Specifications

©Copyright 2001-2013 SD Group (Panasonic, SanDisk, Toshiba) and SD Card Association
Physical Layer Simplified Specification Version 4.10
94
4.13.1.2 Recording Unit (RU)
Each AU is divided into units called "Recording Unit (RU)" (Refer to Figure 4-47). The unit of RU Size
(S
RU
) is 16KBye. The RU Size is a multiple of 16KByte and shall not span across an AU boundary.
Larger RU size may improve performance. The condition and requirement of the minimum RU Size is
defined by Section 4.13.1.8.1. The number of RUs in an AU (N
RU
) is calculated from S
AU
/S
RU
.
4.13.1.3 Write Performance
Figure 4-48 shows the typical data management of the card when the host writes RUs of an AU. When
the host writes to a fragmented AU, the card prepares a new AU by copying the used RUs and writing
the new RUs.. The location A is at the start of the AU boundary and location B is at the end of the AU
boundary. From A to B, the host shall write data to free RUs contiguously and skip used RUs (shall not
skip any free RU). The card may indicate busy to the host, so the host can wait, during the time the
card controller is writing and moving data. The total write time from A to B can be calculated by
summing up the write time of free RUs and the moving time of the used RUs. The number of used RUs
(Nu) is available by counting it over one AU and number of free RUs is expressed by (N
RU
– Nu).
Figure 4-48 : Example of Writing Fragmented AU
The average Performance of a Fragmented AU can be calculated by dividing the number of free RUs
by the total execution time. It is expressed by using Performance Wite (Pw) and Performance Move
(Pm).
Performance of Fragmented AU:
Pm
NuS
Pw
NuNS
NuNS
NuP
RURURU
RURU
+
−
−
=
)(
)(
)(
NuPwPmNuN
PmPwNuN
RU
RU
+−
−
=
)(
)(
..........................(1)
The Performance Write (Pw) is defined as a minimum average write performance over an AU. It is
calculated by taking the average of all sequential RU write operations to one complete AU, which is not
fragmented. Pw is not influenced even if read operations are inserted between write operations.
The Performance Move (Pm) is defined as a minimum average move performance. It is calculated by
taking the average over sequential RU move operations to one complete AU. A move is an internal
operation of the card, so SD clock frequency does not affect the time of the move operation. In case the
card does not have to move RU, Pm should be considered as infinity (1/Pm = 0). Refer to Table 4-57 for
the values defined for each Speed Class.
Note that a Speed Class that supports Class10 shall not use the Pm value stored in the SD Status to
calculate performance in any fragmented AU. Class 10 performance is defined only for entirely free
AUs.
Free..........
Free
Used
UsedFree
Free
UsedFree
Free
Free..........
FreeUsed
Used
Free
Free
UsedFree
Free
RU
DataN..........
Data5
UsedUsed
Data4
Data3
UsedData2
Data1
DataN..........
Data5Used
Used
Data4
Data3Used
Data2
Data1
Move Move
Move
Write
Write
Write
Write
Write
Write
Write
Write
Write
Write
Write
Write
AU (Allocation Unit)
Skip
Existing AU
New AU
Skip
A B