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Analog Devices Inc./Maxim Integrated DS1688S
- Part No.:
- DS1688S
- Manufacturer:
- Analog Devices Inc./Maxim Integrated
- Category:
- Real Time Clocks
- Package:
- 28-SOIC (0.337", 8.56mm Width)
- Datasheet:
-
DS1688S.pdf
- Description:
- IC RTC CLK/CALENDAR PAR 28SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:4,441
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Product details
Overview
DS1688S from Maxim Integrated is a 28-pin SOIC real-time clock (RTC) with integrated NV SRAM control, +3V/+5V dual-supply operation, 64-bit silicon serial number, and battery-backed timekeeping up to 2100 with leap-year compensation. It supports external SRAM backup via VCCO/CEO, provides 32kHz output, and includes power-on elapsed time and power-cycle counters for embedded system maintenance tracking.
For engineers reviewing the DS1688S datasheet, DS1688S pinout, DS1688S application, or DS1688S equivalent, key selection considerations include its dual-voltage RTC+SRAM controller role, 114-byte user NV RAM, PWR interrupt for wake-from-power-off, SQW timing output configuration, and compatibility with legacy DS1287 BIOS implementations.
Technical Context
The DS1688S implements a fully autonomous RTC core with calendar, alarm, century register, and two independent elapsed time counters-one VCC-powered and one VBAT-powered-enabling precise uptime and battery-life monitoring. Its serial interface uses multiplexed AD0–AD7 with ALE/CS/WR/RD control, matching Intel 8080-style bus timing.
Power management features include software-controllable PWR output for system power-on via date/time alarm or KS input, automatic SRAM write protection during VCC transients via CEO assertion, and crystal select bit supporting 6pF or 12.5pF load capacitance crystals for stable oscillation across temperature.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Supply Voltage | +3.0V to +5.0V - enables direct integration into both modern low-voltage and legacy 5V systems without level-shifting. |
| Timekeeping Accuracy | ±2ppm typical at 25°C - ensures <1 second per day drift with 32.768kHz crystal, validated through 2100 with leap-year algorithm. |
| User NV SRAM | 114 bytes - nonvolatile storage retained by VBAT during main power loss, accessible via AD0–AD7 bus without external EEPROM. |
| Serial Number | 64-bit unique silicon ID - factory-programmed for secure device identification in networked or field-deployed equipment. |
| Elapsed Time Counters | Two 32-bit counters: one VCC-powered (system uptime), one VBAT-powered (total battery runtime) - supports warranty analytics and predictive maintenance. |
| Square-Wave Output | 32kHz on SQW pin - configurable via DV1/SQWE/E32K bits; used for power management clocks or microcontroller timer reference. |
| Crystal Load Capacitance | Selectable 6pF or 12.5pF - accommodates common tuning fork crystals without external load capacitor changes. |
Pinout & Package
DS1688S is housed in a 28-pin Small Outline (SO) package, 0.330″ wide, RoHS-compliant (DS1688S+ variant), with standard JEDEC MS-012AC footprint and thermal pad omitted.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| VBAUX (Pin 1) | Auxiliary battery supply | Optional secondary battery input for enhanced backup redundancy; not required if primary VBAT suffices. |
| X1/X2 (Pins 2,3) | Crystal oscillator terminals | Connects to 32.768kHz tuning fork crystal; internal load caps selectable for 6pF or 12.5pF operation. |
| RCLR (Pin 4) | Active-low RAM clear | Hardware reset of 114-byte NV SRAM; asserted low clears all user memory without affecting time/calendar registers. |
| AD0–AD7 (Pins 5–12) | Multiplexed address/data bus | 8-bit bidirectional bus sharing address and data; requires ALE strobe for demultiplexing in 8080-compatible mode. |
| PWR (Pin 13) | Active-low power-on interrupt | Drives external power supply enable; triggered by date/time alarm or KS input; software-controllable for graceful shutdown. |
| GND (Pins 14,19) | Ground reference | Dual ground pins reduce noise coupling between RTC core and SRAM interface sections. |
| KS (Pin 15) | Active-low kickstart input | Hardware wake-up trigger (e.g., keyboard press); initiates PWR assertion and system power-on sequence. |
| CS/WR/RD/ALE (Pins 16,18,20,17) | Bus control signals | Standard 8080-style control set enabling direct connection to microcontroller parallel interfaces without glue logic. |
| PSEL (Pin 21) | Power select input | Configures internal regulators for +3V or +5V operation; tied high for 5V, low for 3V mode. |
| IRQ (Pin 22) | Active-low interrupt request | Asserts on alarm match or elapsed counter overflow; shares function with DS1287 IRQ pin for BIOS compatibility. |
| SQW (Pin 24) | Square-wave output | 32kHz clock output enabled by DV1/SQWE/E32K bits; powers external timers or low-power MCU sleep modes. |
| VCCO (Pin 25) | RAM power-supply output | Switched VCC-derived voltage to external SRAM; automatically transitions to VBAT during main power loss. |
| VCCI (Pin 26) | Main supply input | +3V or +5V primary power; powers RTC core, counters, and interface logic during normal operation. |
