Two markings that differ by one character can mean a different package outline and nothing else. The AT32F421K8U7 and the AT32F421K8U7-4 carry the same 64 KB of flash, the same 16 KB of SRAM and the same 120 MHz core; the first is a 5 mm QFN32 and the second a 4 mm QFN32. When a board is in front of us for a read, that distinction decides how the part is held, how it is decapsulated and, occasionally, whether it survives.
Reading The Suffix
Artery’s part numbering is regular enough to decode without a datasheet once the pattern is familiar.
| Field | Meaning here | Note |
|---|---|---|
| K | QFN32 package | C is 48-pin, R is 64-pin, V is 100-pin |
| 8 | 64 KB flash | 4 is 16 KB, 6 is 32 KB |
| U | QFN body | T is LQFP, P is TSSOP |
| 7 | -40 to +105 °C | industrial range, not automotive |
| -4 | 4 x 4 mm | absent means 5 x 5 mm on this family |
The suffix rule catches people out because the plain U7 appears twice in the family table for different body sizes. On the F421 the 5 mm outline is the default and the 4 mm part is the exception, marked with the dash.
Twenty-Seven Pins, Eleven Channels
Inside a 32-pin leadless body the part still has two USARTs, two SPI/I²S blocks, two I²C buses, one advanced timer with three complementary pairs, five general-purpose timers, a comparator and a twelve-bit converter with eleven external channels. Compared with the 48-pin member, the difference is four ADC inputs and twelve GPIO. Nothing in the memory system changes.
That is worth stating plainly for customers comparing quotes: the K8U7 is not a cut-down die, it is the same die with fewer bonds. The protection layout, the sLib granularity and the boot block are identical.
What A Leadless Body Changes
A QFN has no leads to clip. Every connection to the die has to be made through the pad array underneath, which means the board has to be removed first and the part re-mounted on a carrier before any in-system attempt is meaningful. On a 5 mm body the pads are also smaller than on the 4 mm part, so the carrier design has less margin.
Once the part is on a carrier, the read follows the usual order: protection bytes, unique ID, array, comparison. The smaller body makes decapsulation slightly easier because there is less moulding compound to remove, and slightly riskier because the die occupies a larger fraction of the package.
sLib On A 64 KB Array
The library region is set in 1 KB steps here, and as everywhere in the family it cannot include sector zero. With sixty-four sectors total, a design that wants to protect a core algorithm typically locks the upper half and leaves the lower half for the customer’s application. That split is visible from outside: the readable portion ends at a 1 KB boundary, and everything above it returns 0xFF to a data read while still executing normally.
The four-kilobyte floor applies as well. Whatever is protected, the first four kilobytes remain legible, and we use them to identify the compiler, the startup file and the vector layout before trusting anything else.
Failure Modes We See On This Package
Three recur. The first is mechanical: a pad lifted during removal, which is why we ask for a spare board whenever one exists. The second is a supply that is set from the datasheet’s minimum without accounting for the drop across a carrier, which makes the debug logic unresponsive and looks exactly like permanent protection. The third is a genuinely permanent byte — the advanced access protection mode offered on this series disables the debug interface for good.
The three look identical from a tool prompt and completely different under a microscope, so the diagnosis is physical before it is electrical.
Return Format
A binary image with addresses, a sector map showing the library boundary, and a short statement of what could not be recovered. If the part had been configured with the boot block as user memory, we note the extra 4 KB and deliver the image at its full length.
The 48-pin version is described under copying a locked AT32F421C8T7 heximal. For the 48-pin part in the F425 line, where the same body size carries a USB controller, see extracting locked AT32F425CCT7 software. Questions about a specific board go to our contact page.