SD Bus Interfaces
Memory Cards · 6 specs
Direct Answer
SD Bus Interfaces define the physical and electrical signaling architectures through which SD and microSD cards transfer data with host controllers. Spanning legacy Default/High Speed, Ultra High Speed tiers (UHS-I up to 104 MB/s, UHS-II up to 312 MB/s, UHS-III up to 624 MB/s), and PCIe/NVMe-based SD Express (up to 3,940 MB/s).
Source: SD Association (SDA) Physical Layer Specifications (SD 3.01 / 4.0 / 6.0 / 7.0 / 8.0) · Last reviewed Aug 28, 2026
Specs
| Default Speed (DS) / High Speed (HS) | 12.5 MB/s (25 MHz @ 3.3V) / 25 MB/s (50 MHz @ 3.3V) |
| UHS-I (SD 3.01) | SDR12 (12.5 MB/s), SDR25 (25 MB/s), SDR50 (50 MB/s), DDR50 (50 MB/s), SDR104 (104 MB/s @ 208 MHz 1.8V) |
| UHS-II (SD 4.0) | 156 MB/s (Full Duplex) / 312 MB/s (Half Duplex) via 2nd pin row LVDS (0.26V) |
| UHS-III (SD 6.0) | 312 MB/s (Full Duplex) / 624 MB/s (Full Duplex dual-lane LVDS) |
| SD Express (SD 7.0 / 8.0) | 985 MB/s (PCIe 3.0 x1), 1970 MB/s (PCIe 4.0 x1 / 3.0 x2), up to 3940 MB/s (PCIe 4.0 x2) |
| SD Express Protocol Stack | PCI Express (PCIe) + NVM Express (NVMe v1.3/1.4) over SD pins |
UHS-I vs UHS-II Physical Signaling
- UHS-I (SD 3.0): Operates over the original 9 pins by lowering single-ended I/O signaling voltage from 3.3V down to 1.8V, scaling clock frequencies up to 208 MHz (SDR104 mode) to deliver 104 MB/s theoretical bandwidth (with proprietary overclocked modes like SanDisk DDR200 reaching ~170–200 MB/s).
- UHS-II (SD 4.0): Adds a second row of 8 pins utilizing Low Voltage Differential Signaling (LVDS, 0.26V swing). In Full Duplex mode, two differential pairs transmit simultaneously at 156 MB/s in each direction; in Half Duplex mode, both pairs transmit in the same direction to achieve 312 MB/s.
SD Express: Merging PCIe and NVMe into Removable Cards
Introduced in SD 7.0 (2018) and SD 8.0 (2020), SD Express bypassed traditional SD host controller protocol stacks entirely. By repurposing the second and third pin rows as PCIe differential clock, transmit, and receive lanes running the NVMe storage protocol, SD Express converts memory cards into miniature removable NVMe SSDs capable of speeds up to 3,940 MB/s (nearly 4 GB/s), while remaining backward-compatible with legacy SD host devices at UHS-I speeds.