| CEO (Pin 27) | Active-low RAM chip enable out | Controls external SRAM write enable; asserted during VCC transients to prevent data corruption. |
| CEI (Pin 28) | Active-low RAM chip enable in | Accepts external CE signal to gate DS1688S access to SRAM; enables shared-memory configurations. |
Key Features
| Feature | Design Value |
|---|---|
| DS1287 BIOS compatibility | Direct replacement for legacy PC RTCs in existing firmware; no BIOS modification required for basic time/calendar functions. |
| Automatic SRAM backup control | VCCO/CEO coordination ensures zero-data-loss transition to battery power during VCC dropout, eliminating need for external supervisor ICs. |
| Software-controllable PWR output | Enables full system power sequencing - wake on alarm, execute task, then shut down - reducing standby power in headless embedded devices. |
| Factory-programmed 64-bit serial number | Provides immutable hardware identity for firmware licensing, field failure correlation, and secure boot chain binding. |
| Two independent elapsed time counters | VCC-powered counter tracks operational uptime; VBAT-powered counter logs total battery service life - critical for industrial warranty validation. |
Applications
| Industrial Data Logger | Legacy BIOS-Based Server |
|---|---|
Use Scenario: Standalone environmental monitor logging temperature/humidity every 10 minutes with timestamped entries stored in external SRAM. IC Role / Device Role / Timing Role: Primary RTC and nonvolatile memory controller managing time-stamped data writes and battery-backed retention during AC power loss. Use Value: Eliminates need for separate supervisor and EEPROM; 114-byte NV SRAM stores configuration and last-known state; VBAT-powered elapsed counter validates battery health over 10-year deployment. | Use Scenario: Rack-mounted server using original DS1287-compatible BIOS to maintain accurate system time across reboots and power cycles. IC Role / Device Role / Timing Role: Drop-in RTC replacement providing DS1287 register map and interrupt behavior while adding +3V support and extended SRAM control. Use Value: Enables migration from 5V-only designs to mixed-voltage motherboards without BIOS rewrite; century register and leap-year algorithm ensure correct date handling beyond year 2000. |
| Medical Diagnostic Device | Smart Energy Meter |
Use Scenario: Portable ultrasound unit requiring tamper-proof time stamps on patient reports and audit logs retained during battery swaps. IC Role / Device Role / Timing Role: Secure time source and event logger with factory-unique 64-bit ID for regulatory traceability and firmware version binding. Use Value: 64-bit serial number satisfies FDA 21 CFR Part 11 electronic signature requirements; RCLR pin allows controlled memory wipe during service; PWR output triggers safe shutdown before battery depletion. | Use Scenario: DIN-rail mounted meter recording kWh consumption with time-of-use billing intervals and outage duration logging. IC Role / Device Role / Timing Role: Precision timekeeper and outage-duration tracker using VBAT-powered elapsed counter to log cumulative grid-down minutes. Use Value: Dual elapsed counters differentiate between total runtime (VCC) and battery-only operation (VBAT); 32kHz SQW output synchronizes ADC sampling; IRQ alerts firmware on tariff interval boundaries. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar real-time clock with NV SRAM control applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| DS1689S | Lacks 64-bit serial number, 114-byte NV SRAM, PWR output, and power-cycle counter; retains only core RTC + 32kHz SQW. | Targeted at cost-sensitive designs needing only basic timekeeping and alarm, not system-level power management or secure identification. | Choose DS1689S only if serial number, SRAM control, and wake-on-alarm functionality are unnecessary. |
| DS1742W | Includes 128KB NV SRAM integrated on-die (vs. external SRAM control), no PWR output, no 64-bit serial number, but adds watchdog timer and wider temp range (–40°C to +85°C). | Designed for harsh-environment applications requiring large onboard NV memory and watchdog supervision, not external memory interfacing. | Choose DS1742W when integrated NV RAM capacity and watchdog are prioritized over external SRAM control and secure device ID. |
Compared with DS1689S and DS1742W, the DS1688S uniquely combines factory-serialized identity, external SRAM power/write control, and software-gated system power-on - making it optimal for field-deployed equipment requiring maintenance tracking, secure boot, and legacy BIOS compatibility.
Availability
DS1688S is available at Aetrix Electronics and suitable for industrial data loggers, legacy BIOS-based servers, medical diagnostic devices, and smart energy meters requiring stable component supply, long-term lifecycle assurance, and RoHS-compliant packaging.
Supply support for DS1688S includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.
Manufacturer
Maxim Integrated (now part of Analog Devices) is a U.S.-based semiconductor company specializing in precision analog, mixed-signal, and power management ICs for industrial, computing, and communications applications.
