DS2502

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contains 0s in the same bit positions as the data byte, the programming was successful and the DS2502

will automatically increment its address counter to select the next byte in the EPROM Status data field.

The least significant byte of the new 2-byte address will also be loaded into the 8-bit CRC generator as a

starting value. The bus master will issue the next byte of data using eight write time slots.

As the DS2502 receives this byte of data into the scratchpad, it also shifts the data into the CRC generator

that has been preloaded with the LSB of the current address and the result is an 8-bit CRC of the new data

byte and the LSB of the new address. After supplying the data byte, the bus master will read this 8-bit

CRC from the DS2502 with eight read time slots to confirm that the address incremented properly and the

data byte was received correctly. If the CRC is incorrect, a reset pulse must be issued and the Write Status

command sequence must be restarted. If the CRC is correct, the bus master will issue a programming

pulse and the selected byte in memory will be programmed.

Note that the initial pass through the Write Status flow chart will generate an 8-bit CRC value that is the

result of shifting the command byte into the CRC generator, followed by the 2 address bytes, and finally

the data byte. Subsequent passes through the Write Status flow chart due to the DS2502 automatically

incrementing its address counter will generate an 8-bit CRC that is the result of loading (not shifting) the

LSB of the new (incremented) address into the CRC generator and then shifting in the new data byte.

For both of these cases, the decision to continue (to apply a program pulse to the DS2502) is made

entirely by the bus master, since the DS2502 will not be able to determine if the 8-bit CRC calculated by

the bus master agrees with the 8-bit CRC calculated by the DS2502. If an incorrect CRC is ignored and a

program pulse is applied by the bus master, incorrect programming could occur within the DS2502. Also

note that the DS2502 will always increment its internal address counter after the receipt of the eight read

time slots used to confirm the programming of the selected EPROM byte. The decision to continue is

again made entirely by the bus master, therefore if the EPROM data byte does not match the supplied

data byte but the master continues with the Write Status command, incorrect programming could occur

within the DS2502. The Write Status command sequence can be ended at any point by issuing a reset

pulse.

1-WIRE BUS SYSTEM

The 1-Wire bus is a system which has a single bus master and one or more slaves. In all instances, the

DS2502 is a slave device. The bus master is typically a microcontroller. The discussion of this bus

system is broken down into three topics: hardware configuration, transaction sequence, and 1-Wire

signaling (signal type and timing). A 1-Wire protocol defines bus transactions in terms of the bus state

during specified time slots that are initiated on the falling edge of sync pulses from the bus master.

Hardware Configuration

The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to

drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have an

open drain connection or three-state outputs. The DS2502 is an open drain part with an internal circuit

equivalent to that shown in Figure 7. The bus master can be the same equivalent circuit. If a bi-directional

pin is not available, separate output and input pins can be tied together.

The bus master requires a pullup resistor at the master end of the bus, with the bus master circuit

equivalent to the one shown in Figures 8a and 8b. The value of the pullup resistor should be

approximately 5 kΩ for short line lengths.