10.4.3.4.1 FIFO Flag Usage

In the Axcelerator FIFO, the AFVAL and AEVAL signals are each eight bits. The step size of the signal varies based on the aspect ratio to which the FIFO blocks are configured.

For example, if the FIFO is configured in the 128X36 aspect ratio, the step size is eight. That means, if a 00000011 is programmed on the AEVAL, the almost empty flag asserts after 3*8 = 24 words are written. The step sizes can be calculated from the previous table for other configurations.

The core configuration software automatically adjusts the AF and AE thresholds specified by changing them to the nearest step size. A message is also printed in the log file.

Since eight is the least step size for AFVAL and AEVAL, static flag configuration is not supported for widths below eight.

When the core configurator software is used to configure the FIFO to a depth that is less than the total available depth, the FULL flag does not assert at the depth specified in the software. For example, if the FIFO is configured to 250 × 18, the core configurator provides a total depth of 256, which is the closest size. The FULL flag asserts at 256. A message appears in the Project Manager log file indicating the configuration that is provided, taking all these details into consideration.

When FIFOs are cascaded deep, the data gets written to multiple FIFO blocks. The FIFO Error flags (AFULL_ERR, AEMPTY_ERR, FULL_ERR, EMPTY_ERR) indicate if one or more of the FIFOs are in a different state than expected. You can ignore them if you wish. These ports get generated only if you are cascading FIFOs wide.