Is external crystal required for PSoC 5LP CAN at 1Mbps

[Anonymous]

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.

[ScWi_1017771 · Accepted solution]

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.

[Anonymous]

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.

[ScWi_1017771]

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).