Atmel AT91SAM9 Installation and operating instructions

NAND Flash Support in AT91SAM9

Microcontrollers

1. Scope

The purpose of this application note is to introduce the NAND Flash technology and to

describe how to interface NAND Flash memory to Atmel®AT91SAM9 ARM®Thumb®-

based Microcontrollers that do not feature a NAND Flash Controller. The NAND Flash

logic is driven by the Static Memory Controller on the NCS3 address space.

Sample code is provided the associated zip file, Basic NAND Source Code.zip; the

source code is based on the product libV3.

2. NAND Flash Overview

2.1 General Overview

NAND Flash provides a cost effective alternative to hard drives, especially for portable

and handheld systems. The performance, pricing, and memory size options make it

optimal for storage applications (pictures, audio files, etc.).

The NAND Flash used to illustrate this interface is the K9F2G08U0M, manufactured

by Samsung®Electronics. Figure 2-1 shows the memory organization of this device.

Figure 2-1. K9F2G08U0M Organization

AT91 ARM

Thumb

Microcontrollers

Application

Note

6255B–ATARM–26-Jun-09

Application Note

2.2 NAND Flash vs. NOR Flash

The most important item for memories is the cost per bit which depends on memory cell area per

bit. The cell area of NAND Flash is smaller than that of NOR Flash, making the NAND Flash

more cost effective than NOR Flash.

The first significant difference between NAND and NOR Flash is the hardware interface.

2.2.1 Hardware Interface

NOR Flash has a fully memory-mapped random access interface similar to a RAM, with dedi-

cated address lines and data lines making it “bootable”.

NAND Flash uses a multiplexed I/O interface and additional control signals. It is controlled by

sending commands and addresses through an 8-bit or 16-bit bus to an internal command and

address register.

NOR Flash random-access interface typically composed of 41 pins:

– CE# - chip enable

– WE# - write enable

– OE# - output enable

– D[15:0] - data bus

– A[20:0] - address bus

– WP# - write protect

NAND Flash I/O device-type interface composed of up to 24 pins:

– CE# - chip enable

– WE# - write enable

– RE# - read enable

– CLE - command latch enable

– ALE - address latch enable

– I/O[7:0] or I/O[15:0] - data bus

– WP# - write protect

– R/B# - ready / busy

– RE - Read enable

2.2.2 Array Architecture

NOR Flash is divided into blocks which typically contain many 16-bit wide words. Random

access to stored data words is achieved by placing the selected word address on the address

bus and then reading that data off the data bus. Erase operations are managed at the block level

and words can be programmed after a block has been erased.

NAND Flash is also divided into blocks which contain many pages instead of words (2K +64

bytes). Read and program operations take place on a per-page basis whereas erase operations

takes place on a block basis.

To read or write from NAND Flash, a command sequence is issued to select a block and a page.

After this selection, the entire page can be read or written.

6255B–ATARM–26-Jun-09

Application Note

NAND Flash typically contains blocks that contain errors and cannot be used. A check must be

done by software to list and maintain a table of bad blocks. Data integrity is achieved by using

hardware or software techniques, such as ECC, that check and correct bad data.

2.2.3 Performances

Further differences between NOR and NAND Flash can be found in read/write performances.

Table 2-1 shows random access time for NOR Flash specified at 0.09 µs, whereas NAND ran-

dom access is significantly slower — 25 µs — for the first byte. Once the initial access has been

made, however, the remaining 2,111 bytes are shifted out of NAND at only 30 ns per byte. This

results in a bandwidth of more than 23 Mb/s for 8-bit I/Os or 37 Mb/s for 16-bit I/Os.

The real benefits for NAND Flash can be found in the faster program and erase times, since

NAND provides over five megabytes per second of sustained write performance. The block

erase times are an impressive 2 ms for NAND versus 200 ms for NOR.

2.2.4 Conclusion

Table 2-2 summarizes NAND/NOR advantages and disadvantages.

Clearly, NAND Flash has several significant positive attributes. The one negative attribute is that

it is not well-suited for direct random access.

