NXP Semiconductors LPC24XX UM10237 - Manual (Page 557)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 557 of 792
NXP Semiconductors
UM10237
Chapter 21: LPC24XX SD/MMC card interface
The CRC generator calculates the CRC checksum for all bits before the CRC code. This
includes the start bit, transmitter bit, command index, and command argument (or card
status). The CRC checksum is calculated for the first 120 bits of CID or CSD for the long
response format. Note that the start bit, transmitter bit and the six reserved bits are not
used in the CRC calculation.
The CRC checksum is a 7 bit value:
CRC[6:0] = Remainder [(M(x)
× x
7
) / G(x)]
G(x) = x
7
+ x
3
+ 1
M(x) = (start bit)
× x
39
+ ... + (last bit before CRC)
× x
0
, or
M(x) = (start bit)
× x
119
+ ... + (last bit before CRC)
× x
0
5.3.6 Data path
The card data bus width can be programmed using the clock control register. If the wide
bus mode is enabled, data is transferred at four bits per clock cycle over all four data
signals (MCIDAT[3:0]). If the wide bus mode is not enabled, only one bit per clock cycle is
transferred over MCIDAT0.
Depending on the transfer direction (send or receive), the Data Path State Machine
(DPSM) moves to the WAIT_S or WAIT_R state when it is enabled:
• Send: The DPSM moves to the WAIT_S state. If there is data in the send FIFO, the
DPSM moves to the SEND state, and the data path subunit starts sending data to a
card.
• Receive: The DPSM moves to the WAIT_R state and waits for a start bit. When it
receives a start bit, the DPSM moves to the RECEIVE state, and the data path subunit
starts receiving data from a card.
5.3.7 Data path state machine
The DPSM operates at MCICLK frequency. Data on the card bus signals is synchronous
to the rising edge of MCICLK. The DPSM has six states, as shown in
Figure 21–109 .
Table 485. Command path status flags
Flag Description
CmdRespEnd Set if response CRC is OK.
CmdCrcFail Set if response CRC fails.
CmdSent Set when command (that does not require response) is sent.
CmdTimeOut Response timeout.
CmdActive Command transfer in progress.