RE: My device stops receiving when powered from battery
"John Dammeyer" <[email protected]> Thu, 27 Sep 2018 09:13:33 -0700
| Newsgroups | gmane.comp.hardware.bus.can |
|---|---|
| Message-ID | <[email protected]> |
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Hi,
First of all CAN bus isn't really a truly differential bus. By =
definition if it were differential the current would change direction =
for lows and highs. On CAN bus the current flows during the dominant =
state and doesn't flow at all during the passive state. So we don't =
really think about what the difference in voltage is between the CAN_L =
and CAN_H pins but what their voltage is relative to ground.
=20
That's why the ground is also available on the ODB-II connector because =
your battery operated CAN monitor needs that ground to recognize when a =
signal is recessive. Ie. When both are at 2.5V relative to ground.
=20
Having said that, I did an experiment may years ago with two P82C250 =
drivers both run from 9V (with 5V regulator) batteries and only CAN_H =
and CAN_L connected. The scope ground was never connected to both ends =
to ensure that ground between drivers was isolated. The TTL signals for =
0/1 were properly coupled between the two drivers. I then confirmed =
that operation with Steve Corrigan who was the designer at TI for both =
the RS485 and CAN driver devices. Yes, CAN works without ground but has =
no noise immunity.=20
=20
So back to your problem. You should have termination at both ends of =
the bus. The 3.3V systems will bring CAN_H from 2.5V to just above 3.0V =
while bringing CAN_L from 2.5V down to about 1V. With a scope you would =
see about 2V between the pins during dominant. The receivers are =
designed to see that difference.
=20
Battery --------------------------------------------------------
| | | |=20
3V Reg. 3Vreg 3V Reg 5V Reg
| | | |=20
Termination ---- 3VNode ---3VNode --- 3VNode --- 5VNode --- Termination
Gnd ------------------------------Gnd
=20
Ensure that you actually have continuity between grounds and the ground =
pin of the transceivers. Some devices might have a 100R resistor =
between the CAN ground and the Power ground in order to be able to =
connect the ground from node to node without 10A of motor current =
flowing through 22g CAN ground wire.
=20
So even though they have a common battery or power supply the CAN ground =
can't just be left hanging. Don't assume that the ground from the =
battery is the ground for the CAN bus.
=20
A scope here will be very useful. If you can, send me photos via emails =
directly. johnd at autoartisans dot com
=20
John Dammeyer
=20
=20
=20
=20
From: [email protected] =
[mailto:[email protected]] On Behalf Of FLJ
Sent: September-27-18 3:10 AM
To: [email protected]
Subject: [CANLIST] My device stops receiving when powered from battery
=20
Hello everyone,
=20
as a software developer I'm completely perplexed by my current problem. =
I only have basic HW knowledge and I mostly follow HW diagrams designed =
by others. Until now I guess I was lucky.
=20
I have an embedded device built upon an STM32 F4 evaluation board and =
I'm using a SN65HVD230 CAN tranceiver from texas instruments with it.
=20
I have a battery powered system consisting of 3 CAN nodes using NXP =
TJ1051 tranceivers, I don't have much detail about the MCUs used.
=20
The system all alone communicates over CAN correctly as expected.
=20
If I connect my device to the system's bus, while I power my device from =
an external power supply, everything works fine. I receive all the CAN =
messages I want to from the systems nodes.=20
My device is configured in silent mode, I don't want to send anything.
=20
As soon as I power my device from the same battery as the system, I stop =
receiving CAN messages. My device itself is working when powered from =
the common battery, meaning I can connect with the debugger to it, set =
breakpoints, etc.; only the CAN stops working.
=20
I've read a lot about common-mode voltage in the past few hours, and =
about splitted termination, but I can't understand what the problem is. =
I would've rather expected or understood if the device wasn't working =
when powered externally, but everything on the same battery should've =
worked.
=20
Can anyone help me out please?
=20
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</o:shapelayout></xml><![endif]--></head><body lang=3DEN-CA link=3Dblue =
vlink=3Dpurple style=3D'tab-interval:36.0pt'><div =
class=3DWordSection1><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'>Hi,<o:p></o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>First =
of all CAN bus isn't really a truly differential bus. <span =
style=3D'mso-spacerun:yes'>=C2=A0</span>By definition if it were =
differential the current would change direction for lows and highs. =
<span style=3D'mso-spacerun:yes'>=C2=A0</span>On CAN bus the current =
flows during the dominant state and doesn't flow at all during the =
passive state.<span style=3D'mso-spacerun:yes'>=C2=A0 </span>So we don't =
really think about what the difference in voltage is between the CAN_L =
and CAN_H pins but what their voltage is relative to =
ground.<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>That's =
why the ground is also available on the ODB-II connector because your =
battery operated CAN monitor needs that ground to recognize when a =
signal is recessive. <span class=3DSpellE>Ie</span>. When both are at =
2.5V relative to ground.<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>Having =
said that, I did an experiment may years ago with two P82C250 drivers =
both run from 9V (with 5V regulator) batteries and only CAN_H and CAN_L =
connected.<span style=3D'mso-spacerun:yes'>=C2=A0 </span>The scope =
ground was never connected to both ends to ensure that ground between =
drivers was isolated. <span style=3D'mso-spacerun:yes'>=C2=A0</span>The =
TTL signals for 0/1 were properly coupled between the two drivers.<span =
style=3D'mso-spacerun:yes'>=C2=A0 </span>I then confirmed that operation =
with Steve Corrigan who was the designer at TI for both the RS485 and =
CAN driver devices.<span style=3D'mso-spacerun:yes'>=C2=A0 </span>Yes, =
CAN works without ground but has no noise immunity. =
<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>So back =
to your problem. <span style=3D'mso-spacerun:yes'>=C2=A0</span>You =
should have termination at both ends of the bus.<span =
