Using HC08GZ16 and Running 1MBps CAN off the PLL

[PaulPorter]

Hi all

We are busy investigating the CAN bus which we are going to try and run at 1Mbps. How accurate is the PLL clock? Are its jitter specifications with in the 1% given in the ODVA spec? I have read on ST forums that it is not recommended to run a high speed CAN bus off the PLL due to jitter problems, does Motorola have the same limitation? Our only other option is then to use a 16/32Mhz crystal and not use the PLL.

Any advice/experience would be greatly appreciated

Thanx.

[PaulPorter]

Hi Rocky.

Thanx for the clarification. So it seems then that the only recommended way run the MSCAN module at 1Mbps is to use a externally generated square wave of 16Mhz or more.

Best Regards

[PaulPorter]

Hi RockyRoad

Thanx for the information. How does one then get the CAN system to RUN 1Mbps. For 1Mbps one needs a clock of at least 8Mhz, but because of the Divide by two from the Crystal Clock source you cannot get that unless you use the PLL or a 16/32Mhz crystal.

Where does it say the GZ16 can have a maximum crystal of 8Mhz? I thought at 5V it was 32Mhz. With a 32Mhz crystal you cannot use the PLL, but you wouldnt need to anyway.

[RockyRoad]

Hi Paul -

The oscillators for the CGM modules on HC08's were only designed for the 1 - 8MHz crystal range.

If you look at the Data Sheet in the General Description section:

"* Clock generation module supporting 1-MHz to 8-MHz crystals"

Then, back in the Electrical Characteristics in the 5.0-Volt Control Timing table it has under Frequency of operation that the crystal option is 1 to 8 MHz while the external clock option is dc to 32 MHz. So you can have up to a 32MHz external clock, but it means that you have to feed an externally generated square wave into the OSC1 pin.

- Rocky

[RockyRoad]

I'm not an expert on CAN, but I do know that we don't recommend using the PLL on any of the current parts. In fact, in the GZ16 data sheet there is the note:

If the system clock is generated from a PLL, it is recommended to select the crystal clock source rather than the system clock source due to jitter considerations, especially at faster CAN bus rates.

The GZ16 MSCAN module can select using the crystal oscillator while the rest of the part uses the PLL for a clock source.

Also note that the GZ16 can only use up to an 8MHz crystal.

- Rocky

[rhinoceroshead]

Rocky, can you clarify that last statement?  'We don't recommend using the PLL on any of the current parts.'

Do you mean with the CAN or with the internal clock or not at all?

Does the CAN receiver resynchronize its clock with signal transitions?  I know the CAN specification includes bit stuffing for that reason.

[RockyRoad]

Sorry that I wasn't clear. I did mean that we don't recommend using the PLL as the clock for the MSCAN module.

As I say, I'm not an expert on CAN. I did check with our CAN expert in the group and he confirmed that the current crop of PLL's on our parts do not provide the accuracy because of jitter for CAN.

- Rocky