The DS1688S belongs to Maxim's legacy real-time clock product line, designed specifically to extend the life of DS1287-based platforms with enhanced reliability, dual-voltage operation, and system-level power management features.
FAQ
What is the primary function of the DS1688S in a system design?
The DS1688S serves as a dual-voltage real-time clock with integrated NV SRAM control, providing accurate timekeeping, alarm generation, battery-backed memory management, and system power-on capability. Its core function is to replace legacy DS1287 RTCs while adding features like 64-bit serial identification, VCC/VBAT elapsed time tracking, and automatic external SRAM backup - all within a single 28-pin SOIC package.
Does the DS1688S require an external crystal and battery to operate?
Yes, the DS1688S requires an external 32.768kHz crystal connected to X1/X2 pins and a lithium battery connected to VBAT to maintain time and NV SRAM data during main power loss. Unlike the module variant DS1691, the DS1688S is the bare IC and does not include embedded crystal or battery - those components must be added externally in the PCB layout.
How does the DS1688S handle power transitions to prevent SRAM data corruption?
The DS1688S prevents SRAM data corruption during power transitions using coordinated VCCO and CEO signaling: VCCO supplies switched power to external SRAM, while CEO asserts active-low during VCC droop to disable SRAM writes. This hardware-controlled sequence ensures no partial writes occur during brownout or power-up, eliminating the need for external power supervisors or software intervention in the DS1688S design.
Can the DS1688S be used as a direct replacement for the DS1287 in existing designs?
The DS1688S is pin-compatible and register-compatible with the DS1287 for core time/calendar functions and IRQ behavior, enabling drop-in replacement in most legacy BIOS environments. However, designers must verify PSEL pin configuration (+3V/+5V mode), confirm external SRAM interface timing, and validate that unused pins (e.g., PWR, KS, RCLR) are either tied off or actively managed - as these are new features not present in the DS1287.
What is the purpose of the 64-bit silicon serial number in the DS1688S?
Each DS1688S contains a factory-programmed, globally unique 64-bit silicon serial number used for secure device identification in networked or field-deployed systems. It supports firmware licensing, field failure root-cause analysis, secure boot chain binding, and regulatory compliance (e.g., FDA 21 CFR Part 11). The number is read-only, tamper-proof, and guaranteed non-duplicable across all DS1688S units manufactured by Maxim.
DS1688S Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc./Maxim Integrated
- Series:
- -
- Package/Case:
- 28-SOIC (0.337", 8.56mm Width)
- Packaging:
- Tube
- Product Status:
- Obsolete
- Programmable:
- Not Verified
- Type:
- Clock/Calendar
- Features:
- Alarm, Leap Year, Square Wave Output, Unique ID
- Memory Size:
- 114B
- Time Format:
- HH:MM:SS (12/24 hr)
- Date Format:
- YY-MM-DD
- Interface:
- Parallel
- Voltage - Supply:
- 2.7V ~ 3.3V, 4.5V ~ 5.5V
- Voltage - Supply, Battery:
- -
- Current - Timekeeping (Max):
- -
- Operating Temperature:
- 0°C ~ 70°C
- Mounting Type:
- Surface Mount
- Grade:
- -
- Qualification:
- -
- Supplier Device Package:
- 28-SOIC
DS1688S FAQ
1.How can I place an order for DS1688S through Aetrix?
Please submit a Request for Quotation (RFQ) for DS1688S on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for DS1688S reliable?
The price and inventory of DS1688S are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for DS1688S is usually 5 days.
3.What payment methods are accepted for DS1688S?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for DS1688S transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for DS1688S?
DS1688S orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your DS1688S order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for DS1688S?
For technical support, including DS1688S datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your DS1688S requirements.
6.How does Aetrix verify that DS1688S is sourced from the original manufacturer or authorized distributors?
All DS1688S products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that DS1688S meets industry standards.
7.What is the process for return or replacement of DS1688S?
All DS1688S units undergo pre-shipment inspection (PSI). If there is an issue with DS1688S, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The DS1688S part is unused and in its original packaging.
Return procedure for DS1688S:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
DS1688S Tags
-
MCP7940N-I/SN
Microchip Technology
-
MCP7940MT-I/MNY
Microchip Technology
-
PCF85063ATL/1,118
NXP USA Inc.
-
MCP7940NT-I/SN
Microchip Technology
-
MCP7940NT-I/MS
Microchip Technology
-
MCP7940N-I/MS
Microchip Technology
-
PCF85063AT/AY
NXP USA Inc.
-
PCF85063TP/1Z
NXP Semiconductors
-
PCF85063ATT/AJ
NXP USA Inc.
-
MCP7940NT-E/SN
Microchip Technology
-
MCP7940NT-I/MNY
Microchip Technology
-
MCP79400T-I/SN
Microchip Technology
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