The data is stored between the two, as indicated by the light blue color in the image above. Half of the time the tail will have a higher index than the head. The tail will be forever chasing after the head, that’s how a ring buffer works. The calculation now yields (2 + 8) – 5 = 5, which is the correct answer. The solution is to offset the head with the total number of slots in the FIFO, 8 in this case. Thus, if we perform this simple calculation, we get 2 – 5 = -3, which makes no sense. In the image above, the head is at index 2 while the tail is at index 5. Subtracting the head from the tail only works if the head leads the tail. In the image above, that yields a count of five elements. When the head has a higher index than the tail, we can calculate the number of elements in the ring buffer by subtracting the tail from the head.
#Logicworks 5 tri state buffer free#
The tail pointer will be at the oldest element, while the head points to the next free slot. The slots containing data are colored light blue in the illustration. The tail pointer is still at slot number 0, but the head pointer has moved to slot number 5. The image above shows the same ring buffer after five writes. This is the beauty of the ring buffer, the data doesn’t move, only the pointers do. When either of the pointers is at the highest index, the next write or read will cause the pointer to move back to the lowest index. The tail pointer is incremented every time the user of the FIFO reads an element. For each write, the head pointer moves one place forward. Note that the FIFO would still be empty if both pointers were at another index, for example, 3. This is the initial state of the ring buffer. Both the head and the tail pointer are pointing to element 0, indicating that the FIFO is empty. The image above shows an example FIFO with eight slots. If the head and tail point to the same element, it means that the FIFO is empty. There are other variants, but this is the one we are going to use.
The head always points to the memory slot that will contain the next written data, while the tail refers to the next element that will be read from the FIFO. These two pointers are the head and tail pointers. For the rest of this article, we will refer to these counters are pointers. In VHDL, this will be an index to an array cell. These counters refer to an offset from the start of the memory space where the data is stored. Two counters are used to keep track of the location and the number of elements in the FIFO.
New elements stay at the same memory location from the time of writing until it is read and removed from the FIFO. How a ring buffer worksĪ ring buffer is a FIFO implementation that uses contiguous memory for storing the buffered data with a minimum of data shuffling. Because even though they all perform the same basic queue and dequeue tasks, they can be vastly different when taking the details into account. But still, many engineers prefer to implement their own FIFOs.
There exist many free FIFO implementations online, as well as FIFO generators like Xilinx LogiCORE. What kind of interface do you need? Are you limited by resources? Should it be resilient to over-read and overwrite? Is latency acceptable? Those are some of the questions that arise in my mind when asked to create a FIFO. There are many design decisions you will have to make when implementing a FIFO. In this article, we will create a ring buffer in VHDL to implement a FIFO in block RAM. Circular buffers are popular constructs for creating queues in sequential programming languages, but they can also be implemented in hardware.
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