spiOverJtag: README.md draft

spiOverJtag/README.md

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+# spiOverJtag
+
+*Lattice ECP3/ECP5*, *Gowin GW2/GW5*, and *Cologne Chip GateMate*
+have an internal interface to access external SPI flash via *JTAG*.
+In contrast, accessing SPI flash on *Xilinx*, *Efinix* and
+*Altera Cyclone* FPGAs requires loading a dedicated bitstream that
+bridges *JTAG* and *SPI* interfaces.
+
+`spiOverJtag` contains bridge bitstreams used by `openFPGALoader` to access
+external SPI flash over JTAG on supported Xilinx, Altera, and Efinix devices.
+
+>## Important notes
+>
+>- *spiOverJtag* supports Single-Wire mode only (`MOSI`/`MISO`) for maximum
+>  board compatibility.
+>- Existing bridge bitstreams are versioned in the repository. Rebuild is
+>  typically required only when adding support for a missing FPGA model/package.
+
+## Dependencies
+
+- `make`
+- `gzip`
+- `python3`
+- Python package: `edalize`
+- Vendor toolchains (depending on target family):
+  - Xilinx: Vivado and/or ISE
+  - Altera: Quartus
+  - Efinix: Efinity
+
+Notes:
+- `build.py` is used for **Xilinx** and **Altera** targets.
+- `efinix_build.py` is used for **Efinix** targets.
+
+## Build generalities
+
+All steps mentioned in next sections must be performed from `spiOverJtag`
+sub-directory.
+
+Generated outputs:
+
+- *Xilinx*: `spiOverJtag_<part>.bit.gz`
+- *Efinix*: `spiOverJtag_efinix_<part>.bit.gz`
+- *Altera*: `spiOverJtag_<part>.rbf.gz`
+
+Build command examples:
+
+```bash
+# Xilinx Artix7 35t
+make spiOverJtag_xc7a35t.bit.gz
+# Altera Cyclone10CL 016
+make spiOverJtag_10cl016484.rbf.gz
+# Efinix Trion T13 F256
+make spiOverJtag_efinix_t13f256.bit.gz
+```
+
+Clean temporary/build files:
+
+```bash
+make clean
+```
+
+## Add support for a new device
+
+### 1. Register the part in `Makefile`
+
+Add the short part name into the appropriate list:
+- `XILINX_PARTS`
+- `ALTERA_PARTS`
+- `EFINIX_PARTS`
+
+### 2. Update build script mappings
+
+For *Xilinx* (`build.py`):
+
+Approach for *Xilinx* FPGAs depends on family:
+- for `Artix`, `Kintex 7` and `Spartan 7` size and packages are provided via
+  `packages` dict.
+- for others devices `pkg_name` (*Vivado*) or `tool_options` (*ISE*) must be
+  updated to add the relationship between FPGA model/size/package and
+  device/package.
+
+For *Altera* (`build.py`):
+
+The `full_part` dict must be updated to match the short format and a
+*Quartus*-compatible device name.
+
+For *Efinix* (`efinix_build.py`):
+
+The script contains pin mapping data for *Trion* and *Titanium* devices.
+
+The principle is to update `efinix_pinout` with a new sub-dict that provides
+the pin name for each SPI IO.
+
+The `timing_models` dict must also be updated to add an entry for the new FPGA.
+
+> Note:
+> For *Titanium* devices, ensure the package mapping exists in
+> `efinix_pinout["Titanium"]` and the corresponding timing model exists in
+> `timing_models`.
+
+### 3. Add constraints (if required)
+
+*spiOverJtag* already contains many constraint files.
+Usually, devices in the same family with the same package share the same
+JTAG and SPI pinout (for example, *Xilinx Artix7* devices with *ftg256*
+can reuse `constr_xc7a_ftg256`, regardless of device size).
+This step is only required when a package is not yet available for a given
+FPGA family.
+
+Add the constraint file expected by the mapping:
+
+- *Xilinx Vivado*: `constr_<name>.xdc`
+- *Xilinx ISE*: `constr_<name>.ucf`
+- *Altera*: no explicit constraint file in this directory (handled by Quartus
+  and script flow)
+- *Efinix*: generated/handled by `efinix_build.py`; update script data for new
+  package pinouts when needed
+
+### 4. Build spiOverJtag bitstream
+
+Run the target build command and verify the compressed file is generated:
+
+```bash
+# Xilinx/Efinix
+make spiOverJtag_<new-part>.bit.gz
+# or for Altera
+make spiOverJtag_<new-part>.rbf.gz
+```
+
+At the end of the tool execution, a new bitstream should be present in the
+current directory.
+
+> Note:
+>The directory `tmp_<part>/` (or `tmp_efinix_<part>/`) is the working
+>directory and may be removed after bitstream generation.