What is an arbiter?
An arbiter is a very common block used on HW designs.
I think I can find the best example of an arbiter at home. When my two kids were teenagers, I had only one car. On Friday and Saturday evenings, there was usually a conflict over who got to use the car. Usually, it was on me to decide (arbiter) who got the car. Not an easy task. (I still have only one car. It just happens that my kids are not teenagers anymore, and praise the Lord, they have their own cars).
HW boards are not that different. There are at least two cases where a shared (and valued, and expensive) resource is ‘wanted’ by several users:
- Common memory: It is very usual to see boards where fast memory (i.e. DDR) is shared between a processor and an FPGA. Obviously, memory cannot answer two masters at once. The processor, and the FPGA, ask the arbiter for permission to access the memory. Even if it is done at very high speed and simultaneously for our perception, in reality, the FPGA and the processor have to take turns to own the memory. NOTE: This was only a simple example, as there are many other applications where even more than two ‘intelligent’ devices (processors, DSPs, GPUs, FPGAs, ASICs) take turns accessing a common memory.
- Common bus: Many times multiple masters (and slaves) communicate through a common bus. Electrically speaking the bus is very simple. It is no more than a set of copper connections, or wires (sometimes with bus drivers). But bus ownership is important since it is impossible for two masters to own the bus at the same time (examples of multi-master buses: I2C, PCI). If two masters tried to ‘talk’ at the same time there would be a collision, and the data on the bus would be corrupted.
The arbiter is the piece of HW who decides who gets to use the common, valued resource, at any given time. However, and unlike the soccer arbiter, the VHDL arbiter will never take one of the devices out of the ‘game’ (well… almost never. There are cases where HW arbiters decide that one device is behaving badly and decide to get it out of the game. One example is a pluggable card accessing a common bus, that can be unplugged at a critical moment, and if the arbiter didn’t recognize the failure, the bus could remain stuck over the -now missing- device).
The arbiter receives two types of signals from its clients:
- Request: Asserted by each one of the devices that want to own the common resource. There can be many request signals asserted at the same time, as is the case when many devices ask for permission to own the common resource. On the other hand, there can be only one, and even no request signals asserted. The latter would be the case when at a specific moment, no one needs the shared resource.
- Grant: Asserted by the arbiter, there is one grant signal for each master. Usually, there will be ONLY ONE grant signal asserted at any given time.
In the next installment, we will see two examples of arbiters implemented in VHDL.