DS1305 end-of-life: candidate replacements and what to verify

Part: Maxim DS1305 (now Analog Devices), serial alarm real-time clock with NV SRAM and trickle charger.
Status: Distributor-flagged Obsolete at DigiKey (no longer manufactured); only remnant and authorized-aftermarket supply left. (Confirmed 2026-06-25; re-check live stock the day you order.) Source: DigiKey DS1305

What it is

The DS1305 is a jellybean serial RTC that has quietly sat on mature industrial, instrumentation, and embedded boards for two decades. It keeps time and date, carries 96 bytes of NV RAM, runs two programmable time-of-day alarms, and includes a programmable trickle charger plus a VBAT backup input. (Specs confirmed against the ADI datasheet.)

Why it matters

This is exactly where silent obsolescence bites. An RTC is rarely the part anyone worries about, right up until it goes zero-stock and the reorder bounces. And the DS1305 carries a trap that cross-reference tools gloss over: it is an SPI/3-wire part (the SERMODE pin selects the mode). The modern RTCs the forums point you to (DS3231, DS1307, PCF8523) are I2C. That is not a footprint difference you patch. It is a different bus: new pinout, new firmware driver, a PCB change, and a requalification.

Suggested replacements (validate in YOUR system, not pound-for-pound)

These are candidates to evaluate, not drop-ins. Form-fit-function depends on your design: the bus, the register map your firmware assumes, the board, and the qualification envelope. The same part can pass in one system and fail in another. Validate each in your own design.

The honest call

If the design is frozen around the DS1305's SPI/3-wire interface and register map, the cheapest real path is usually an authorized continuation or a last-time-buy that covers remaining build plus spares, not a modern I2C RTC that quietly forces a firmware and PCB change. Only reach for a TCXO part like the DS3234 on SPI if a redesign is already on the table. Treat any of these as a requalification, not a substitution.

Before you buy

• Confirm DS1305 lifecycle status and remaining stock the day you order (DigiKey, Mouser, ADI). Status moves.
• Confirm your firmware's assumed interface (3-wire vs SPI) and register map against the candidate.
• Confirm pinout and package on the exact ordering suffix (16-pin DIP or 20-pin TSSOP; no SO package exists).
• Check the SERMODE pin state on your board (GND = 3-wire mode, VCC = SPI mode).
• Confirm the candidate's lifecycle; do not migrate to the next part that is about to go obsolete.

Where to look

Distributor lifecycle pages for remaining stock and any last-time-buy terms; authorized aftermarket (Rochester Electronics) for an authentic continuation when requalification is the expensive path.