P24C512-TOH-MKR in a Three-Tier EEPROM Plan for a Medical Infusion Pump

A volumetric infusion pump carries four kinds of data that look alike on a BOM line and behave nothing alike in service. Calibration constants change once at the factory. Therapy settings change when a nurse edits a protocol. The drug library changes when the pharmacy releases a new formulary. The event log grows every time the pump is used and must survive a battery swap on a ward trolley.

Those four payloads do not need the same memory, and treating them as one requirement is how pumps end up with a part too small for the formulary or oversized for headroom nobody uses. The PUYA I2C EEPROM line covers the range from 2 Kbit to 1,024 Kbit with the same command set, which makes the choice a tier decision rather than a vendor decision.

The P24C512-TOH-MKR sits in the third tier: 512 Kbit arranged as 64 Kbytes in 128 byte pages, on an I2C bus at 400 kHz or 1 MHz from 1.7 V to 5.5 V, rated from -40 °C to +105 °C in the K grade (p. 1, p. 6, p. 16). The call is simple. For a pump that keeps the formulary and the audit trail in one device, this is a clean fit; for a pump that logs raw pressure waveforms, it is not the right answer, and the tier above exists for that case.

Three tiers inside one product line

The line splits by array size and, less obviously, by page size. Page size decides whether a log record fits one transaction, and it is the number most often left out of a comparison table.

The tier boundaries are not arbitrary. Tier one exists because a motor trim table is a few dozen words, and a 2 Kbit part next to the driver is cheaper than routing the calibration bus back to the main board. Tier two exists because 32 byte pages hold a settings block yet lose only that block when a write is interrupted. Tier three exists because the formulary plus the log do not fit in tier two, and because a 128 byte page matches one library entry written by the pharmacy tool.

Above tier three the question changes. The 1,024 Kbit part keeps the same 256 byte page discipline and the same two byte word address, so nothing in the protocol changes, but the array is twice the size and no pump in this class fills it with formatted text.

What each tier buys you

The 2 Kbit part stores 256 bytes with 8 byte pages. The page boundary bites: a sixteen byte calibration block spans two pages, and a page write that runs past it wraps to the start of the same page rather than advancing, so the driver must split the transfer. It comes in the smallest bodies in the line, including a five-lead package, and carries 4 kV human body model ESD rather than the 6 kV of the larger members (its own datasheet).

The 64 Kbit part stores 8 Kbytes with 32 byte pages and adds a 128 bit serial number in a separate writable area, which is what a consumable identification scheme actually needs (its own datasheet). A pump that keeps only settings and a summary of recent therapies can live here, and the smaller array keeps a full dump short.

The 512 Kbit part stores 64 Kbytes with 128 byte pages, and the page size is the reason it fits the third tier. One library entry, or one log record with its header, lands inside a single page write, so the write either happens or does not. The part also carries a second page of 128 bytes that can be written once and then locked permanently in read-only mode: the lock instruction uses device type 1011b with address bit A10 set, and afterwards the device answers NoACK to any further write to that page (p. 13). A status read tells firmware whether the lock is already set (p. 15). That page is where the pump's identity belongs: model, serial, build date, calibration hash.

The 1,024 Kbit part stores 128 Kbytes with 256 byte pages (its own datasheet). The extra room is real, but a 256 byte page only pays off when records are large or when the log is written as a raw stream.

Two electrical facts decide the board work around the memory. The first is that the bus needs its pull-ups and nothing else: SDA is open drain and may be wired-OR with other devices, while the address pins and the write control pin are internally pulled down, so a floating pin is a defined logic low and no resistor is needed on them (p. 2, p. 3). The AC table characterises the bus with a 1.3 kΩ pull-up at 2.5 V and at 5.5 V, and 10 kΩ at 1.7 V. They are test conditions rather than recommendations, but they bracket the timing table (p. 6). The datasheet carries no application circuit, so VCC decoupling is the only other external part.

The second is standby current, which matters more than any of the above in a battery-powered pump: 1.0 µA at 3.3 V and 85 °C, 6.0 µA at 5.5 V and 105 °C, and 10 µA at 5.5 V and 125 °C (p. 5). Read current is 0.5 mA and write current 2.5 mA at 400 kHz (p. 5), so average draw depends on how often the log is flushed.

