Interface - SanDisk SDSDB-32-201-80 - Industrial Grade Flash Memory Card Product Manual

Application Note
Read (TAAC + NSAC)
Write (TAAC + NSAC) * R2W_FACTOR
The factors used in calculating the values in Table 1—TAAC, NSAC, and R2W_FACTOR—can be read directly
from the CSD register of the MultiMediaCard and SD Card.
The TAAC factor's unit is time, and the NSAC factor has units of 100 clocks. You can convert TAAC units to
clock cycles by multiplying by the frequency of the clock and calculate the time-outs in units of clock cycles if
desired. Alternatively, given the frequency of the clock, you can convert the NSAC units to time and calculate the
time-outs in units of time.
The R2W_FACTOR is a read-to-write factor and has no units. A design engineer can use the time-out values
derived from the CSD register to make the design compatible with all MultiMediaCards and SD cards regardless of
customer brand.

Interface

The MultiMediaCard and SD Card support multiple busses. Both cards support the 1-bit SPI bus that includes bus
pins DATin, DATout, CLK, and CS. The SPI bus is generally found on Motorola and other major MCU
manufacturer products.
The SD Card also supports a 4-bit and a 1-bit SD bi-directional bus mode. SD bus pins are CLK, CMD, and DAT
in 1-bit mode and CLK, CMD, and DAT[0:3] in 4-bit mode.
The MultiMediaCard also supports the 1-bit bi-directional MMC bus mode that has CLK, CMD, and DAT bus pins.
The CMD and DAT pins are bi-directional on the SD 1-bit, SD 4-bit, and MMC 1-bit.
The maximum burst rate achievable with the SD Card and MultiMediaCard depends on the clock speed and bus
mode. The burst rate is the data transfer rate between the card's buffer and host.
Table 2. MultiMediaCard and SD Card Clock Speed and Burst Rate
Product
MultiMediaCard
SPI Bus mode
MMC 1-bit mode
SD Card
SPI Bus mode
SD 1-bit mode
SD 4-bit mode
The write and read throughput rates of the SD Card and MultiMediaCard are slower than the burst rate because each
card includes the busy time to write data from the card's buffers to its internal Flash RAM, and busy time to read
data from the internal Flash RAM to the card's buffer. Since most designs use this write and read busy time to
complete other processes, choosing a 1- or 4-bit bus mode can have a 4x speed effect on the time spent servicing the
SD Card.
A-2
Maximum Clock Speed and Burst Rate
Clock Speed
20 MHz
20 MHz
25 MHz
25 MHz
25 MHz
SanDisk Secure Digital (SD) Card Product Manual, Rev. 1.9 © 2003 SANDISK CORPORATION
100ms
250 ms
Burst Rate
2.5 MB/s
2.5 MB/s
3.125 MB/s
3.125 MB/s
12.5 MB/s