1
Copyright
Cirrus Logic, Inc. 2000
(All Rights Reserved)
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
http://www.cirrus.com
AN184
Application Note
EP72XX INTERFACE TO MULTIMEDIACARD
AUG ‘00
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TABLE OF CONTENTS
1. INTRODUCTION ....................................................................................................................... 3
2. BACKGROUND ........................................................................................................................ 3
3. MMC MODE VIRTUAL PERIPHERA L SOLUTION ......... ...... ....... ...... ....... ...... ....... ..................3
3.1 Sample Code: MMC Interface Connection ........................................................................4
4. SPI MODE VIRTUAL PERIPHERAL ........................................................................................6
4.1 Sample Code: Sending Data to MMC Using SPI Mode .....................................................7
5. CONCLUSION .......................................................................................................................... 8
6. REFERENCES .......................................................................................................................... 8
LIST OF FIGURES
Figure 1. MMC in MMC mode.........................................................................................................5
Figure 2. Waveform Measurement Results of send_mmc_byte. ....................................................5
Figure 3. MMC in SPI Mode............................................................................................................6
Contacting Cirrus Logic Support
For a complete listing of Direct Sales, Distributor, and Sales Representative contacts, visit the Cirrus Logic web site at:
http://www.cirrus.com/corporate/contacts/
Prelimi nar y product info rmation descr i bes products which are in production, b ut f or which full characteriza t i on da t a i s not yet avail ab le . Ad vance p roduct infor -
mation describes products which are in development and subject to development changes. Cirrus Logic, Inc. has made best efforts to ensure that the information
contained in this document is accurate and reliable. However, the information is subject to change without notice and is provided “AS IS” without warranty of any
kind ( ex pres s or i mplie d). No re spon sibility is as sume d by Cirru s Logic, Inc. for the use of this information, nor for infringements of patents or other rights of third
parties. This document is the property of Cirrus Logic, Inc. and implies no license under patents, copyrights, trademarks, or trade secrets. No part of this publi-
cation may be copied, reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photographic, or otherwise)
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sale of any items without the prior written consent of Cirrus Logic, Inc. The names of products of Cirrus Logic, Inc. or other vendors and suppliers appearing in
this document may be trademar ks or servi ce marks of their re specti ve owners which may be reg istered in some jur isdict ions. A list of Cirrus Lo gi c, Inc. trade -
marks and service marks can be found at http://www.cirrus.com.
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1. INTRODUCTION
This application note describes a simple interface
between a Cirrus Logic EP72XX microcontroller
and a MultiMediaCard.
The MultiMediaCard (MMC) is a universal low-
cost data storage and communication media. It is
designed to cover a w ide area of applications such
as electronic toys, organizers, PDAs, cameras,
smart phones, digital recorders, and digital players
(such as MP3).
The MMC communication is based on a 7-pin
serial bus designed to operate in a low voltage
range. A MMC communication protocol is defined
as part of the standard and is referred to as MMC
mode. For compatibility, an alternate
communication protocol (a subset of MMC) is
based on the SPI™ (Serial Peripheral Interface)
standard. This application note will describe a
method to support both modes.
2. BACKGROUND
For digital audio applications such as portable MP3
players, the digital content is typically stored in
NAND Flash memory. The MMC offers an
alternative storage media and has the adva ntage of
being removable and portable. For operation using
the SPI protocol, the host must be able to read data
at a typical MP3 data rate of 128 kilo-bits-per-
second (kbits/s).
For programming the MMC, 128 kbits/s rate is
insufficient to provide short download times. A
typical portable MP3 system may include a USB
port that accepts MP3 data files from a host PC.
Since USB is a relatively fast interface with bit
rates approaching 5 Mbits/s average rate (due to
operating system overhead), the main limitation to
download speed is the speed at which the MMC
can be programmed. First versions of MMC cards
can be programmed at ~1 Mbits/s, or around
125 kbytes/s. Newer generations of MMC cards are
expected to reach 2.5 Mbits/s, or ~400 kbytes/s
The EP72xx mic rocontrol lers have a spe ed-l imite d
SPI port. Though adequate for reading content (at
least up to 128 kbits/s), it is too slow for writing.
However, the high speed of the ARM® processor
allows one to create a “virtual peripheral” to
support both SPI and MMC modes. A software
“bit-bang” method provides the necessary signals
for data and command, while a chip select provides
the clock. Transfer speed using this method
approaches 3 Mbits/s. For future applications, the
SD MMC (secure MMC) bus has added 3 more
data lines so that 4-bits can be transferred per clock
(instead of one). This should eventually improve
download speeds approaching that of traditional
NAND flash.
3. MMC MODE VIRTUAL
PERIPHERAL SOLUTION
Two GPIO pins are used for MMC_Data and
MMC_Command. A spare chip select line is used
for MMC_Clock. Section 3.1, “Sample Code:
MMC Interface Connection” contains sample code
that illustrates a possible connection to the MMC
interface. In this case, GPIO pin PA7 is used for
MMC_Data and PA6 is used for MMC_Command.
A schematic is shown in Figure 1.
To maximize transfer speed, the routine to send
bytes to the MMC card should be written in
assembly language. For illustrative purposes, the
sample code is written in C with in-line assembly.
Note that the code is "unrolled", i.e. it does not use
a loop counter to set the number of bits transmitted
(in this case, one byte or 8 bits). Doing so adds
substantially more overhead. As shown, it takes 23
instructions to execute the main routine. This
routine, when running out of cache, should be able
to burst at 74/23 MHz, or approximately 3 Mbs
(see Figure 2). Using a loop counter will add two
extra instructions per iteration, plus another cycle
for the conditional branch, for a total of 8 * (3 + 2)
= 40 cycles, or about ½ the speed of the unrolled
loop.
