DS1647-120+

Real Time Clock Overview

The DS1647-120+ is a 512k x 8 nonvolatile static RAM integrated with a real-time clock, crystal, power-fail control circuit, and lithium energy source. It operates within a supply voltage range of 4.5V to 5.5V and supports byte-wide access with an address access time of 120ns. The device features double-buffered clock registers for year, month, date, day, hours, minutes, and seconds in BCD format, valid through 2099. It provides over 10 years of data retention without power and includes automatic leap year compensation. The component is housed in a 32-pin EDIP package and functions as a direct replacement for standard SRAM while adding timekeeping capabilities.

Real Time Clock Feature Summary

• Integrates 512k x 8 NV SRAM with full-function real-time clock and lithium battery backup
• Clock registers reside in the eight top RAM locations (7FFF8h–7FFFFh) and are accessed via standard RAM interfaces
• Automatic leap year compensation and BCD-coded time/date registers valid through 2099
• Hardware power-fail write protection activates when VCC drops below 4.0V–4.5V

Real Time Clock Applications

• Embedded systems requiring persistent timekeeping and nonvolatile data storage
• Industrial controllers needing ROM/EPROM/EEPROM substitution with read/write nonvolatility
• Data logging devices utilizing the integrated RTC for timestamping events

Real Time Clock Procurement Notes

Verify that the specific order suffix corresponds to the lead-free/RoHS-compliant variant (+). Confirm the package type matches the required board layout; the DS1647-120+ utilizes a 32-pin EDIP module. Ensure the design accounts for the internal lithium source activation, which occurs only upon initial application of VCC above the power-fail threshold. Check for availability of compatible PowerCap modules if surface-mount integration is required instead of the DIP module.

Real Time Clock Selection Notes

Select the DS1647-120+ for applications requiring 512k-bit density with integrated timekeeping. The -120 speed grade indicates a 120ns access time suitable for standard 5V microcontroller buses. For surface-mount designs, consider the DS1647P variant paired with a separate PowerCap module to avoid thermal damage to the internal battery during reflow soldering. Ensure the system operates within the 0°C to +70°C temperature range for guaranteed accuracy specifications.

Real Time Clock Part Context

The DS1647 series belongs to the Dallas Semiconductor/Maxim Integrated line of Timekeeping RAMs. These devices combine standard JEDEC-compatible SRAM pinouts with RTC functionality, allowing designers to replace multiple discrete components (SRAM, RTC, battery holder) with a single IC. The DS1647-120+ represents the lead-free version of the original DS1647-120, maintaining identical electrical and functional characteristics while complying with environmental directives.

Real Time Clock Replacement Considerations

The DS1647 is pin-for-pin compatible with other densities in the DS164XP family, such as the DS1645 or DS1646, provided the memory organization differences are accounted for in software. When replacing standard SRAM, ensure the host system handles the specific control register writes required to configure the RTC bits.

Real Time Clock FAQ

How is the real-time clock accessed?

The clock registers are mapped to the eight highest addresses (7FFF8h to 7FFFFh) and are accessed using standard RAM read/write cycles. They are double-buffered to prevent reading incomplete data during updates.

What triggers the power-fail write protection?

Write protection activates when the VCC supply falls below the power-fail point (VPF), typically between 4.0V and 4.5V. This prevents data corruption during low-voltage conditions by deselecting the device.

How is the internal lithium battery activated?

The internal lithium energy source is disconnected at shipment. It is automatically enabled for battery backup operation only when VCC is first applied at a level greater than the power-fail threshold (VPF).