00:25.592 align:center line:-1 position:50% size:43% that was evidence of arbitration. 00:00:25.592 --> 00:00:29.663 align:center line:-1 position:50% size:69% What happens when the arbitration is happening is, 00:00:29.663 --> 00:00:31.632 align:center line:-1 position:50% size:50% there are multiple nodes in a system, 00:00:31.632 --> 00:00:38.772 align:center line:-1 position:50% size:57% as you see in this physical timing diagram. 00:00:38.772 --> 00:00:43.210 align:center line:-1 position:50% size:46% You have node A, node B, node C, all the way up to node N. 00:00:43.210 --> 00:00:45.078 align:center line:-1 position:50% size:36% They are individual nodes. 00:00:45.078 --> 00:00:48.882 align:center line:-1 position:50% size:40% They have their own clocking. 00:00:48.882 --> 00:00:53.787 align:center line:-1 position:50% size:42% They each look out on the bus, see if the bus is idle. 00:00:53.787 --> 00:00:56.690 align:center line:-1 position:50% size:38% If they all see the bus is idle, 00:00:56.690 --> 00:01:04.331 align:center line:-1 position:50% size:49% any of them can start generating bits almost at the same time. 00:01:04.331 --> 00:01:06.199 align:center line:-1 position:50% size:26% Follow that red dot. 00:01:06.199 --> 00:01:10.437 align:center line:-1 position:50% size:49% Node A generated a bit onto the bus. 00:01:10.437 --> 00:01:13.407 align:center line:-1 position:50% size:48% It has to travel the length of the bus. 00:01:13.407 --> 00:01:17.477 align:center line:-1 position:50% size:52% Node N looks out on the bus and says, "Looks idle to me." 00:01:17.477 --> 00:01:23.750 align:center line:-1 position:50% size:62% It hadn't received the bit from node A yet, and that bit travels all the way back to node A. 00:01:23.750 --> 00:01:28.588 align:center line:-1 position:50% size:49% Then, after it arrives back at node A, 00:01:28.588 --> 00:01:35.395 align:center line:-1 position:50% size:40% node A that generated a bit at approximately 75% to 80%, 00:01:35.395 --> 00:01:38.365 align:center line:-1 position:50% size:63% this is the sample point we talked about earlier, 00:01:38.365 --> 00:01:44.304 align:center line:-1 position:50% size:39% it delays looking at its own bit that it thinks it put on the bus 00:01:44.304 --> 00:01:49.509 align:center line:-1 position:50% size:43% to see if it has the same polarity. 00:01:49.509 --> 00:01:53.613 align:center line:-1 position:50% size:45% This is an illustration of two buses talking at the same time. 00:01:53.613 --> 00:01:59.586 align:center line:-1 position:50% size:44% There's a budget for this timing, and it's determined by, first of all, 00:01:59.586 --> 00:02:01.788 align:center line:-1 position:50% size:33% just a propagation delay. 00:02:01.788 --> 00:02:07.961 align:center line:-1 position:50% size:24% Typical for copper is about 5 ns/m. 00:02:07.961 --> 00:02:11.832 align:center line:-1 position:50% size:52% Then, in addition, there's a delay of the transceiver itself. 00:02:11.832 --> 00:02:16.570 align:center line:-1 position:50% size:53% That's the small white box that's drawn above each of those nodes 00:02:16.570 --> 00:02:21.108 align:center line:-1 position:50% size:47% as well as processing time of the controller itself of each node. 00:02:21.108 --> 00:02:23.810 align:center line:-1 position:50% size:32% That's called loop delay. 00:02:23.810 --> 00:02:25.445 align:center line:-1 position:50% size:38% It's two times the loop delay, 00:02:25.445 --> 00:02:34.988 align:center line:-1 position:50% size:58% which, typical is about 250 ns for a typical CAN transceiver and controller. 00:02:34.988 --> 00:02:38.358 align:center line:-1 position:50% size:34% You've got two times that plus two times the length. 00:02:38.358 --> 00:02:44.431 align:center line:-1 position:50% size:47% If you assume a length in a vehicle could be up to 10 m, 00:02:44.431 --> 00:02:53.507 align:center line:-1 position:50% size:57% there's 100 ns, so 5 ns/m times 10 m, double it because it travels back and forth, 00:02:53.507 --> 00:03:00.380 align:center line:-1 position:50% size:41% then 2 times the loop delay, and you're up to about 600 ns. 00:03:00.380 --> 00:03:07.187 align:center line:-1 position:50% size:66% You can see there's a long delay that can happen where multiple nodes can talk at the same time. 00:03:07.187 --> 00:03:14.027 align:center line:-1 position:50% size:48% Let's take a look at this next graphic and see how arbitration works. 00:03:14.027 --> 00:03:17.297 align:center line:-1 position:50% size:53% This is why they call it dominant-bit low. 00:03:17.297 --> 00:03:25.238 align:center line:-1 position:50% size:56% Any node that pulls the bus to a low level, the differential bus is low. 00:03:25.238 --> 00:03:28.708 align:center line:-1 position:50% size:60% Here, I show an example of node 1, 2, and 3. 