Serial EEPROM for Plug & Play Monitors

The latest addition to the range of Application Specific Memories made by SGS-THOMSON Microelectronics is a 1Kbit serial EEPROM specifically designed for the new breed of "Plug & Play" monitors. Organised as 128 x 8-bits, the ST24LC21 is fully compatible with the VESA Data Display Channel (DDC) standard, communicating both in DDC1 (Transmit Only) and DDC2B (I2C Bidirectional) modes. The device allows a direct connection between the PC host and the monitor using the standard video cable and 15-pin VGA connector.

Plug & Play systems are characterised by the ability to reconfigure themselves automatically whenever a new PC card or peripheral is added to the system. Central to the Plug & Play concept is that the peripherals include non-volatile memories containing identification information. This information is used by the PC host to configure its video and graphics subsystems and select appropriate software drivers.

The Video Electronics Standard Association (VESA) DDC standard specifies how this information is accessed. DDC1 is a unidirectional data channel that continuously transmits 128 bytes of Extended Display Identification Data (EDID) from the monitor to the host, while DDC2B is a bidirectional data channel based on the I2C protocol. In DDC1 mode, the ST24LC21's clock signal is normally obtained via the Vertical Sync signal available within the monitor, while in DDC2B mode the Data (SDA) and Data Clock (SCL) lines can be directly connected to pins 12 and 15, respectively, of the VGA connector. An SGS-THOMSON ESDA6V1 Transil array can be used to provide maximum protection against ESD and latchup at these inputs.

Operating over a wide 2.5 - 5.5V supply voltage range, the ST24LC21 also offers a hardware write control feature in the bidirectional (read/write) mode. In this case, pin 7 becomes a Write Control input (WC) and a write command will only be executed if WC is taken high before the start of the command sequence and held high throughout the command.

Available in 8-pin plastic DIP and SO packages, the ST24LC21 is aimed primarily at Plug & Play monitors and other PC peripherals but is also suitable for identifying any slave connected to the serial Plug & Play bus.

The number one supplier worldwide of EPROM and serial EEPROM, SGS-THOMSON is also the world's second largest supplier of non-volatile memories.

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