Decode, or no decode

The same byte, two meanings

Switch the decode bit and watch what the eight cells are labelled. That is the whole feature: the byte is unchanged and what the chip does with it is not.

Decoder on. The chip reads the low four bits as a character and looks it up in its own font, called code B. Bits 4 to 6 are ignored entirely. You get sixteen characters:

0 1 2 3 4 5 6 7 8 9 - E H L P and a blank.

That is enough for a clock, a counter, a voltage, and - for out of range. It is also why Error on a seven-segment display is so often spelled with E, H, L and P in it — those four are the letters the decoder happens to have.

Decoder off. The eight bits are the eight segment lines, one each, and all 128 patterns are available. A, b, C, d, F, n, o, r, t, u, degree signs, bare bars, half a box. The cost is that you now own the font.

The bit order

This is the part that catches people, and it is worth stating flatly:

On a MAX7219, bit 7 is the decimal point, bit 6 is segment A, and it counts down to segment G at bit 0.

Almost every other seven-segment part counts the other way — segment A at bit 0, climbing to the point at bit 7. The TM1637 display in the same shop does. Every 74HC595 tutorial does. So a segment table copied from one of those into a MAX7219 sketch compiles, uploads, and draws the reflection of what you wanted.

There is no warning because there is nothing wrong: the byte is valid, the chip lights the segments it names, and they are the wrong segments. Symmetric characters — 0, 1, 8 — survive the mirroring and look correct, which is what makes the fault confusing to diagnose. Press the wrong-order button in the figure above and step through the characters to see which ones give the game away.

Per digit, not per chip

The decode-mode register is one byte at address 0x09, and each bit is one digit. So the modes can be mixed:

// Decoder on for the low four digits, off for the top four.
lc.setScanLimit(0, 7);
sendRaw(0x09, 0x0F);   // 0b00001111

0x0F is the useful one. It gives you four decoded number digits and four raw ones, which is how a unit or a symbol goes beside a reading on the same display — 23.5 in the decoder's font and a degree sign drawn by hand.

LedControl does not expose the decode-mode register directly. setDigit and setChar go through the decoder; setRow writes a raw byte and quietly turns that digit's decoding off first. If you want the mixed arrangement above, setRow on the digits you want raw and setDigit on the rest gets you there without touching 0x09 yourself.

Which to use

Use the decoder. It is smaller, faster to write, immune to the bit-order trap, and it is what the chip is for.

Turn it off for the specific digit that needs a shape the font does not have, and leave the rest alone. A whole display in raw mode is a font you are maintaining for no reason.