Skip to content

How-to: render a custom 96×160 frame

With both checksums cracked, a full still-image transfer can be forged end-to-end. Any client-side look (brightness, grayscale, etc.) must be baked into the pixels first. There is no device opcode for it (see Still images → attributes are client-side).

🔧 Steps

  1. Draw your content on a 96×160 canvas. Apply brightness/grayscale/etc. here.

  2. Convert each pixel to RGB565 big-endian:

    v = ((R>>3)<<11) | ((G>>2)<<5) | (B>>3);   bytes = [v>>8, v & 0xFF]

  3. Concatenate row-major → a 30,720-byte buffer (548 × 56-byte blocks + 1 × 32-byte tail).

  4. Announce (0x40):

    40 00 00 08 CF 02 00 A5 5A 10 00 01 C5 B1 01

    type [9]=0x10 (image); CRC [12,13]=C5 B1 = CRC16-MODBUS over [0x10,0x00,0x01].

  5. Settle ~300 ms (the module must process the announce).

  6. Setup / commit (0x41, PK_ADD_PIC):

    41 00 00 07 21 03 00 A5 5A 0C 78 00 C3 93     ; 0x7800 = 30,720 = 96×160

  7. Settle ~30 ms (the module must arm the ADD_PIC commit).

  8. Data blocks (0x41): 548 blocks of 56 bytes + 1 final 32-byte tail block, ACK-gated:

    • offset = running byte offset, little-endian in bytes[1,2] (0, 56, 112, … 30,688)
    • byte[3] = 0x38 (56) for the 548 blocks; 0x20 (32) for the tail
    • byte[6] = 0x00, then the payload bytes
    • checksum (bytes[4,5], 16-bit LE) = (0x41 + offLo + offHi + len + Σpayload) & 0xFFFF — compute after laying down the payload
    • wait for each block's byte[6]=0x55 ack before sending the next; resend ≤4× if missed

  9. Finish (0x42): 42 00 00 38 7A (rest zero-padded).

  10. Do NOT send a trailing view switch. PK_ADD_PIC already displays the committed frame and it stays; a trailing 0x0D (PK_TOGGLE_PIC) advances past it.

pad()

OS-level HID libs prepend a 0x00 report-ID byte, then zero-fill to 64 data bytes.

⚠️ Why the settles and ack-gating matter

Skip the settles and the frame lands in scratch, acks 549/549, and never displays (old picture stays). Blast the blocks with no flow control and dropped bytes flip pixel alignment → red/blue banding. Both are covered on Display commit (PK_*) and Chunking & pacing. This exact path already renders a live Spotify card — see Now-playing.