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My assumption is the SoC can support both protocol OSPI SDR and DDR. With an embedded system designer, if he can choose between SDR or DDR mode of OSPI flash device to deploy on his board, what is the point to consider? I mean the advanced feature such as speed, bandwidth, latency, persistence, etc...

In my opinion, DDR mode always outperform SDR mode so where we can use SDR mode?

PS: in my question, both SoC and OSPI flash device can support DDR and SDR mode. Then the choice is not limited by the hardware. Just want to understand the scheme which mode will be chosen and it is better more than the other.

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2 Answers 2

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It depends on qspi clocking for the peripheral. For example, the ddr rate may too high for the flash at max peripheral clock, and the next slower option is slower than the max rate on sdr.

I ran into this with the stm32f7.

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  • \$\begingroup\$ thank you. it is one real point we need to care :) \$\endgroup\$
    – Hai Anh
    Jun 1, 2021 at 1:27
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Here is what p.29 of AN4760 (Rev 3) by STMicroelectronics says about DDR/SDR modes for QSPI:

DDR mode permits to sample at rising and falling edge of each clock cycle and enables the possibility to double the throughput. DDR mode allows boosting the throughput and the execution performances; it is also very useful when the system clock (HCLK) is low and does not allow the QUADSPI to operate at maximum speed.

• SDR: data sent on CLK falling edge and sampled on CLK rising edge.
• DDR: data sent on both CLK edges, during the address, alternate or data phases. The sample is done one half CLK later.

When using DDR (dual data rate), also known as DTR (dual transfer rate) the user should consider the following:
• The communication start triggering and the configuration procedure are the same as in SDR.
• The command is sent every clock cycle like in SDR mode.
• The alternate, data and address phases are sent on both edges of the clock.
• The dummy cycles are counted every clock cycle like in SDR mode.

Please, when you read this post, refer to the most recent document's revision for more reliable information.

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