The LC prefix marks a low-voltage member of an EPROM family, and the 621 is its one-kilobyte 18-pin variant. There is no converter, no serial port and no data EEPROM: 1024 words of program memory, 80 bytes of RAM, 13 I/O, and a package that opens in an afternoon.
EPROM At Low Voltage Is Still EPROM
The medium decides the method. This part stores code in an erasable array, supplied either as a windowed ceramic device or as a plastic OTP, and the protection element sits outside the matrix as a fuse. Nothing about the low-voltage rating changes that.
What the low voltage changes is the design around it: a part specified to run at low supply is usually in a battery product, and a battery product is usually a device whose firmware is dominated by power management.
One Kilobyte Of Power Management
Sleep, wake, measure, transmit-less, sleep again. The image on a part like this is a sequence of peripheral shutdowns and restarts, and the interesting content is the ordering: which block is enabled before which, and how long the part waits for an oscillator to settle.
That ordering is what a reconstruction has to preserve. A rebuilt device that enables the converter before the reference has settled produces a first reading that is wrong by a fixed amount, and every subsequent reading is right — the fault is one sample wide and easy to miss.
The LP Oscillator Option
A low-voltage part is often clocked from a low-frequency crystal to keep the sleep current small, and that choice appears in the configuration word as an oscillator mode. A rebuilt part with the wrong mode runs at a completely different rate, and on a device whose whole purpose is a long timer, that is the difference between a week and a day.
We read the mode from the configuration word, confirm it against the timing in the firmware, and measure the actual frequency on a toggled pin.
Watchdog At Low Voltage
The watchdog on a low-voltage part is the last line of defence against a firmware hang, and its timeout is set by a prescaler that the code writes. When the supply sags, the watchdog is also the first thing to behave oddly.
On a rebuilt device we test the watchdog twice: once at nominal voltage and once at the low-battery threshold, allowing it to time out and observing the reset. A part whose watchdog does not fire at low voltage is a part that will hang in the field instead of recovering.
Reading A Windowed Part
If the donor arrives as a windowed ceramic package, the erasure history matters. A part that has been cleared and reprogrammed has been through at least one thermal cycle at the window, and the array may show the characteristic soft bits that come with repeated erasure.
We read such parts at two biases and report any block that changes.
When The Customer Has Only One Sample
A single OTP sample with no sister part means there is no independent reference for a doubtful block. In that case we read the block at three biases, compare against the surrounding addressing structure, and flag anything that cannot be corroborated.
Being explicit about which bytes are corroborated and which are not is more honest than delivering a clean-looking file with hidden uncertainty.
Thirteen Pins And No Converter
Two pins for power, one for programming and reset, and ten for the application. Without a converter the firmware has no analogue path to manage, which simplifies verification considerably: the rebuilt part is a digital device with a timing specification.
We measure the timing on the donors and on the rebuild, and we compare the wake-up latency specifically, because that is where the low-voltage design spends its care.
The Fuse, Photographed
Before the die is touched, the protect element is photographed. On an EPROM part it is a visible structure outside the array, and its state is evidence: a part that arrived protected and a part that arrived open are different jobs with different histories.
That photograph is delivered with the image.
Reading And Comparing
The array reads in two passes from 0000h to 03FFh, the configuration word at 2007h is decoded, the device ID is recorded. On a part this size we also compare the two passes word by word and re-probe any block that disagrees.
Delivery
Binary, HEX with configuration, a listing, the fuse photograph, the decoded configuration table, and the measured wake-up latency.
The 512-word version is the PIC16LC620 page, the 2K part in the same package is the PIC16C622A job, and the 18-pin part with a converter is the PIC16C710 page. Quotes go through the services desk.