00:03:28.708 --> 00:03:32.412 align:center line:-1 position:50% size:51% By the way, it operates as a wired AN. 00:03:32.412 --> 00:03:37.284 align:center line:-1 position:50% size:50% The start of it, they all started talking, they generated the same level, 00:03:37.284 --> 00:03:42.022 align:center line:-1 position:50% size:52% they all looked out on the bus about 75% into a bit time and they say, 00:03:42.022 --> 00:03:44.958 align:center line:-1 position:50% size:32% "Well, that's the polarity of the bit I put out there. 00:03:44.958 --> 00:03:46.960 align:center line:-1 position:50% size:32% I'm going to keep talking." 00:03:50.263 --> 00:03:53.366 align:center line:-1 position:50% size:51% Here, I show two bits at the same time. 00:03:53.366 --> 00:03:58.138 align:center line:-1 position:50% size:61% The next two bits, they all put recessive levels out there or 1s. 00:03:58.138 --> 00:04:02.109 align:center line:-1 position:50% size:46% Again, they looked out there, "That's the polarity I put out there." 00:04:04.111 --> 00:04:05.979 align:center line:-1 position:50% size:48% I keep clicking on the wrong button. 00:04:05.979 --> 00:04:09.116 align:center line:-1 position:50% size:32% Next 2 bits, same thing. 00:04:09.116 --> 00:04:13.954 align:center line:-1 position:50% size:43% During bit field number 6, node 1 put a recessive bit there, 00:04:13.954 --> 00:04:17.924 align:center line:-1 position:50% size:58% node 2 and 3 put 2 dominant bits out there. 00:04:17.924 --> 00:04:22.596 align:center line:-1 position:50% size:52% Node 6 looks out there, and if you look at the red trace down at the bottom, 00:04:22.596 --> 00:04:25.999 align:center line:-1 position:50% size:58% it follows what node 2 and 3 put on the bus. 00:04:25.999 --> 00:04:31.972 align:center line:-1 position:50% size:48% Node 1 says, "I didn't put a low level or a dominant level on the bus." 00:04:31.972 --> 00:04:37.878 align:center line:-1 position:50% size:52% That node turns into a listen-only mode at that point. 00:04:37.878 --> 00:04:43.950 align:center line:-1 position:50% size:60% The next bit, node 2 an 3 are still contending for dominance on the bus. 00:04:43.950 --> 00:04:46.286 align:center line:-1 position:50% size:34% In this case, node 2 wins. 00:04:46.286 --> 00:04:49.256 align:center line:-1 position:50% size:47% Node 3 goes to a listen-only mode. 00:04:49.256 --> 00:04:57.097 align:center line:-1 position:50% size:58% Now, node 2 has full capability to transmit throughout the rest of the frame. 00:04:57.097 --> 00:05:01.001 align:center line:-1 position:50% size:40% Back to the waveform we saw just a few minutes ago. 00:05:01.001 --> 00:05:08.208 align:center line:-1 position:50% size:50% Why did, during that arbitration time, did those bits go to a lower low level? 00:05:08.208 --> 00:05:11.912 align:center line:-1 position:50% size:37% It's because multiple nodes were pulling more current, 00:05:11.912 --> 00:05:15.482 align:center line:-1 position:50% size:62% simultaneously pulling the bus to a lower level. 00:05:15.482 --> 00:05:17.984 align:center line:-1 position:50% size:49% It's still sampled as a dominant level. 00:05:17.984 --> 00:05:19.486 align:center line:-1 position:50% size:30% It crosses a threshold. 00:05:19.486 --> 00:05:25.959 align:center line:-1 position:50% size:62% It really doesn't matter that it's a lower low level or a more dominant level. 00:05:25.959 --> 00:05:28.562 align:center line:-1 position:50% size:44% It's still seen as a dominant level. 00:05:28.562 --> 00:05:35.268 align:center line:-1 position:50% size:51% At the end of the frame, that bit was the acknowledgement bit. 00:05:35.268 --> 00:05:40.140 align:center line:-1 position:50% size:38% Once a frame is transmitted, all nodes are monitoring it. 00:05:40.140 --> 00:05:45.579 align:center line:-1 position:50% size:54% If they see it as a valid frame, they all pull on the bus at the same time, 00:05:45.579 --> 00:05:49.149 align:center line:-1 position:50% size:38% pull more current out, drives it to a lower low level. 00:05:49.149 --> 00:05:53.286 align:center line:-1 position:50% size:52% This just makes it convenient when you're monitoring it with a scope. 00:05:53.286 --> 00:05:55.822 align:center line:-1 position:50% size:34% To see this activity, you can immediately see 00:05:55.822 --> 00:06:01.728 align:center line:-1 position:50% size:51% is there arbitration happening and is the frame being acknowledged. 00:06:01.728 --> 00:06:06.700 align:center line:-1 position:50% size:65% Back to what we talked about very, very early on, waveform update rate. 00:06:06.700 --> 00:06:09.569 align:center line:-1 position:50% size:55% Sometimes arbitration can be infrequent, 00:06:09.569 --> 00:06:15.575 align:center line:-1 position:50% size:57% in which case a fast waveform update rate reveals that to you very, very quickly.