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NOTE: The nCS5 byte field in MEMCFG2 = 0x3E. Chip
select 5 is not cached and the write buffer is dis-
abled.)
3.1 Sample Code: MMC Interface Connection
// Routine to send a bit stream to MMC card.
// Data to send is in R0, address of nCS5 is in R1, and GPIO address is in R2.
// For this example, R1=0x5000.0000, R2=0x8000.0000 (port A data register).
// It is also assumed that PA7 is an output and pins PA[6:0] are input prior
// to calling this function. Data is sent MSB first.
void
send_mmc_byte(char mmc_data, int port, int GPIO)
{__asm
{
strb r0, [r2] // get value and place in GPIO
strb r0, [r1] // strobe cs5
mov r0, r0, lsl #1 // shift next bit left which places it in PA7.
strb r0, [r2] // Repeat 7 more times.
strb r0, [r1]
mov r0, r0, lsl #1
strb r0, [r2]
strb r0, [r1]
mov r0, r0, lsl #1
strb r0, [r2]
strb r0, [r1]
mov r0, r0, lsl #1
strb r0, [r2]
strb r0, [r1]
mov r0, r0, lsl #1
strb r0, [r2]
strb r0, [r1]
mov r0, r0, lsl #1
strb r0, [r2]
strb r0, [r1]
mov r0, r0, lsl #1
strb r0, [r2]
strb r0, [r1]
}
}
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3.3V
nCS5
PA[7]
PA[6]
51K51K U?
MMC Connector
1
2
3
4
5
6
7
RSV
CMD
Vss
Vdd
CLK
Vss2
DAT[0]
Figure 1. MMC in MMC mode
Figure 2. Waveform Measurement Results of send_mmc_byte.
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4. SPI MODE VIRTUAL PERIPHERAL
To implement a SPI port, three GPIO signals are
needed: chip select, data out, and data in. As with
the MMC interface, nCS5 is used for the SPI data
clock. A schematic is shown in Figure 3. A C
subroutine with in-line assembly to write an 8-bit
byte of data to the MMC is shown below. Unlike
the previous sample code example, the routine in
Section 4.1, “Sample Code: Sending Data to MMC
Using SPI Mode” uses a loop counter. Better
performance can be achieved if the loop is
unrolled. This is left as an exercise for the reader.
Both Virtual Peripheral examples were written
using the ARM Tools Version 2.50. Code results
were benchmarked on a Cirrus Logic EP7209
evaluation board running the Angel Debug
Monitor.
In some cases, it is desired to be able to support
both modes of MMC operation, i.e., SPI and MMC
modes. In this case, it is necessary to provide a
power switch to the MMC power supply. Upon
power up, the MMC defaults to MMC mode. To set
for SPI mode, a command is issued to the card.
Once in SPI mode, the MMC cannot revert back to
MMC Mode until power is recycled.
nCS5
3.3V
U?
MMC Connector
1
2
3
4
5
6
7
nCS
DI
Vss
Vdd
SCLK
Vss2
DO
PA[7]
PA[6]
PE[0]
Figure 3. MMC in SPI Mode
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4.1 Sample Code: Sending Data to MMC Using SPI Mode
// Routine to send a bit stream to the MMC card using the SPI mode.
// Data to send is in R0, address of nCS5 in R1 (in this case, 0x5000.0000),
// and R2 contains address of GPIO (Port A).
// It is also assumed that PA7 is an output and pins PA[6:0] are input prior
// to calling this function. In addition, the MMC chip select must be
// asserted (set to low) prior to using this routine. If a PA bit is used
// for CS, added instructions and registers are required to mask off the CS
// bit and will affect transfer speed.
// Data is sent MSB first.
void
send_spi_byte(char spi_data, int port, int GPIO)
{__asm
{
mov r3, #0x8 //loop counter
s1:
strb r0, [r2] // output bit to PA7
strb r0, [r1] // and strobe nCS5
mov r0, r0, lsl #1 // get next bit by shifting left once
subs r3, r3, #1 // are all bits transferred?
bne s1 // no. branch back
}
}
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5. CONCLUSION
Using the fast instruction set of the ARM7 and by
careful design of software, it is possible to generate
a software virtual peripheral that can interface to a
MMC memory card using either the MMC or SPI
interface protocol. Speeds up to 3 Mbit/s can be
achieved in this manner, which is adequate for
most applications. For best performance, follow
these guidelines:
-Data read or written should use the multiple
block mode for data transfer. Some MMC cards
will not support multiple block mode with the SPI
interface
-Use MMC mode if possible.
-Be aware that it can take as long as 2.5 mS after
a command is sent to the MMC before it is ready
to send data. This is referred to as the NAC
access time delay.
-Keep a flat file structure on the MMC so that
blocks are contiguous thereby minimizing NAC
-Unroll all assembly loops
-Try to design with the fastest and widest
program memory possible. This will improve
overall speed.
6. REFERENCES
Information on the MMC specifications can be
found at the MultiMediaCard Association Web site
www.mmca.org.
Data sheets for MMC cards can be found at several
locations. Start by trying www.sandisk.com.
Information on Cirrus Logic’s ARM-based
microcontrollers, including data sheets on the
EP7209/12 can be found at www.cirrus.com.
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• Notes •
