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Hi,
We are planning to use the CAN peripheral of a PSoc 5LP (CY8C5888AXI-LP096). We would like to run the CAN bus at a baud rate of 1Mbps. On page 12 of AN52701, it states the following:
"The CAN protocol does not transmit a clock to synchronize the bits. Synchronization between nodes is done for every
bit transmitted, during the Sync Segment, as Figure 5 on page 4 shows. This requires the use of a highly accurate
oscillator for baud rates greater than 125 Kbps.
The CAN protocol specifies that the clock accuracy must be less than or equal to 0.5%. An error less than 0.1% can
be achieved in PSoC by using an external crystal."
Does this mean that an external crystal is required in order to run the CAN bus at a baud rate of 1 Mbps? Or, is the external crystal just recommended? In other words, is robust CAN communication at 1Mbps possible without an external crystal?
Thanks for your help.
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Robust CAN communication requires "clock accuracy less than or equal to 0.5%". Period. There is no requirement that this come from a crystal oscillator (how would it even know?)
It's easy to achieve better than 0.5% accuracy with a crystal, which is why use of a crystal is recommended. However if you can provide 0.5% or better accuracy in some other way then that's fine.
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Robust CAN communication requires "clock accuracy less than or equal to 0.5%". Period. There is no requirement that this come from a crystal oscillator (how would it even know?)
It's easy to achieve better than 0.5% accuracy with a crystal, which is why use of a crystal is recommended. However if you can provide 0.5% or better accuracy in some other way then that's fine.
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Thanks for your feedback. Do you know if the internal oscillator of the PSoc 5LP (CY8C5888AXI-LP096) is capable of this level of accuracy?
Thanks.
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Nope, unless you are using USB mode and locking to the USB bus (not familiar - suppose it uses a frequency reference? You would have to have a continuous USB connection).
From Document Number: 001-84932 Rev. *L, table 11-80 which describes the IMO (Internal Main Oscillator):
Looks like the accuracy range is +/-7%, best is -1%..+1% at 3MHz and the 0C to 70C temp range, UNLESS you use 24MHz - USB mode, with oscillator locking to USB bus (I suppose the USB bus provides an external frequency reference).