Table 2-1. Differences in Performance

Characteristics

NAND Flash

K9F2G08U0M

NOR Flash

AT49BV16x4-90

Random access read

25 µs (first byte)

30 ns each for remaining 2111

bytes

0.09 µs

Sustained read speed

(sector basis) 37 Mbytes/s 11 Mbytes/s

Random write speed 300 µs/2,112 bytes 20 µs / bytes

Sustained write speed (sector

basis) 5 Mbytes/s 0.05 Mbytes/s

Erase block size 128 Kbytes 64 Kbytes

Erase cycles 100,000 to 1,000,000 10,000 to 100,000

Erase time per block 2 ms 200 ms

Table 2-2. NAND/NOR Comparison

NAND NOR

Advantages

Fast writes

Fast erases

Lower bit cost

Higher density

Random access

Byte writes possible

Disadvantages

Slow random access

Byte writes difficult

Bad blocks management and ECC

required

Slow writes

Slow erase

Applications

File (disk) applications

Voice, data, video recorder

Any large sequential data

Execute directly from non volatile

memory

6255B–ATARM–26-Jun-09

Application Note

NAND is available in large capacities and is the lowest cost Flash memory available today.

NAND is used in virtually all removable cards for cost/density reasons: USB Cards, Memory

Stick, MMC Multimedia Card, SD Secure Digital, CF Compact Flash.

3. Bad Block Management and Error Corrected Code (ECC)

3.1 Definition of “Bad Block”

By default, NAND devices contain invalid blocks which have one or more invalid bits.

Furthermore, since the first memory block (physical block address 00h) in NAND devices is

guaranteed to be free of defects (up to 1,000 PROGRAM/ERASE cycles), the first 8 Kb of Flash

memory can safely be used for system bootstrapping functions.

3.2 Software Considerations

To avoid writing to and reading from bad memory blocks, system software must create a map of

invalid memory blocks. If the application code executes from RAM rather than Flash memory,

system software bad-block mapping is only necessary at boot time and during Flash storage

updates.

All device locations are erased (FFh for X8, FFFFh for X16) except locations where the invalid

block information is written prior to shipping. The invalid block status is defined by the 1st byte

(X8 device) or 1st word (X16 device) in the spare area.

The 1st or 2nd page of every invalid block has non-FFh(X8) or non-FFFFh(X16) data at the col-

umn address of 2048 (X8 device) or 1024 (X16 device). Since the invalid block information is

also erasable in most cases, it is impossible to recover the information once it has been erased.

Therefore, the system must be able to recognize the invalid block(s) based on the original invalid

block information and create the invalid block table via the flow chart in Figure 3-1.

Figure 3-1. Bad Block Recognition Flow Chart

6255B–ATARM–26-Jun-09

Application Note

Important Note: Any intentional erasure of the original invalid block information is prohibited.

3.3 ECC

NAND devices are subject to data failures that occur during device operation. To ensure data

read/write integrity, system error-checking and correction (ECC) algorithms must be imple-

mented. Depending on the AT91 product, the ECC algorithm must be calculated by software or

can be generated by the embedded hardware ECC controller. The ECC controller is capable of

single bit error correction and 2-bit random detection. When NAND has more than 2 bits of

errors, the data cannot be corrected. This controller allows ECC management without CPU inter-

vention and thus improves the total bandwidth of the system.

4. NAND Flash Signals

4.1 Bus Operation

The bus on NAND Flash devices is internally multiplexed. Data I/O, addresses, and commands

all share the same pins. I/O pins. I/O[15:8] are used only for data in the x16 configuration.

Addresses and commands are always supplied on I/O[7:0].

The command sequence normally consists of a command latch cycle, an ADDRESS LATCH

cycle, and a DATA cycle — either READ or WRITE.

4.2 Control Signals

The signals CE#, WE#, RE#, CLE and ALE control Flash device READ and WRITE operations.

CE# is used to enable the device. When CE# is low and the device is not in the busy state, the

Flash memory accepts command, data, and address information.

When the device is not performing an operation, the CE# pin is typically driven HIGH and the

device enters standby mode. The memory enters standby if CE# goes HIGH while data is being

transferred and the device is not busy.

A subset of NAND Flash supports the CE# “Don’t Care” operation allowing the NAND Flash to

reside on the same asynchronous memory bus as other Flash or SRAM devices. Other devices

on the memory bus can then be accessed while the NAND Flash is busy with internal opera-

tions. This capability is important for designs that require multiple memory devices on the same

bus.

4.3 Commands (ALE = 0, CLE = 1)

All the NAND operations (except READ STATUS and RESET commands) consist of a com-

mand write cycle followed by address write cycle. The READ STATUS command does not have

an address write cycle. The command is transferred into the NAND command register followed

by the start address, for the read or program operation, latched into the address register.

Commands are written to the command register on the rising edge of WE# when:

• CE# and ALE are low

• CLE is high

Commands are input on I/O[7:0] only. For devices with a x16 interface, I/O[15:8] must be written

with zeros when issuing a command.

