Module latched 

Bitplane framebuffer for an 8-bit latched HUB75 interface.

This module provides a framebuffer that stores colour data as separate bit-planes rather than the threshold-based frames used by crate::latched::DmaFrameBuffer. Each plane holds one bit of every colour channel, giving PLANES planes total (typically 8 for full 8-bit colour). Row addressing is carried by four trailing Address bytes per row, identical to the non-bitplane latched layout.

Hardware Requirements

Requires a parallel output peripheral capable of clocking 8 bits at a time, plus an external latch circuit to hold the row address and gate the pixel clock (same circuit as the non-bitplane latched variant).

HUB75 Signal Bit Mapping (8-bit words)

Two distinct 8-bit words are streamed to the panel:

1. Address / Timing (Address) – row-select and latch control.
2. Pixel Data (Entry) – RGB bits for two sub-pixels plus OE/LAT shadow bits.

Address word (row select & timing)
┌──7─┬──6──┬─5─-┬─4─-┬─3-─┬─2-─┬─1-─┬─0-─┐
│ OE │ LAT │    │ E  │ D  │ C  │ B  │ A  │
└────┴─────┴───-┴───-┴───-┴───-┴───-┴───-┘

Entry word (pixel data)
┌──7─┬──6──┬─5──┬─4──┬─3──┬─2──┬─1──┬─0──┐
│ OE │ LAT │ B2 │ G2 │ R2 │ B1 │ G1 │ R1 │
└────┴─────┴────┴────┴────┴────┴────┴────┘

Bits 7-6 (OE/LAT) occupy the same positions in both words so the control lines stay valid throughout the DMA stream.

Bitplane BCM Rendering

The framebuffer is organised into PLANES bit-planes. Plane 0 carries the MSB (bit 7) of each colour channel, plane 1 carries bit 6, and so on down to plane 7 which carries the LSB (bit 0).

To produce correct brightness via Binary Code Modulation, configure the DMA descriptor chain so that each plane’s data is output (scanned) a number of times equal to its bit-weight:

plane 0 (bit 7) → output 2^7 = 128 times
plane 1 (bit 6) → output 2^6 =  64 times
plane 2 (bit 5) → output 2^5 =  32 times
  …
plane 7 (bit 0) → output 2^0 =   1 time

That is, each plane is scanned 2^(7 - plane_index) times. The weighted repetition counts sum to 255, reproducing the full 8-bit intensity range. See https://www.batsocks.co.uk/readme/art_bcm_1.htm for background on BCM.

Memory Usage

Memory scales linearly with PLANES: the buffer contains PLANES copies of the row data (one per bit-plane). Unlike the threshold-based crate::latched::DmaFrameBuffer whose frame count grows as 2^BITS - 1, this layout uses exactly PLANES planes regardless of colour depth.

Each row is COLS data bytes plus 4 address bytes, so total size is PLANES * NROWS * (COLS + 4) bytes.

Structs

DmaFrameBuffer

The entire BCM Frame Buffer (Contiguous Memory)

Row

A single BCM row payload for 8-bit latched output.