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MPC8313 PCI Bridge

Question asked by DENIS FOURNIER on Jan 9, 2015
Latest reply on Jan 14, 2015 by DENIS FOURNIER

I interfaced a DSP (TMS320C6424) with a Freescale CPU (MPC8313E) using the 32 bit PCI Interface. Assuming that both devices 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 devices are fully PCI compliant.

I did not use any serial resistors on the PCI signals as the PCI specification don't require any termination due to the week driver. The nets are about 4 inches long with impedance controlled to 55 ohms +/- 10%. The MPC8313 is configured as PCI host and the TMS320C6424 as PCI device.

There is actually no problem. Everything work perfectly fine. But I probed some signals on the PCI interface to check the waveforms and they are mostly very badly shaped: Down to -1.05V undershoot and up to +4.24V overshoot. The PCI driver is apparently too strong and the waveform certainly does not meet the absolute maximum rating of the receiver chips.

I think that +4.24V overshoot may be harmful for the internal PCI clamping diode and some precautions need to be considered. A 33 ohms series resistors may be required on both end of each net. To make the interface PCI compliant, I will then need to add 54 series resistors on each of the devices.

In the event that I don't have enough space on the PCB to add 108 series resistors to reshape the waveform to match the PCI specifications, can I 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 MPC8313E PCI interface?