Why this part sits where it sits

The pump's memory map, drawn as pages rather than one blob, makes the choice obvious. The 64 Kbyte array is 512 pages of 128 bytes. The first pages hold a working copy of the identity. The next block holds the formulary, sized by the pharmacy tool rather than by the firmware team. The rest is a rotating log, written to the next free page and wrapped at the end of the region.

That layout is only safe because of two datasheet facts. A page write never advances past its page: if more than 128 bytes are clocked in, the address rolls over to the start of the same page and earlier data is overwritten (p. 12). And the part ends a write on its own internal timer, 5 ms maximum, refusing all input until it finishes (p. 1, p. 6). Together they mean a record cannot silently corrupt its neighbour, which is the property a regulated log needs.

Endurance decides whether the log can rotate or has to be clever. The array is rated for 1,000,000 write cycles per cell in page mode at 25 °C and 3.3 V, with retention of 100 years at 55 °C (p. 6). A pump that writes one record per therapy event never approaches that per cell if it rotates across the whole log region instead of rewriting page zero. A pump that writes to the same address every minute wears that page out inside a normal service life. The difference is a pointer, not a part number.

Where tier three stops being the right answer deserves stating plainly. If the formulary lives in the MCU's flash and the EEPROM only carries settings, the 64 Kbit tier is enough and cheaper in board area. If the log must hold raw pressure waveforms, 64 Kbytes fills faster than the service interval, and the 1,024 Kbit tier or a different memory class is the honest answer. If the identity must be unforgeable rather than merely read-only, a lockable page is a deterrent, not a cryptographic guarantee.

One caution belongs here. The datasheet carries the standard notice that PUYA assumes no responsibility for use of its products for any particular purpose (p. 22). The part is not intended to stand in for a medical qualification; the validation file belongs to the pump maker, not to the device.

The upgrade path when requirements move

Upward, the move is to the 1,024 Kbit members, the P24CM01 and its B revision, both documented as 1,024 Kbit with 256 byte pages (their own datasheet). The command set, the two byte word address and the device address format are unchanged, so the driver change is the page size constant and the array bound. The page-size change needs a test: a log writer tuned for 128 byte pages splits records differently at 256 bytes, and a wrap test at the region end is the only way to catch it.

Downward, the move is to the 64 Kbit P24C64C, whose 32 byte pages force a log record to be split or shortened (its own datasheet). That is a firmware change with a real consequence: an interrupted multi-page write leaves a partial record, so the record format needs a length and a checksum in its first bytes.

Sideways within the line, the 2 Kbit P24C02A covers the small payloads, and the ordering code decides the body and the temperature grade: the TO code denotes a five-lead TSOT23-5 body and the K grade spans -40 °C to +105 °C, while the E grade extends to +125 °C (p. 16 of its datasheet). The same grades and the same tape and reel option apply across the line, which lets one purchasing line cover all four tiers.

Two ordering details should be settled before the BOM freezes. The grade letter is a temperature promise rather than a label: K means -40 °C to +105 °C and E means -40 °C to +125 °C (p. 16). The shipping option matters on a line that places thousands of units a week, because R is tape and reel while T is tube (p. 16).

Frequently asked questions

Sourcing note

The P24C512 is PUYA silicon. The ordering code carries the operating voltage, the plating, the package, the density, the grade and the carrier: M is 1.7 V to 5.5 V, H is RoHS with no halogen and no antimony, K is -40 °C to +105 °C, and R is tape and reel (p. 16). The Rev 1.5 legend on that page lists PDIP8, SOP8, TSSOP8, DFN8 and UDFN8 as the bodies covered by the revision, so send us the suffix you intend to freeze and we will confirm the body against the current revision before you release the BOM.

Two sourcing questions come up on every pump programme. On lifecycle, ask us for the PCN and EOL policy in writing against the volume you forecast, because a pump that ships for ten years needs a statement, not an assurance. On second source, the four tiers share a protocol but not a footprint, so they are a design escape rather than a true alternative; if your quality file demands an equivalent from a second line, say so at RFQ and we will quote one against the same specification.