Markus Zehnder
100 lines
3.4 KiB
Markdown
100 lines
3.4 KiB
Markdown
# LCD Protocol
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This page contains the current state of the reverse engineered AOOSTAR display protocol.
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See [Linux shell commands](shell_commands.md) how you can switch the display on and off with standard Linux commands.
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- **Resolution:** 960 × 376
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- **Manufacturer:** Synwit
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- **Connected over USB UART** with a proprietary serial communication protocol:
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- **USB device ID:** `416:90A1` (as shown by `lsusb`)
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- **Linux device (example on Debian):** `/dev/ttyACM0`
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- **1,500,000 baud**, 8N1 (likely ignored; actual USB transfer speed is much higher)
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## Display Off
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**Request:**
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<img src="img/lcd_off.png" alt="LCD off EBNF" width="485" height="70">
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<details>
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```
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@startebnf lcd_off
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lcd_off = 0xAA, 0x55, 0xAA, 0x55, 0x0A, 0x00, 0x00, 0x00 ;
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@endebnf
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```
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</details>
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**Response:**
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- Success: character `A`
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- Error: _unknown_
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## Display On
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**Request:**
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<img src="img/lcd_on.png" alt="LCD on EBNF" width="485" height="70">
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<details>
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```
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@startebnf lcd_on
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lcd_on = 0xAA, 0x55, 0xAA, 0x55, 0x0B, 0x00, 0x00, 0x00 ;
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@endebnf
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```
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</details>
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**Response:**
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- Success: character `A`
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- Error: _unknown_
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Note:
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- When switching the display on, the last displayed image is immediately shown.
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## Display Image
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**Request:**
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<img src="img/send_image.png" alt="Send image EBNF" width="868" height="423">
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<details>
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```
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@startebnf send_image
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send_image = img_cmd_start, { data_chunk }, img_cmd_end ;
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img_cmd_start = 0xAA, 0x55, 0xAA, 0x55, 0x05, 0x00, 0x00, 0x00, 0x04, 0x00, 0x0F, 0x2F, 0x00, 0x04, 0x0B, 0x00 ;
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data_chunk = chunk_header, chunk_offset, rgb565_chunk ;
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chunk_header = 0xAA, 0x55, 0xAA, 0x55, 0x08, 0x00, 0x00, 0x00 ;
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chunk_offset = ? 32 bit offset in little-endian format ? ;
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rgb565_chunk = 47 * ? byte image data in RGB565 format from given index ?;
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img_cmd_end = 0xAA, 0x55, 0xAA, 0x55, 0x06, 0x00, 0x00, 0x00 ;
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@endebnf
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```
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</details>
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**Response:**
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- Success: character `A`
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- Error: _unknown_
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Notes:
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- When sending an image to the screen, the image must be in **RGB565** format (16 bits per pixel).
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- `asterctl` performs all graphic operations on an RGB888 image buffer.
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- `asterctl` automatically converts the image to RGB565 when sending it to the display.
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- The 1.5 Mbps baud rate set in the client is ignored, as actual USB bulk transfer achieves much higher throughput.
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For reference, at the nominal serial rate (~1,500,000 baud), it would take approximately 6 seconds to transfer a full image of 721,920 bytes (960 × 376 × 2):
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- Display protocol: payload per chunk = 47 bytes; header per chunk = 12 bytes
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- Number of chunks: 721,920 / 47 ≈ 15,360 chunks
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- Total transmitted data: 15,360 chunks × 59 bytes/chunk = 906,240 bytes
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- Serial frame format: 1 start bit + 8 data bits + 1 stop bit = 10 bits/byte
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- Effective byte rate: 1,500,000 bits/sec / 10 bits/byte = 150,000 bytes/sec
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- Transfer time: 906,240 bytes / 150,000 bytes/sec ≈ 6 seconds
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- **Performance:**
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- Displaying the first fullscreen image takes around 1.3 seconds.
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- Once the new image is fully transferred and the end-header command is sent, the display firmware switches to the new image.
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- **Partial Updates:**
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- `asterctl` uses a frame cache to send only changed chunks after the initial image is displayed, greatly speeding up partial screen updates.
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- The chunk size is 47 bytes, determined from the original app. It is unknown if other chunk sizes are supported.
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- There are no fractional chunks: 960x376 x 2 bytes/pixel / 47 bytes/chunk = 15360 chunks
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