TMS320C6424 32 bit PCI interface to Freescale MPC8313E

Thanks Senthil,
I was suspecting something like this.

In the PCI V2.2 specifications, a PCI bridge usually has relatively weak output driver and assume a trace impedance of 65 ohms with no termination. In this case, series resistors are not required.

In the MPC8313E Hardware Specification, Freescale indicates that the AC test load used for the PCI signals is formed of a 50 ohms trace and 50 ohms termination to NVcc/2. This does not correspond to PCI specifications and will also required series resistors to achieve current limiting required for Reflected wave switching.

This answers my question as if the PCI driver is current limited for the Reflected Wave Switching operation of the PCI interface. The answer is No and external resistors must be used to match the trace impedance on both devices.

Thanks,
Regards,

In the event that we don't have enough space on the PCB to add 54 series resistors on both end of the PCI interface to reshape the waveform to match the PCI specifications, can we assume that the clamping diode on the receiver will be sufficient to absorb the energy of the overshoot and undershoot without the risk of damaging the chip on a long term?

In other words, the serial damping resistors are they mandatory on the C6424 PCI interface?

Thanks,

Hello Denis,

I am not sure the receiver would have the capability to absorb the reflected energy. Please check with the receiver device vendor to clarify this.

To my knowledge, PCI spec does not call for any series termination. You can perform simulation to find out optimal series resistor if required.

Regards,
Senthil

Hi Senthil,

The MPC8313 is the host and the TMS320C6424 is the device. So, the C6424 is the receiver.

You are correct, the PCI spec does not call for any series termination in the case were the chips (Host and device) are PCI compliant.
In the PCI V2.2 specifications, a PCI bridge usually has relatively weak output driver and assume a trace impedance of 65 ohms with no termination. In this case, series resistors are not required. But apparently, neither of the chips MPC8313 or TMS320 are fully PCI compliant.

For example, on AD31, when a data is transfered from the TMS320 to the MPC8313. There is an overshoot of +3.78V on the receiver (MPC8313) and an overshoot of +4.36V on the TMS320 resulting from the echo return from the unterminated net.

In the PCI specifications, the driver must drive the signal half way to the high level until the echo return from the unterminated net and pull the signal to the high logical level (+3.3V). This is called "Reflected Wave Switching". In this case, there is no overshoot on the signal and no termination is required.

Regards,
Denis

Hello Senthil,

By the way, I am evaluating simulators to avoid the net termination problem.
I am thinking about HyperLynx but I am not sure it is the best software for the job.

Do you have any suggestion?

Thanks,
Denis

Great,
Thank you very much,

Hi Senthil,

About the PCI interface. Freescale have some overshoot tolerance specifications in addition to the "Absolute Maximum Rating".
The absolute maximum rating for the PCI interface is : -0.3V to Vdd +0.3V
The overshoot tolerance for the PCI interface is : -0.7V to Vdd +20% for a maximum period of 3ns.

Is there a similar tolerance for the TMS320C6424?

regards,
Denis