最後更新: 2019-06-25

SD卡分類

三種類型@2018:

• SD          # 128MB~2GB, FAT16
• SDHC      # 高容量SD卡 (4GB ~ 32GB), FAT32
• SDXC      # 超大容量SD卡(64GB ~ 2TB), exFAT

* 新格式不向後兼容那些舊的讀卡的主機設備

速度等級和UHS速度等級

* 定義SDHC / SDXC卡最低速度保證

Speed Class

....
4         4MB/s
10       10MB/s

* 最低寫入速度

UHS Speed Class

UHS-I

U1         10MB/s
U3         30MB/s

* 最低寫入速度

Video Speed Class

UHS-II

V30        30MB/s
V60        60MB/s
V90        90MB/s

* 最低寫入速度

A1 與 A2

A = Application Performance Class

 * 注明了最低 IO

[A1]

Min. Random Read: 1500 IOPS
Min. Random Write 500 IPOS
Min. Sustained Sequential Write: 10MB/s

[A2]

Min. Random Read: 4000 IOPS
Min. Random Write 2000 IPOS
Min. Sustained Sequential Write: 10MB/s

 1-bit SD mode (Separate command and data channels)
 4-bit SD mode (parallel transfers)
 SPI mode (Serial Peripheral Interface Bus)

Pinout

9 | 1 2 3 ... |7 8|

Four-Bit SD Bus

1     CD/DAT3     Card detection / Connector data line 3
2     CMD     Command/Response line
3     Vss1     Supply voltage (earth)
4     Vdd      Power supply
5     CLK
6     Vss2     Supply voltage
7     DAT0     I/O/PP
8     DAT1     I/O/PP
9     DAT2     I/O/PP
 
 Max clock rate: 20Mz
 1.8 V/3.3 V
20–100 mA at a supply voltage of 3.3 V

 Class 2     2 MB/sec
 Class 4     4 MB/sec
 
 Ultra-High Speed (UHS) designation is available on some SDHC and SDXC cards.
 
 Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode.

在 USB Card Reader 上讀 Card 比 Pi Card Slot 快的現象

原因 :
usb card reader 是行 ~50Mhz

config.txt  

init_emmc_clock=100000000
# aised the eMMC base clock to 100 MHz
# 80 MHz by default.

sdhci-bcm2708.c
# forced 50 MHz clock in the driver

cluster size that matches the erase region of the physical memory on the card