style=3D'mso-spacerun:yes'>=C2=A0 </span>The 3.3V systems will bring =
CAN_H from 2.5V to just above 3.0V while bringing CAN_L from 2.5V down =
to about 1V.<span style=3D'mso-spacerun:yes'>=C2=A0 </span>With a scope =
you would see about 2V between the pins during dominant. <span =
style=3D'mso-spacerun:yes'>=C2=A0</span>The receivers are designed to =
see that difference.<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DCode><span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 =
</span>Battery =
--------------------------------------------------------<o:p></o:p></p><p=
class=3DCode><span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 </span>|<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
</span>|<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 </span>|<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 </span>| <o:p></o:p></p><p class=3DCode><span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0</span><span =
lang=3DNL style=3D'mso-ansi-language:NL'>3V Reg.<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0 </span>3Vreg<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0 </span><span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0</span>3V Reg<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0 </span>5V =
Reg<o:p></o:p></span></p><p class=3DCode><span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 </span>|<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
</span>|<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 </span>|<span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 </span>| <o:p></o:p></p><p class=3DCode>Termination ---- 3VNode =
---3VNode --- 3VNode --- 5VNode --- Termination<o:p></o:p></p><p =
class=3DCode><span =
style=3D'mso-spacerun:yes'>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 </span><span =
style=3D'mso-spacerun:yes'>=C2=A0</span><span =
class=3DSpellE>Gnd</span><span style=3D'mso-spacerun:yes'>=C2=A0 =
</span>------------------------------<span =
class=3DSpellE>Gnd</span><o:p></o:p></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>Ensure =
that you actually have continuity between grounds and the ground pin of =
the transceivers.<span style=3D'mso-spacerun:yes'>=C2=A0 </span>Some =
devices might have a 100R resistor between the CAN ground and the Power =
ground in order to be able to connect the ground from node to node =
without 10A of motor current flowing through 22g CAN ground =
wire.<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>So even =
though they have a common battery or power supply the CAN ground can't =
just be left hanging.<span style=3D'mso-spacerun:yes'>=C2=A0 =
</span>Don't assume that the ground from the battery is the ground for =
the CAN bus.<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>A scope =
here will be very useful. <span =
style=3D'mso-spacerun:yes'>=C2=A0</span>If you can, send me photos via =
emails directly.<span style=3D'mso-spacerun:yes'>=C2=A0 </span><span =
class=3DSpellE>johnd</span> at <span class=3DSpellE>autoartisans</span> =
dot com<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span style=3D'font-size:11.0pt;color:#1F497D'>John =
Dammeyer<o:p></o:p></span></p><p class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><p =
class=3DMsoNormal><span =
style=3D'font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p><div=
style=3D'border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm =
4.0pt'><div><div style=3D'border:none;border-top:solid #B5C4DF =
1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=3DMsoNormal><b><span =
lang=3DEN-US =
style=3D'font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-f=
ont-family:"Times New =
Roman";mso-ansi-language:EN-US'>From:</span></b><span lang=3DEN-US =
style=3D'font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-f=
ont-family:"Times New Roman";mso-ansi-language:EN-US'> =
[email protected] =
[mailto:[email protected]] <b>On Behalf Of =
</b>FLJ<br><b>Sent:</b> September-27-18 3:10 AM<br><b>To:</b> =
[email protected]<br><b>Subject:</b> [CANLIST] My device stops =
receiving when powered from battery<o:p></o:p></span></p></div></div><p =
class=3DMsoNormal><o:p> </o:p></p><div><p class=3DMsoNormal>Hello =
everyone,<o:p></o:p></p><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p =
class=3DMsoNormal>as a software developer I'm completely perplexed by my =
current problem. I only have basic HW knowledge and I mostly follow HW =
diagrams designed by others. Until now I guess I was =
lucky.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p class=3DMsoNormal>I =
have an embedded device built upon an STM32 F4 evaluation board and I'm =
using a SN65HVD230 CAN tranceiver from texas instruments with =
it.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p class=3DMsoNormal>I =
have a battery powered system consisting of 3 CAN nodes using NXP TJ1051 =
tranceivers, I don't have much detail about the MCUs =
used.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p =
class=3DMsoNormal>The system all alone communicates over CAN correctly =
as expected.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p =
class=3DMsoNormal>If I connect my device to the system's bus, while I =
power my device from an external power supply, everything works fine. I =
receive all the CAN messages I want to from the systems =
nodes. <o:p></o:p></p></div><div><p class=3DMsoNormal>My device is =
configured in silent mode, I don't want to send =
anything.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p =
class=3DMsoNormal>As soon as I power my device from the same battery as =
the system, I stop receiving CAN messages. My device itself is working =
when powered from the common battery, meaning I can connect with the =
debugger to it, set breakpoints, etc.; only the CAN stops =
working.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p =
class=3DMsoNormal>I've read a lot about common-mode voltage in the past =
few hours, and about splitted termination, but I can't understand what =
the problem is. I would've rather expected or understood if the device =
wasn't working when powered externally, but everything on the same =
battery should've worked.<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div><div><p =
class=3DMsoNormal>Can anyone help me out =
please?<o:p></o:p></p></div><div><p =
class=3DMsoNormal><o:p> </o:p></p></div></div></div></div></body></h=
tml>
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