Microarchitectures are often designed with one optimal RAM standard in mind. As A15 seems to be architecturally
very similar to A14, a new RAM standard was probably out of the cards.
Like Intel & AMD & Qualcomm, Apple needs to update its SoC's IMC (integrated memory controller) for LPDDR5. I don't know if Apple designs in-house memory controllers or licenses the IP from Synopsys et al, but that also could be a bottleneck (for either choice: it's taking time to design it or their vendor isn't ready for what Apple wants in LPDDR5).
That is, it's not just replacing the RAM chip (which is a commodity), but also notably updating the SoC.
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Out of curiosity for iPhone memory controller updates,
iPhone 4S: first to use LPDDR2
iPhone 5S: first to use LPDDR3
iPhone 6S: first to use LPDDR4
iPhone 8 / X: first to use LPDDR4X
iPhone 8, X, SE 2nd Gen, XS, XR, 11, 12, and now 13 have remained on LPDDR4X, though frequencies & timings and what-not have probably improved. And, to be frank, LPDDR4
X is a minor revision to LPDDR4, so in reality we've been on the same memory standard since the 2015 iPhone 6S.
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It mirrors the rest of the industry: LPDDR5
just arrived to market
in early 2020: 20 months.
It's also still scaling in manufacturing & binning, like all new RAM standards:
LPDDR4X: 4266 Mbps
LPDDR5 in 2020: 5500 Mbps
LPDDR5 in 2021: 6400 Mbps (spec's peak data rate)
LPDDR5X in 2022: up to 8533 Mbps (spec's peak data rate)
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It could have happened: Intel apparently certified Tiger Lake for LPDDR5, though I can't think of a single major device that shipped with LPDDR5.
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It's also volume. Apple seemingly needs one memory standard for all A15 devices; that's unlike Qualcomm that certifies multiple standards for a single SoC (
hell, even a single phone model). Most Android phones with high-speed LPDDR5 are flagships and, overall, Android flagships sell in
lower volume than iPhones.
That is, Apple needs 100 million of A15 SoCs + high-frequency LPDDR5 RAM. Does that LPDDR5 capacity exist?
