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I'm designing a CAN-based platform using the Microchip MCP2551 transmitter. The circuit looks like that: enter image description here

Notice the unconnected pin VREF. I tested the circuit on a prototype and CAN is working. What is the purpose of that pin VREF, which is described in the datasheet as voltage output reference? Could you suggest some applications using VREF? If I don't use it is it ok to left it open?

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  • \$\begingroup\$ It's alright to leave the Vref open. \$\endgroup\$ – Nick Alexeev Jul 16 '15 at 16:09
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The CAN bus needs to be terminated with \$120\Omega\$ in order to work properly. There are now two basic ways how this can be done. The easiest thing would be to connect \$120\Omega\$ between CAN_H and CAN_L. This has the drawback however, that the DC bias level of the bus is not defined properly. In order to have this achieved you have to use split termination, and this is where VREF becomes relevant.

According to the datasheet, VREF is somewhere around VCC/2. If you connect \$60\Omega\$ resistors from CAN_H and CAN_L to VREF you have established proper termination of the bus and furthermore you have a defined bias level of VREF/2 if the transceiver is in its recessive state.

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  • \$\begingroup\$ Are you suggesting that two 60 Ohms to VREF are a better termination than 120 Ohm? What if at the other end there is a 120 Ohm - would they be incompatible? \$\endgroup\$ – judoka_acl Jul 16 '15 at 16:36
  • \$\begingroup\$ I would not call it a better termination, you only gain a defined level on the bus if all devices are recessive. As long as you don't need this feature, stay with the single 120Ohm resistor. But even if you use split termination on one side, you can still have 120Ohm on the other. For the electromagnetic wave there is absolutely no difference between those two. \$\endgroup\$ – christoph Jul 16 '15 at 19:11
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The VREF pin in the MCP2551 has the same functionality as the SPLIT pin in the MCP2561 (see MCP2551 to MCP2561 migration). It is used to stabilize common mode in biased split termination configuration, see an example in page 6 of the datasheet MCP2561.

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Most probably the \$V_{\text{ref}}\$ applications are:

  • The current regulation engine
  • Resistive network voltage divider method
  • Digital-to-analog converter method.
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    \$\begingroup\$ Could you give some references, because in internet I found nothing related to CAN VREF and the applications mentioned. \$\endgroup\$ – judoka_acl Jul 16 '15 at 8:11

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