Differences

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--- fpgapad [2025/01/22 14:52] – created arisotura
+++ fpgapad [2025/03/19 22:59] (current) – arisotura
@@ Line 20: / Line 20: @@
 Thus, the FPGApad was born.
-It proved to be a very convenient way to upload code to the gamepad: I can do so from my computer over USB, and I don't have to worry about Flash write cycles, since the FPGA uses SDRAM. It also proved invaluable as a debug output: I could simply send data over the SPI bus and have the FPGA forward them to my computer. This finally allowed me to get my LCD init sequence working, and even more.
+It proved to be a very convenient way to upload code to the gamepad: I can do so from my computer over USB, and I don't have to worry about Flash write cycles, since the FPGA uses SDRAM. It also proved invaluable as a debug output: I could simply send data over the SPI bus and have the FPGA forward it to my computer. This finally allowed me to get my LCD init sequence working, and even more.
@@ Line 34: / Line 34: @@
 [[https://github.com/Arisotura/spi_flash|Flash emulator codebase]]
-The codebase is a bit of a mess. I had started adding support for write commands, but there are still issues with it as of now.
+The codebase is a bit of a mess.
 (TODO: add the rest)
+===== FPGA/computer interface =====
+The FPGA communicates with the computer over USB (on the US1 port of the ULX3S). The FPGA exposes a serial interface running at 3 megabauds, 8 bits, no parity, 1 stop bit.
+The communication protocol is binary: send one command byte, send required address/length bytes if any, then send or receive data bytes.
+The following commands are supported:
+^ Command ^ Description ^
+| 0x30    | Get version |
+| 0x31    | Read memory |
+| 0x32    | Write memory |
+**Command 0x30**
+Returns the current protocol version (0x01).
+**Command 0x31**
+Reads data from SDRAM.
+This command takes 4 parameter bytes: 3 for the address (MSb's first), 1 for the length to read. Address and length are expressed in 8-byte chunks.
+After the parameter bytes are sent, data from the requested memory address is returned.
+**Command 0x32**
+Write data to SDRAM.
+This command takes 4 parameter bytes: 3 for the address (MSb's first), 1 for the length to write. Address and length are expressed in 8-byte chunks.
+After the parameter bytes are sent, data bytes should be sent to be written to memory. A 0x01 byte is returned when the write has completed.
+**Debug output**
+When the gamepad runs, the FPGA also sends the following data to the computer:
+  * SPI command bytes, and address bytes for commands that take addresses
+  * All SPI bytes received after SPI command 0xF2
+  * Byte 0xE2 when gamepad Vcc drops to zero, to indicate a reset
+===== FPGA/gamepad interface =====
+The FPGA connects to the gamepad's SPI bus, where it emulates Flash memory.
+The following SPI commands are currently supported:
+^ Command ^ Description ^
+| 0x02    | Page program |
+| 0x03    | Read |
+| 0x04    | Write disable |
+| 0x05    | Read status (basic status bits supported) |
+| 0x06    | Write enable |
+| 0x20    | Subsector erase |
+| 0x9E/0x9F | Read chip ID (fixed 20,BA,19,00,00,...) |
+| 0xB7    | Enter 4-byte address mode |
+| 0xD8    | Sector erase |
+| 0xE9    | Leave 4-byte address mode |
+| 0xF2    | Debug |
+Command 0xF2 is an addition to the normal Flash command set. It allows to abuse the SPI bus as a debug output.
+When receiving command 0xF2, the FPGA simply forwards all subsequent bytes to the computer until the SPI chipselect line goes high.
+The data sent via command 0xF2 follows a simple format: a simple 16-bit header is sent MSB-first after the command byte to indicate how the subsequent data should be parsed.
+^ Bits ^ Description ^
+| 0-13 | Data length in bytes |
+| 14-15 | Data handling; 0=hexdump, 1=file, 2=string, 3=string |
+Data handling value 1 means the data should be dumped to a binary file. Other values mean it should be displayed in the terminal, as a hex dump or directly as a string.