WM72016 – Secure F-RAM with Gen-2 RFID and Serial Port

Rev. 1.4

May 2011

Page 12 of 32

Wrap Enable: Asserting the WRPEN control bit to a logic one enables the USER memory wrapping feature.

The wrap enable feature allows the stored address pointer to wrap back to the factory-set initial stored address

value of 0x006. In this manner, the WM72016 memory acts as a circular buffer. Clearing the WRPEN control

bit disables wrapping resulting in a write-once memory.

In this case, when the Working Stored Address

reaches the end of user memory, no additional unaddressed write cycles will be possible. The WRPEN and

AUTOLOCK control bits are mutually exclusive – only one of the two control bits may be asserted at any

given time.

Auto Lock Enable: Asserting the AUTOLOCK control bit to a logic one enables memory locking of the USER

memory span between the start of USER memory and the Working Stored Address. The AUTOLOCK and

WRPEN control bits are mutually exclusive – only one of the two control bits may be asserted at any given

time. The automatic locking feature can only be used when AUTOINCR is asserted to a logic one.

Auto Increment: Asserting AUTOINCR control bit to a logic one enables the Working Stored Address

increment function. Upon receiving an unaddressed write cycle, the WM72016 increments the pointer stored

in the Working Stored Address register to point to the next free memory location then writes the cover-coded

data word to the respective memory location.

This functionality removes any requirement for a RFID

interrogator to determine where free USER memory is located and manipulating the memory pointer itself.

WORKING STORED ADDRESS

To better utilize the F-RAM‟s fast write capability, memory has been architected using an optional Working

Stored Address register. The stored address function enables automation of the storage of large blocks of user

data, such as pedigree or tracking information. This feature enables a RFID interrogator the ability to use a

standard Gen2 WRITE command using a designated address of 0x3FFF (0xFF7F EBV-formatted) as a redirect

pointer to use the contents of the Working Stored Address register – this is referred to as an unaddressed write

(UNADDR_WRITE). The Working Stored Address is a USER memory address pointer used to address the

first available USER memory data word as shown in Figure 12. The Working Stored Address is a read/write

register located at address 0x003 in USER memory. It may be used to address USER memory only – it is an

address pointer to a memory location within the USER memory bank and cannot be used to address other

memory banks or memory regions in the WM72016. It may be manually updated by simply writing to USER

memory address 0x003 or will automatically increment when the AUTOINCR control bit in the Control/Status

register is asserted to a logic one and an unaddressed write command is received.

LCK

INIT

EN

RFU

ADDR[9:0]

PLCK

Figure 11. Working Stored Address Register

Table 6: Working Stored Address – Bit Definitions

Bit

Mnemonic

Function

Initial

Value

15

LOCK

Memory locking of this register.

LOCK

PERMALOCK

DESCRIPTION

0

0

Register unlocked

0

1

Register permanently unlocked

1

0

Register writeable only from the

SECURED state

1

1

Register permanently locked

0

14

PERMALOCK

0

13:11

RFU

Reserved for future use

0

10

INITEN

When asserted to a logic ‘1’, sets the contents of the Initial Stored

Address register with the value defined in the 10-bit address field in bits

9 through 0 written to this register using a Gen2 write instruction.

0

9:0

ADDR

Working stored address pointer

006