An iButtonTM is a chip housed in a stainless steel enclosure. The electrical interface is reduced to the absolute minimum, i.e., a single data line plus a ground reference. The energy needed for operation is either ì stolen? from the data line (ì parasitic power? ) or is taken from an embedded lithium cell. The logical functions range from a simple serial number to password-protected memory, to 64 kbits and beyond of nonvolatile RAM or EPROM, to a Temperature iButton, to a real time clock plus 4 kbits of nonvolatile RAM. Common to all iButtons is a globally unique registration number, the serial 1-WireTM protocol, presence detect, and communication in discrete time slots. Table 1 gives an overview of the available devices.For read operations all devices are satisfied with a 5 k
pullup resistor to supply energy and to terminate the 1-Wire bus. iButton devices based on nonvolatile RAM (DS1991 to DS1996) can also be written using this same interface. Due to their different technology, EPROM-based iButtons (DS1982 to DS1986) also require pulses of up to 12V for programming. Since they cannot be erased, EPROM iButtons are referred to as Add-Only Memories. Another device, the DS1920 Temperature iButton, gets its energy for temperature conversion through a low-impedance, active pullup to 5V. Different requirements for writing or special functions are the reason for several types of interfaces.