3 ms·

Can someone explain why this uses DDR2 instead of a more modern DDR4?

phlipski 5y ago
I'm gonna speculate that no one sells DDR4 in bare die form. There are however vendors that sell DDR2 bare die for SIP's.
KZerda 5y ago
Offhand, one of the big issues that comes to mind is latency. DDR4 has a much higher latency than DDR2 between requesting and receiving, so could be potentially slower in a simpler core that doesn't have as big of a cache, and less aggressive cacheing/fetching algorithms.
duskwuff 5y ago
DDR4 would require a significantly larger and more complex memory controller, and wouldn't improve performance. (This isn't exactly a high-performance SoC -- while no clock speed is listed, I suspect it's in the sub-GHz range.)
bradfa 5y ago
There's a bunch of reasons, here's a few I've heard or thought of myself: * DDR2 is generally 1.8V. There's probably already a need for 1.8V logic within this SOC, so the power rail is "free." DDR3, DDR3L, and all the variants of DDR4 use a lower voltage. No one wants another power supply rail just to power some internal-to-the-SOC memory if they don't have to. * DDR2 is easily available in single die x16 (data bus width) 64MB (512Mb) sizes. DDR3 may be available in this size, but DDR4 is almost certainly not available or would require buying dies which don't sell a lot so becomes risky. * DDR2 in single-die configurations is super simple to hook up. * DDR2 in this size die isn't going away any time soon and may likely be the cheapest way to get a smaller amount of RAM. * SIP vendors often can negotiate very good and stable pricing for RAM, which is a big benefit to the PCB design integrator as they then don't have as crazy of price fluctuations which are common in the RAM market. This makes the SIP more attractive to some customers (the PCB designer, not the end user). * DDR2 can likely keep this speed of CPU fed with data fast enough that the RAM won't be the bottleneck for many workloads.
- foxfluff 5y ago
  Fwiw Allwinner/Sochip do sell a CPU with 128MB of DDR3 in the package (S3). You do need to provide 1.5V supply for the DRAM.

I'm gonna speculate that no one sells DDR4 in bare die form. There are however vendors that sell DDR2 bare die for SIP's.

KZerda 5y ago

Offhand, one of the big issues that comes to mind is latency. DDR4 has a much higher latency than DDR2 between requesting and receiving, so could be potentially slower in a simpler core that doesn't have as big of a cache, and less aggressive cacheing/fetching algorithms.

duskwuff 5y ago

DDR4 would require a significantly larger and more complex memory controller, and wouldn't improve performance. (This isn't exactly a high-performance SoC -- while no clock speed is listed, I suspect it's in the sub-GHz range.)

bradfa 5y ago

There's a bunch of reasons, here's a few I've heard or thought of myself: * DDR2 is generally 1.8V. There's probably already a need for 1.8V logic within this SOC, so the power rail is "free." DDR3, DDR3L, and all the variants of DDR4 use a lower voltage. No one wants another power supply rail just to power some internal-to-the-SOC memory if they don't have to. * DDR2 is easily available in single die x16 (data bus width) 64MB (512Mb) sizes. DDR3 may be available in this size, but DDR4 is almost certainly not available or would require buying dies which don't sell a lot so becomes risky. * DDR2 in single-die configurations is super simple to hook up. * DDR2 in this size die isn't going away any time soon and may likely be the cheapest way to get a smaller amount of RAM. * SIP vendors often can negotiate very good and stable pricing for RAM, which is a big benefit to the PCB design integrator as they then don't have as crazy of price fluctuations which are common in the RAM market. This makes the SIP more attractive to some customers (the PCB designer, not the end user). * DDR2 can likely keep this speed of CPU fed with data fast enough that the RAM won't be the bottleneck for many workloads.

foxfluff 5y ago
Fwiw Allwinner/Sochip do sell a CPU with 128MB of DDR3 in the package (S3). You do need to provide 1.5V supply for the DRAM.