PCI arbiter doubt

According to PCI 2.1, grant should be removed when FRAME is asserted.

Best regards

Zara

Zara wrote:

I don't have 2.1 handy, but I'm looking at 2.2, which AFAIK didn't change the arbitration signifcantly. Certainly it does NOT suggest that GNT should be removed when FRAME is asserted, although that is allowed.

GNT can be deasserted on any cycle, but if the bus is idle, there needs to be a one cycle delay before asserting a different device's GNT.

If GNT is deasserted and FRAME is asserted on the same clock, the device retains the bus until the transaction completes (just as if GNT is deasserted during the transaction).

But the arbiter shouldn't automatically remove GNT whenever FRAME is asserted, unless you want to limit the device's bus occupancy. Once GNT is deasserted, the device is only allowed to stay on the bus for the duration of the latency timer. So if no other device is requesting the bus, it's usually better to leave GNT asserted ("bus parking").