Wednesday, November 12, 2014

Migrating to Vivado (Part 2)

The title says it all.

I have managed to massage the C65GS source so that it synthesis in Vivado.

This involved some silly things that Vivado should really support, like casting ports in a module. Don't worry about what that means. Just understand that it is something really simple, and Vivado chokes on it.

However, that dwarfs into insignificance with the real problem: Vivado fails to detect when to use BRAM (the little RAM blocks in the FPGA), and instead tries to implement them using logic.  This means that Vivado thinks I need an FPGA 5x bigger than I have, even though ISE can synthesise it to fit in less than 1/2 the current one.

Oddly, it does realise that two small memories should be BRAMs, as the following output from Vivado shows (you might need to make your display really wide to see the gory detail).

Start RAM, DSP and Shift Register Reporting
---------------------------------------------------------------------------------

Block RAM:
+------------+-------------------------+------------------------+---+---+------------------------+---+---+--------------+--------+--------+------------------------+
|Module Name | RTL Object              | PORT A (depth X width) | W | R | PORT B (depth X width) | W | R | OUT_REG      | RAMB18 | RAMB36 | Hierarchical Name      | 
+------------+-------------------------+------------------------+---+---+------------------------+---+---+--------------+--------+--------+------------------------+
|framepacker | thumnailbuffer0/ram_reg | 4 K X 8(READ_FIRST)    | W |   | 4 K X 8(WRITE_FIRST)   |   | R | Port A and B | 0      | 1      | framepacker/extram__26 | 
|viciv       | rasterbuffer1/ram_reg   | 2 K X 9(READ_FIRST)    | W |   | 2 K X 9(WRITE_FIRST)   |   | R | Port A and B | 1      | 0      | viciv/extram__42       | 
+------------+-------------------------+------------------------+---+---+------------------------+---+---+--------------+--------+--------+------------------------+

Note: The table shows RAMs generated at current stage. Some RAM generation could be reversed due to later optimizations. Multiple instantiated RAMs are reported only once. "Hierarch
ical Name" reflects the hierarchical modules names of the RAM and only part of it is displayed.
Distributed RAM: 
+--------------+--------------------------+--------------------+----------------------+--------------------------------+----------------------+
|Module Name   | RTL Object               | Inference Criteria | Size (depth X width) | Primitives                     | Hierarchical Name    | 
+--------------+--------------------------+--------------------+----------------------+--------------------------------+----------------------+
|gs4510        | shadowram0/ram_reg       | Implied            | 128 K X 8            | RAM256X1S x 4096               | gs4510/ram__25       | 
|iomapper__GC0 | kickstartrom/ram_reg     | Implied            | 16 K X 8             | RAM256X1S x 512                | ram__26              | 
|framepacker   | videobuffer0/ram_reg     | Implied            | 4 K X 8              | RAM64X1D x 128  RAM64M x 128   | framepacker/ram__28  | 
|ethernet      | rxbuffer0/ram_reg        | Implied            | 4 K X 8              | RAM64X1D x 128  RAM64M x 128   | ethernet/ram__29     | 
|ethernet      | rrnet_rxbuffer/ram_reg   | Implied            | 4 K X 8              | RAM64X1D x 128  RAM64M x 128   | ethernet/ram__30     | 
|ethernet      | txbuffer0/ram_reg        | Implied            | 4 K X 8              | RAM64X1D x 128  RAM64M x 128   | ethernet/ram__31     | 
|sdcardio      | sdsectorbuffer0/ram_reg  | Implied            | 512 X 8              | RAM64X1D x 16  RAM64M x 16     | sdcardio/ram__32     | 
|sdcardio      | sdsectorbuffer1/ram_reg  | Implied            | 512 X 8              | RAM64X1D x 16  RAM64M x 16     | sdcardio/ram__33     | 
|sdcardio      | f011sectorbuffer/ram_reg | Implied            | 512 X 8              | RAM64X1D x 16  RAM64M x 16     | sdcardio/ram__34     | 
|uart_monitor  | membuf_reg               | Implied            | 16 X 8               | RAM16X1S x 8                   | uart_monitor/ram__35 | 
|uart_monitor  | historyram0/ram_reg      | Implied            | 1 K X 177            | RAM256X1S x 708                | uart_monitor/ram__37 | 
|viciv         | chipram0/ram_reg         | Implied            | 128 K X 9            | RAM128X1D x 9216               | viciv/ram__38        | 
|viciv         | colourram1/ram_reg       | Implied            | 64 K X 8             | RAM128X1D x 4096               | viciv/ram__39        | 
|viciv         | paletteram0/ram_reg      | Implied            | 1 K X 32             | RAM256X1S x 128                | viciv/ram__40        | 
|viciv         | charrom1/ram_reg         | Implied            | 4 K X 8              | RAM256X1S x 128                | viciv/ram__41        | 
|viciv         | sprite_x_reg             | Implied            | 8 X 8                | RAM16X1S x 8                   | viciv/ram__42        | 
|viciv         | sprite_data_offsets_reg  | Implied            | 8 X 6                | RAM32M x 1                     | viciv/ram__43        | 
|viciv         | sprite_colours_reg       | Implied            | 8 X 8                | RAM16X1S x 8                   | viciv/ram__44        | 
|viciv         | sprite_y_reg             | Implied            | 8 X 8                | RAM16X1S x 8                   | viciv/ram__45        | 
|viciv         | buffer1/ram_reg          | Implied            | 512 X 8              | RAM64X1D x 16  RAM64M x 16     | viciv/ram__46        | 
+--------------+--------------------------+--------------------+----------------------+--------------------------------+----------------------+

All the rest are being implemented as various types of fabric-based RAM.

I have asked for help on the Xilinx Community Forums to see if anyone knows whats going wrong, and how I can work around it.

But for now, it looks like I am using ISE for a bit longer still.

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