Product manual

Application Note

Read (TAAC + NSAC) 100ms

Write (TAAC + NSAC) * R2W_FACTOR 250 ms

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 Industrial Grade 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 Industrial Grade 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 Industrial Grade 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 Industrial Grade 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 Industrial Grade SD Card Clock Speed and Burst Rate

Product Maximum Clock Speed and Burst Rate

MultiMediaCard Clock Speed Burst Rate

SPI Bus mode 20 MHz 2.5 MB/s

MMC 1-bit mode 20 MHz 2.5 MB/s

SD Card

SPI Bus mode 25 MHz 3.125 MB/s

SD 1-bit mode 25 MHz 3.125 MB/s

SD 4-bit mode 25 MHz 12.5 MB/s

The write and read throughput rates of the Industrial Grade 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 Industrial Grade SD Card.