Project IceStorm – Bitstream File Format Documentation

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.

General Description of the File Format

The bitstream file starts with the bytes 0xFF 0x00, followed by a sequence of zero-terminated comment strings, followed by 0x00 0xFF. However, there seems to be a bug in the Lattice "bitstream" tool that moves the terminating 0x00 0xFF a few bytes into the comment string in some cases.

After the comment sections the token 0x7EAA997E (MSB first) starts the actual bit stream. The bitstream consists of one-byte commands, followed by a payload word, followed by an optional block of data. The MSB nibble of the command byte is the command opcode, the LSB nibble is the length of the command payload in bytes. The commands that do not require a payload are using the opcode 0, with the command encoded in the payload field. Note that this "payload" in this context refers to a single integer argument, not the blocks of data that follows the command in case of the CRAM and BRAM commands.

The following commands are known:

0payload=1: CRAM Data
payload=3: BRAM Data
payload=5: Reset CRC
payload=6: Wakeup
payload=8: Reboot
1Set bank number
2CRC check
4Set boot address
5Set internal oscillator frequency range
payload=0: low
payload=1: medium
payload=2: high
6Set bank width
7Set bank height
8Set bank offset
9payload=0: Disable warm boot
payload=16: Enable cold boot
payload=32: Enable warm boot

Use iceunpack -vv to display the commands as they are interpreted by the tool.

Note: The format itself seems to be very flexible. At the moment it is unclear what the FPGA devices will do when presented with a bitstream that use the commands in a different way than the bitstreams generated by the lattice tools.

Writing SRAM content

Most bytes in the bitstream are SRAM data bytes that should be written to the various SRAM banks in the FPGA. The following sequence is used to program an SRAM cell:

The bank width and height parameters reflect the width and height of the SRAM bank. A large SRAM can be written in smaller junks. In this case height parameter may be smaller and the offset parameter reflects the vertical start position.

There are four CRAM and four BRAM banks in an iCE40 FPGA. The different devices from the family use different widths and heights, but the same number of banks.

The CRAM banks hold the configuration bits for the FPGA fabric and hard IP blocks, the BRAM corresponds to the contents of the block ram resources.

The ordering of the data bits is in MSB first row-major order.

Organization of the CRAM

Mapping of tile config bits to 2D CRAM

The chip is organized into four quadrants. Each CRAM memory bank contains the configuration bits for one quadrant. The address 0 is always the corner of the quadrant, i.e. in one quadrant the bit addresses increase with the tile x/y coordinates, in another they increase with the tile x coordinate but decrease with the tile y coordinate, and so on.

For an iCE40 1k device, that has 12 x 16 tiles (not counting the io tiles), the CRAM bank 0 is the one containing the corner tile (1 1), the CRAM bank 1 contains the corner tile (1 16), the CRAM bank 2 contains the corner tile (12 1) and the CRAM bank 3 contains the corner tile (12 16). The entire CRAM of such a device is depicted on the right (bank 0 is in the lower left corner in blue/green).

The checkerboard pattern in the picture visualizes which bits are associated with which tile. The height of the configuration block is 16 for all tile types, but the width is different for each tile type. IO tiles have configurations that are 18 bits wide, LOGIC tiles are 54 bits wide, and RAM tiles are 42 bits wide. (Notice the two slightly smaller columns for the RAM tiles.)

The IO tiles on the top and bottom of the chip use a strange permutation pattern for their bits. It can be seen in the picture that their columns are spread out horizontally. What cannot be seen in the picture is the columns also are not in order and the bit positions are vertically permuted as well. The CramIndexConverter class in encapsulates the calculations that are necessary to convert between tile-relative bit addresses and CRAM bank-relative bit addresses.

The black pixels in the image correspond to CRAM bits that are not associated with any IO, LOGIC or RAM tile. Some of them are unused, others are used by hard IPs or other global resources. The iceunpack tool reports such bits, when set, with the ".extra_bit bank x y" statement in the ASCII output format.

Organization of the BRAM

This part of the documentation has not been written yet.

CRC Check

The CRC is a 16 bit CRC. The (truncated) polynomial is 0x1021 (CRC-16-CCITT). The "Reset CRC" command sets the CRC to 0xFFFF. No zero padding is performed.