pub const PACK_DIR_0: u32 = 0;
Pack from LSB first. For image data, 0x11, 0x22, 0x33, 0x44, it will appear as 0x44332211 in RX FIFO. For stat data, 0xAAAA, 0xBBBB, it will appear as 0xBBBBAAAA in STAT FIFO.