6255B–ATARM–26-Jun-09

Application Note

4.4 Address (ALE = 1, CLE = 0)

Addresses are written to the address register on the rising edge of WE# when:

• CE# and CLE are low

• ALE is high

Addresses are input on I/O[7:0] only. For devices with a x16 interface, I/O[15:8] must be written

with zeros when issuing an address. Generally all five ADDRESS cycles are written to the

device.

4.5 Data (ALE = 0, CLE = 0)

Data is written to the data register on the rising edge of WE# when CE#, CLE, and ALE are low.

Data is input on I/O[7:0] for x8 devices, and I/O[15:0] on x16 devices.

4.6 Ready / Busy

The R/B# output provides a hardware method of indicating the completion of a PRO-

GRAM/ERASE/READ operation. The signal is typically high, and transitions to low after the

appropriate command is written to the device. A dedicated PIO should be assigned to this signal

with a pull-up resistor for proper operation. Alternatively, the READ STATUS command can be

used by the software.

4.7 Example

The following waveforms shows the successive accesses: COMMAND Latch, ADDRESS Latch

and DATA Output with a “CE don’t Care” NAND. Notice that no command can be sent to the

NAND Flash during tR, because it is busy.

Figure 4-1. Page READ Operation

6255B–ATARM–26-Jun-09

Application Note

5. AT91 EBI NandFlash Logic

The NAND Flash logic is driven by the Static Memory Controller (SMC) on the NCS3 address

space. Programming the CS3A field in the EBI_CSA Register in the Bus Matrix User Interface to

the appropriate value enables the NAND Flash logic. For details on this register, refer to the Bus

Matrix User Interface section in the product datasheet. Access to an external NAND Flash

device is then made by accessing the address space reserved to NCS3 (i.e., between

0x40000000 and 0x4FFF FFFF).

The NAND Flash Logic drives the read and write command signals of the SMC on the NANDOE

and NANDWE signals when the NCS3 signal is active.(refer to the Static Memory Controller

section in the product datasheet).

The address latch enable and command latch enable signals on the NAND Flash device are

driven by address bits A22 and A21 of the EBI address bus.The command, address or data

words on the data bus of the NAND Flash device are distinguished by using their address within

the NCS3 address space. The chip enable (CE) signal of the device and the ready/busy (R/B)

signals are connected to PIO lines.

Two NAND Flash types exists, those who are “CE don’t care” and those who are not.

For “CE don’t care” NAND, the chip enable state is don’t care during the busy period preceding

the data read cycle. Thus allowing this flash to be connected to active memory buses such as

the AT91 memory bus.

For standard NAND, the CE signal remains asserted even when NCS3 is not selected, prevent-

ing the device from returning to standby mode. In this case, a PIO line should be dedicated to

drive the Chip Enable signal.

Unlike the AT91SAM9261, where A21 and A22 are not specifically dedicated to NAND flash

ALE and CLE signals, on the AT91SAM9260 A21/ALE and A22/CLE are the signals mandatory

to drive the NAND flash and ECC controller. Another combination of addresses prevent using

the ECC controller with the NAND Flash.

Notes: 1. Any free PIO can be used for this purpose

2. For standard NAND

3. For AT91SAM9261 the address bit A21 is arbitrarily dedicated to CLE

4. For AT91SAM9261 the address bit A22 is arbitrarily dedicated to ALE

Table 5-1. EBI Signals Example for AT91SAM9261

Name Function Type Active Level

NANDCS NAND Flash Chip Select Line Output Low

NANDOE NAND Flash Output Enable Output Low

NANDWE NAND Flash Write Enable Output Low

CLE(A21) (3) Command Latch Enable Output High

ALE(A22) (4) Address Latch Enable Output High

PIOx/CE Chip Enable (1)(2) Output Low

PIOy/RDY/BSY Ready/Busy (1) Input Low

6255B–ATARM–26-Jun-09

Application Note

Notes: 1. Any free PIO can be used for this purpose

2. For standard NAND

Figure 5-1. “CE don’t care” and Standard NAND Flash Application Example

Table 5-2. EBI Signals Example for AT91SAM9260

Name Function Type Active Level

NANDCS NAND Flash Chip Select Line Output Low

NANDOE NAND Flash Output Enable Output Low

NANDWE NAND Flash Write Enable Output Low

A22/CLE Command Latch Enable Output High

A21/ALE Address Latch Enable Output High

PIOx/CE Chip Enable (1)(2) Output Low

PIOy/RDY/BSY Ready/Busy (1) Input Low

Table 5-3. ALE/CLE Management

ALE CLE

AT91SAM9261 Memory

Address Offset

AT91SAM9260 Memory

Address Offset NAND Register Selected

0 0 0x000000 0x000000 DATA register

0 1 0x200000 0x400000 COMMAND register

1 0 0x400000 0x200000 ADDRESS register

1 1 0x600000 0x600000 Undefined (Don’t use)

