Differences between DDR and DDR2
Contents
DDR vs. DDR2
Double Data Rate (DDR) and Double Data Rate 2 (DDR2) are generations of synchronous dynamic random-access memory (SDRAM) defined by JEDEC. These technologies enable the transfer of data on both the rising and falling edges of the clock signal, effectively doubling the data rate compared to standard SDRAM. DDR2 succeeded the original DDR specification in 2003 to address limitations in clock speed and power consumption found in the first generation.[1]
Comparison table
| Feature | DDR | DDR2 |
|---|---|---|
| Release year | 2000 | 2003 |
| Prefetch buffer | 2-bit | 4-bit |
| Standard voltage | 2.5 V | 1.8 V |
| Pin count (Desktop) | 184 pins | 240 pins |
| Internal clock frequency | 100–200 MHz | 100–266 MHz |
| Data transfer rate | 200–400 MT/s | 400–1066 MT/s |
| On-Die Termination (ODT) | Not present | Integrated |
Architecture and prefetch
The most significant architectural change between the two generations is the prefetch buffer size. DDR uses a 2-bit prefetch, meaning the memory array accesses two bits of data for every clock cycle. DDR2 increased this to a 4-bit prefetch. This design allows the external data bus of DDR2 to operate at twice the frequency of its internal memory clock. As a result, DDR2 can achieve higher data transfer rates without the need for the internal memory cells to run at higher speeds, which would otherwise lead to manufacturing difficulties.[2]
While DDR2 offers higher bandwidth, it often has higher CAS latency than DDR. DDR modules typically have latencies between 2 and 3 clock cycles. In contrast, DDR2 latencies usually range from 3 to 5 cycles. The higher clock speeds of DDR2 generally compensate for this latency increase in terms of total system performance.
Electrical characteristics
The two generations differ in their power requirements and signal management. Standard DDR modules operate at 2.5 volts. DDR2 reduced this requirement to 1.8 volts. This lower voltage results in less heat generation and lower power consumption for the system. This change made DDR2 particularly useful for laptop computers and servers where thermal management is a priority.[3]
DDR2 also introduced On-Die Termination (ODT). In the DDR generation, termination resistors were placed on the motherboard. By moving these resistors onto the memory die itself, DDR2 reduces signal reflections and timing errors. This integration improved signal integrity at the higher frequencies required for faster data rates.
Physical design
DDR and DDR2 modules are not physically compatible. A DDR module for a desktop computer has 184 pins, while a DDR2 module has 240 pins. The pins on a DDR2 module are smaller and more densely packed. To prevent incorrect installation, the physical notch on the bottom of the module is located in a different position on DDR2 compared to DDR. These physical differences mean that a motherboard designed for DDR cannot accept DDR2 modules, and vice versa. Laptop variants (SO-DIMMs) also differ in pin count; DDR SO-DIMMs have 200 pins, while DDR2 SO-DIMMs have 200 pins but use a different notch alignment.[4]
References
- ↑ JEDEC Solid State Technology Association. (2003). "DDR2 SDRAM Standard (JESD79-2)."
- ↑ Mueller, Scott. (2015). "Upgrading and Repairing PCs." 22nd Edition. Que Publishing. pp. 320-325.
- ↑ Crucial. (2023). "DDR vs. DDR2 vs. DDR3: What is the Difference?" Tech Support Whitepaper.
- ↑ Kingston Technology. (2005). "DDR2 Memory Technology Whitepaper."
