Dear PCI Users, We are an company active in the AUDIO domain development. We are dealing with the PCI for many years, in particular with the AMCC 5933 and now with the Philips Trimedia processor (with a Master/Slave PCI interface included). We are faced to the following problem: We need to read into the PC memory only 1 or 2 DWords every 1.3ms from our PCI board. In most cases, these reads are performed in about 1us. But on some PC, the PCI Read takes until 4ms to terminate correctly (with the data transferred). During all this time, our CPU stalls which is not acceptable for a real-time process. After having performed different investigations, we have discovered a relation with the AGP bus transactions. When this "latency" occurs on the PCI bus, the AGP transfers about 512kByte of data distributed into bursts of 1us (32 DWord) each. The result is a PCI Wait state on our Read of the corresponding AGP access duration. This behaviour (Read delayed with Waitstate) can be observed with other PCI devices. Please, find a Screen shot of this PCI transfert with wait state. The 4 upper signals are of the AGP type and shows the relation with the PCI transaction. <<PCI latency related to AGP transaction Screen Shot.jpg>> Our test environment is as follow: Windows 2000 with Service Pack 2 and 256MB of memory. Motherboard ASUS CUV4X-DLS (SCSI and Network on-board) 2 x PIII @ 1GHz. Graphic adapter AGP Matrox G450 dual-head. PCI Mykerinos board (our Audio board). All other PCI slots are empty. Our questions: - Is such latency on the PCI bus is a known problem and what is it possible to do to avoid it? - Is it possible to configure the AGP bus, or the PC BIOS or something else, to be able to terminate a PCI transaction between one of this AGP burst in a reasonable latency (for example <10us like described in the PCI 2.2 for write transaction) ? An answer would be very appreciated. Best Regards. Olivier Auberson Hardware Engineer ------------------------------------------ MERGING TECHNOLOGIES www.merging.com ------------------------------------------ phone: +41 21 946 04 44 fax: +41 21 946 04 45
PCI latency related to AGP transaction Screen Shot.jpg
