Project IceStorm aims at documenting the bitstream format of Lattice iCE40 FPGAs and providing simple tools for analyzing and creating bitstream files. This is work in progress.
Regarding the Span-4 and Span-12 Wires a RAM tile behaves exactly like a LOGIC tile. So for simple applications that do not need the block ram resources, the RAM tiles can be handled like a LOGIC tiles without logic cells in them.
A pair or RAM tiles (odd and even y-coordinates) provides an interface to a block ram cell. Like with LOGIC tiles, signals entering the RAM tile have to be routed over local tracks to the block ram inputs. Tiles with odd y-coordinates are "bottom" RAM Tiles (RAMB Tiles), and tiles with even y-coordinates are "top" RAM Tiles (RAMT Tiles). Each pair of RAMB/RAMT tiles implements a SB_RAM40_4K cell. The cell ports are spread out over the two tiles as follows:
| SB_RAM40_4K | RAMB Tile | RAMT Tile |
|---|---|---|
| RDATA[15:0] | RDATA[7:0] | RDATA[15:8] |
| RADDR[10:0] | - | RADDR[10:0] |
| WADDR[10:0] | WADDR[10:0] | - |
| MASK[15:0] | MASK[7:0] | MASK[15:8] |
| WDATA[15:0] | WDATA[7:0] | WDATA[15:8] |
| RCLKE | - | RCLKE |
| RCLK | - | RCLK |
| RE | - | RE |
| WCLKE | WCLKE | - |
| WCLK | WCLK | - |
| WE | WE | - |
The configuration bit RamConfig PowerUp in the RAMB tile enables the memory. This bit is active-low in 1k chips, i.e. an unused RAM block has only this bit set. Note that icebox_explain.py will ignore all RAMB tiles that only have the RamConfig PowerUp bit set.
In 8k chips the RamConfig PowerUp bit is active-high. So an unused RAM block has all bits cleared in the 8k config bitstream.
The RamConfig CBIT_* bits in the RAMT tile configure the read/write width of the memory. Those bits map to the SB_RAM40_4K cell parameters as follows:
| SB_RAM40_4K | RAMT Config Bit |
|---|---|
| WRITE_MODE[0] | RamConfig CBIT_0 |
| WRITE_MODE[1] | RamConfig CBIT_1 |
| READ_MODE[0] | RamConfig CBIT_2 |
| READ_MODE[1] | RamConfig CBIT_3 |
The read/write mode selects the width of the read/write port:
| MODE | DATA Width | Used WDATA/RDATA Bits |
|---|---|---|
| 0 | 16 | 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 |
| 1 | 8 | 14, 12, 10, 8, 6, 4, 2, 0 |
| 2 | 4 | 13, 9, 5, 1 |
| 3 | 2 | 11, 3 |
The NegClk bit in the RAMB tile negates the polarity of the WCLK port, and the NegClk bit in the RAMT tile negates the polarity of the RCLK port.
A logic tile sends the output of its eight logic cells to its neighbour tiles. A RAM tile does the same thing with the RDATA outputs. Each RAMB tile exports its RDATA[7:0] outputs and each RAMT tile exports its RDATA[15:8] outputs via this mechanism.