vme_bus.vhd: handle fast dtack reaction (for ASYNC_DTACK)

If g_ASYNC_DTACK is set to true (the default is false), /DTACK is asynchronously driven by /DS. In that case, the release of /DTACK must be read from the async driver to handle fast changes.

vme_bus.vhd: handle fast dtack reaction (for ASYNC_DTACK)
If g_ASYNC_DTACK is set to true (the default is false), /DTACK is asynchronously driven by /DS. In that case, the release of /DTACK must be read from the async driver to handle fast changes.
2d7930f8 · Tristan Gingold · 8e083d62 · 2d7930f8
Commit 2d7930f8 authored Jan 13, 2022 by Tristan Gingold
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vme_bus.vhd hdl/rtl/vme_bus.vhd +4 -1

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--- a/hdl/rtl/vme_bus.vhd
+++ b/hdl/rtl/vme_bus.vhd
@@ -691,7 +691,10 @@ begin
            -- Once the responding Slave has driven DTACK* or BERR* low, it
            -- MUST NOT release them or drive DTACK* high until it detects
            -- both DS0* and DS1* high.
-            if vme_ds_n_i = "11" then
+            if vme_ds_n_i = "11"
+              or (g_ASYNC_DTACK
+                  and vme_dtack_async_ctrl = '1' and vme_dtack_n_out = '0')
+            then
              vme_berr_n_o    <= '1';

              -- Rescind DTACK.