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DS2223T Datasheet(PDF) 2 Page - Dallas Semiconductor |
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DS2223T Datasheet(HTML) 2 Page - Dallas Semiconductor |
2 / 10 page 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. |
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