LongTimekeeper Bottom

LongTimeKeeper

LongTimeKeeper is a drop-in replacement for the STMicroelectronics M48T58 / M48T58Y TIMEKEEPER® NVRAM (8 K × 8 battery-backed SRAM with real-time clock) in its 28-pin 600 mil PCDIP "CAPHAT" package.

The original PCDIP part has its lithium cell sealed in the package. When the cell runs flat, the chip loses both the clock and the SRAM contents, and the whole IC has to be replaced. LongTimeKeeper uses the surface-mount M48T58Y in SOH28 instead. That package normally takes an ST SNAPHAT® battery/crystal top. Here the SNAPHAT is replaced by:

• a discrete 32.768 kHz crystal, and
• a user-replaceable 1/2 AA 3.6 V lithium cell in a Keystone holder. The cell is a Tadiran SL-850/S from the SL-800 XOL (eXtended Operating Life) series, rated 1.2 Ah. This series has a very low self-discharge rate.

The cell has a much larger capacity than the original internal cell, and you can swap it without desoldering anything or losing the board's socketed footprint.

How it works

The adapter is a stack of two PCBs:

Signal mapping: every DIP-28 pin of the original package (A0–A12, DQ0–DQ7, Ē1, E2, Ḡ, W̄, FT, VCC, VSS) is routed 1:1 to the matching pin of the SOH28 device, so the host system sees an unmodified M48T58. Both boards are 2-layer, 1.6 mm FR-4, and fit within the original DIP-28 footprint (about 18.4 × 36.3 mm).

The projects were created with KiCad 10.

Bill of materials

Prices are indicative, VAT included (TTC, 20 % French VAT), for 1 unit, checked in October 2026 at DigiKey France, except where noted. Distributor prices change often, so check them before ordering.

¹ TME price (0.48 € excl. VAT, sold in packs of 10). ² Estimated. ³ Per board, from a batch of 5 at JLCPCB, with shipping and VAT included. See PCB cost estimate. ⁴ 1001Piles price (France, VAT included). Order the /S variant (standard terminals, no solder tabs) to fit the Keystone 108 holder. The /T variant has solder tabs and does not fit.

PCB cost estimate

Both boards are small (about 18.4 × 36.3 mm, 6.7 cm² each), 2-layer, 1.6 mm FR-4, and use standard rules. They qualify for the cheapest prototype offers. Prices are indicative and VAT included (20 %). The two designs are ordered as two separate jobs.

JLCPCB is the cheapest option, but the low price is mostly lost if you order only one module, because shipping is a fixed cost. AISLER is about four times more expensive per module, but it ships faster within the EU and charges no import fees. Use a quote calculator for an exact price: some options increase it, such as ENIG finish (flatter pads, which make the SOH28 easier to solder), a non-green soldermask or express shipping.

Notes on component choice

• Crystal: the schematic only specifies "32.768 kHz" on a 3215 footprint. The M48T58 datasheet does not give a load capacitance for the SNAPHAT crystal. Check the clock accuracy after assembly, and if needed try a lower-load (6–7 pF) 3215 crystal.
• Battery: use the Tadiran SL-850/S long-life cell (SL-800 series), not a standard-series cell such as the TL-5902. Its very low self-discharge rate suits a back-up that only draws a few µA for many years. The footprint and 3D model use the 1/2 AA (ER14250) size. Use a 3.6 V primary lithium cell. Do not use a rechargeable 3.7 V Li-ion 14250 cell, because its voltage is too high for the VBAT input.
• DIP legs: standard square header pins match the 3D model. Turned (machined) pins are gentler on the original socket.

Battery life estimate

The cell supplies the M48T58Y only while the host is switched off (VCC below the switchover voltage V_SO). When the host is on, the chip runs from VCC and draws no current from the cell.

Inputs

The calculation follows the AN1012 formula, with the cell's self-discharge added to the battery current:

life (years) = capacity ÷ (8 760 h × (1 − VCC duty cycle) × I_BAT + yearly self-discharge)

With a VCC duty cycle of 0 % (host always off), the M48T58Y draws 537 nA × 8 760 h ≈ 4.7 mAh/year at 20 °C and 1 130 nA × 8 760 h ≈ 9.9 mAh/year at 70 °C.

Estimate (host always off, 100 % battery backup)

¹ The self-discharge figures are specified at room temperature. At 70 °C the cell's real self-discharge is higher, so this row overestimates the life.

Conclusion. Even with the host always off, the calculated life is far beyond the cell's rated life. The practical limit is therefore the cell's rated life, not its capacity: Tadiran rates the XOL series for up to 40 years of operation. Expect about 40 years at room temperature. For comparison, AN1012 (Table 13) gives the original M48T58 with its internal 48 mAh cell about 9.4 years (typical) at 20 °C and 4.8 years at 70 °C when the host is always off.

Factors that shorten this life:

• Temperature. Both the M48T58Y current and the cell's self-discharge rise with temperature, for example inside a closed, warm cabinet.
• Leakage. Flux residue or humidity on the board can create leakage paths. Clean the boards after soldering.

Monitoring. At each power-up, the M48T58Y sets its battery low flag (BL, bit D6 of register 1FFDh) when the cell is below about 2.5 V. Li-SOCl₂ cells keep a flat voltage of about 3.6 V almost until they are empty, so treat this flag as the signal to replace the SL-850.

Assembly hints

1. Solder Y1 and U7 (SOH28) on the bottom board. Do not wave-solder the SOH28 package (see the ST datasheet).
2. Fit the two 1 × 14 headers on the bottom board, with the long pins pointing down, to form the DIP-28 legs.
3. Solder the Keystone 108 holder on the Up board, then the 4 link pins. Stack the Up board on the bottom board and solder the pins.
4. Insert the SL-850/S cell with the polarity marked on the holder, then snap on the 108C cover.
5. Plug the module into the original M48T58 socket, with pin 1 at the notched end.

Datasheets

• M48T58 / M48T58Y – STMicroelectronics
• AN1012 – Predicting the battery life and data retention period of NVRAMs and serial RTCs – STMicroelectronics
• Keystone 108 battery holder
• Keystone 108C cover
• Tadiran SL-850 (XOL – extended operating life). This page is taken from the Tadiran Lithium Batteries Product Data Catalogue and also shows the SL-861.
• Tadiran TL-5902. This is the standard-series datasheet, kept for reference.

License

Copyright © 2026 Guimli.

This hardware design is licensed under the CERN Open Hardware Licence Version 2 – Strongly Reciprocal (CERN-OHL-S-2.0).

You may redistribute and modify this source and make products using it under the terms of the CERN-OHL-S v2. This source is distributed WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY, SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE. See the CERN-OHL-S v2 for applicable conditions.

Third-party documents in Datasheet/ and 3D models from manufacturers remain the property of their respective owners and are not covered by this licence.