DS2223/DS2224

080598 2/10

OPERATION

All communications to and from the EconoRAM are

accomplished via a single interface lead. EconoRAM

data is read and written through the use of time slots. All

data is preceded by a command byte to specify the type

of transaction. Once a specific transaction has been

initiated, either a read or a write, it must be com-

pleted for all memory locations before another

transaction can be started.

1–WIRE SIGNALLING

The EconoRAM requires strict protocols to insure data

integrity. The protocol consists of three types of signal-

ling on one line: Write 0 time slot, Write 1 time slot and

Read Data time slot. All these signals are initiated by the

host.

READ/WRITE TIME SLOTS

The definitions of write and read time slots are illustrated

in Figures 1 through 3. All time slots are initiated by the

host driving the data line low. The falling edge of the data

line synchronizes the EconoRAM to the host by trigger-

ing a delay circuit in the EconoRAM. During write time

slots, the delay circuit determines when the EconoRAM

will sample the data line. For a read data time slot, if a “0”

is to be transmitted, the delay circuit determines how

long the EconoRAM will hold the data line low overriding

the 1 generated by the host. If the data bit is a “1”, the

EconoRAM will leave the read data time slot

unchanged.

COMMAND BYTE

The command byte to specify the type of transaction is

transmitted LSB first from the host to the EconoRAM

using write time slots. The first bit of the command byte

(see Figure 4) is a logic 1. This indicates to the Econo-

RAM that a command byte is being written. The next two

bits are the select bits which denote the physical

address of the EconoRAM that is to be accessed (set to

00 currently). The remaining five bits determine whether

a read or a write operation is to follow. If a write operation

is to be performed, all five bits are set to a logic 1 level. If

a read operation is to be performed, any or all of these

bits are set to a logic 0 level. All eight bits of the com-

mand byte are transmitted to the EconoRAM with a sep-

arate time slot for each bit.

READ OR WRITE TRANSACTION

Read or write transactions are performed by initializing

the EconoRAM to a known state, issuing a command

byte, and then generating the time slots to either read

EconoRAM contents or write new data. Each transac-

tion consists of 264 time slots. Eight time slots transmit

the command byte, the remaining 256 time slots trans-

fer the data bits. (See Figure 5.) Once a transaction is

started, it must be completed before a new transaction

can begin.

To initially set the EconoRAM into a known state, 264

Write Zero time slots must be sent by the host. These

Write Zero time slots will not corrupt the data in the Eco-

noRAM since a command byte has not been written.

This operation will increment the address pointer inter-

nal to the EconoRAM to its maximum count value. Upon

reaching this maximum value, the EconoRAM will

ignore all additional Write Zero time slots issued to it and

the internal address pointer will remain locked at the top

count value. This condition is removed by the reception

of a Write One time slot, typically the first bit of a com-

mand byte.

Once the EconoRAM has been set into a known state,

the command byte is transmitted to the EconoRAM with

eight write time slots. This resets the address pointer

internal to the EconoRAM and prepares it for the

appropriate operation, either a read or a write.

After the command byte has been received by the Eco-

noRAM, the host controls the transfer of data. In the

case of a read transaction, the host issues 256 read time

slots. In the case of a write transaction, the host issues

256 write time slots according to the data to be written.

All data is read and written least significant bit first.

Although the DS2224 has the first 32 bits replaced by

lasered ROM rather than SRAM, it requires 256 write

time slots for a complete write transaction. The data

being sent during the first 32 write time slots has no

effect on the DS2224 other than advancing the internal

address pointer. As stated previously, it is not possible

to change from read to write or vice versa before a trans-

action is completed.