Distributed Arbitration Priority Uncertainty

Up until this point, it has been stated that the devices in the distributed arbitration system have fixed priority. The one closest to the central arbiter has the highest priority, and they decrease in order from there.

However, there is a possibility during a small window of time, where no one is using the bus so /BusBusy is equal to 1, that a higher priority device makes a request but does not win the arbitration, such that a lower priority device ends up with control of the bus. This is a side-effect of daisy-chaining the BusGrant signals.

Consider Figure 9.

• Device $i$ is closer to the CA, Device $n$ is further from the CA.
• The time window where priority can be violated will be $(n-i)\cdot t_{\text{DA}}$ and is called the uncertainty time, which is the time for the grant to be passed through the DAs between the two devices making requests.
• The existence of this window where priority does not behave as expected is called priority uncertainty.
• Uncertainty occurs when a high-priority device requests the bus after the BusGrant signal has passed it but before it reaches a lower-priority device, leading to potential priority inversion.

• If Device $i$ makes the request at the time marked $t_{\text{Request i1}}$ (before the BusGrant signal reaches it), then it will win the arbitration and take control of the bus, as the request was made before the BusGrant_0 token was received.
• However, if Device $i$ makes the request at the time marked $t_{\text{Request i2}}$ (after the BusGrant signal has passed it), it will not get the bus.
  • The grant token will propagate from the CA through the DAs up to DA $i$.
  • When DA $i$ sees the signal, Request_i has not been asserted yet, so it passes it on.
  • Once DA $i$ has passed on the BusGrant signal, there is a period of time where the grant may be propagating through devices that are not asking for the bus before it reaches Device $n$.
  • If the request by Device $i$ is made during this time, no device has control of the bus, but it cannot take control as it has already passed the token along.
  • This uncertainty period ends when the token reaches Device $n$ and it takes control of the bus.