D[7:0]

ALE

NCS7/NANDWE

NCS6/NANDOE NOE

NWE

A[22:21]

CLE

AD[7:0]

PIO R/B

EBI

"CE don't Care"

NAND Flash

NCS3/NANDCS

D[7:0]

ALE

NCS7/NANDWE

NCS6/NANDOE NOE

NWE

A[22:21]

CLE

AD[7:0]

PIO R/B

EBI

NAND Flash

PIO

NCS3/NANDCS Not Connected

6255B–ATARM–26-Jun-09

Application Note

6. AT91 System Initialization for a K9F2G08U0M Device

6.1 Clocks

The system is running at full speed, this means 198 MHz for the processor and 99 MHz for the

Bus. The EBI NCS3 is to be assigned for NAND Flash usage.

6.2 PIOs

6.2.1 Standard NAND

Two PIO lines are needed for CE and RDY/BSY. NANDOE (PC0) and NANDWE (PC1) are to

be configured for NAND Flash usage.

Note: 1. Any free PIO can be used for this purpose

6.2.2 “CE don’t care” NAND

One PIO line only is needed, for RDY/BSY. NCS3 has the NANDCS function. On the

AT91SAM9260 the NCS3 is multiplexed with a PIO line allowing standard NAND Flash and “CE

don’t care” device support without any hardware modification.

Note: 1. Any free PIO can be used for this purpose

Table 6-1. System Configuration

Description Settings Register/field Value

System

PLL frequency 198 MHz PMC_PLLAR 0x20603F09

Processor / Bus Clock 198 / 99 MHz PMC_MCKR 0x00000102

EBI Chip Select Assignment NAND EBI_CSA

EBI_CS3A 0x8

Table 6-2. Peripheral Configuration for Standard NAND on AT91SAM9261

Description Settings AT91 libV3 Function

NANDOE and NANDWE are

respectively PC0 and PC1 Output

AT91F_PIO_CfgPeriph(AT91C_BASE_PIOC, (AT91C_PC0_SMOE

| AT91C_PC1_SMWE), 0);

PC14 is CE (1) Output AT91F_PIO_CfgOutput(AT91C_BASE_PIOC, AT91C_PIO_PC14);

PC15 is RDY/BSY (1) Input AT91F_PIO_CfgInput(AT91C_BASE_PIOC, AT91C_PIO_PC15);

Enable PIO clock (1) AT91F_PIOC_CfgPMC ();

Table 6-3. Peripheral Configuration for “CE don’t Care” NAND on AT91SAM9260

Description Settings AT91 libV3 function

NCS3 is CE Output

AT91F_PIO_CfgPeriph(AT91C_BASE_PIOC,

AT91C_PC14_NCS3_NANDCS,0);

PC13 is RDY/BSY (1) Input AT91F_PIO_CfgInput(AT91C_BASE_PIOC, AT91C_PIO_PC13);

Enable PIO clock (1) AT91F_PIOC_CfgPMC ();

6255B–ATARM–26-Jun-09

Application Note

6.3 SMC Timings

The K9F2G08U0M is a 256 MB device connected with an 8-bit data bus width.

An accurate one-to-one comparison is necessary between NandFlash and SMC waveforms for

a complete SMC configuration. Figure 6-1 and Figure 6-2 show two cases that highlight all the

required timings.

Figure 6-1. COMMAND LATCH and ADDRESS LATCH Cycle

Figure 6-2. SERIAL ACCESS Cycle after READ

These timings are summarized in Table 6-4.

ALE/CLE

NCS

NWE_SETUP NWE_PULSE NWE_HOLD

NWE

NCS_WR_SETUP NCS_WR_PULSE NCS_WR_HOLD

NWE_CYCLE

D[31:0]

NCS

NRD_SETUP NRD_PULSE NRD_HOLD

NRD

D[31:0]

NCS_RD_SETUP NCS_RD_PULSE NCS_RD_HOLD

NRD_CYCLE

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