[Oberon] RAM capacity for Oberon
skulski at pas.rochester.edu
Thu Jan 10 00:24:40 CET 2019
thank you for the response. I want to add that ZBT is both faster and more predictable than ASRAM, though it is a bit trickier to use. The pipelined ZBT memory can run at 200 MHz, with 2 clock latency. It means that data is available 2 clocks after the address. It effectively translates into 100 MHz throughput, if you skip one clock. In other words, you can run the ZBT part at 200 MHz, and make it appear as if the data was presented on the next clock after the address. Internally it is running at 200 MHz, but for the clients it is running at 100 MHz. This is faster than ASRAM, which effectively can run at 25 MHz.
Concerning memory pricing, there was a bit of misunderstanding. Let me post actual prices for 25 pieces from last Fall. These are the cheapest, or the most reasonable devices which I found. The price-per-megabyte can be calculated from the prices given below. It turns out $12 per MB for ASRAM, and $9 per MB for ZBT. Other memory types are cheaper.
1) Synchronous ZBT: ISSI IS61NLP102418B-200B3LI, 1M * 18, 3ns, $17.79/ea DigiKey.
2) Asynch ASRAM: ISSI IS61WV51216EDBLL-10TLI, 512k * 16, 10 ns, $12.3/ea DigiKey.
3) Cellular RAM: Winbond W957D6HBCX7I, 133 MHz, 8 M * 16, 70 ns, $5.30/ea Arrow.
4) HyperRAM: ISSI IS66WVH16M8ALL-166B1LI, 166 MHz, 16 M * 8, 36 ns, $6.24/ea Digikey.
5) SDRAM: AS4C32M16SB-7TCN 32M x 16 143MHz 5.4ns $11.70/ea Digikey.
6) LPDDR: MT46H64M16LFBF-5 IT:B 64M x 16 10.66/ea Digikey.
7) DDR3L: MT41K256M16TW-107 AIT:P DDR3L 4Gb 256M x 16 14.79/ea Digikey.
As you can see, the DDR3L is the least expensive per MB. It also requires the most complex firmware and it probably will be very ineffective w/o caching.
I chose the ZBT for my RiskFive project, and I am now watching with amazement how students cannot make sense of it.
Part of the student's problem is the RISC5 Verilog code, which is extremely terse and unforgiving for understanding.
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