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Abstract

A logic device having a state machine for serially transferring data between an AT-compatible mother board and a keyboard having a microcontroller for scanning the keyboard matrix, without the need for a microcontroller on the mother board. A timer is provided to signal a transmission time-out error, and to indicate the start delay before beginning to transfer data when transmitting to the keyboard. Transmission to the keyboard begins by asserting the clock and serial data bit to the keyboard, and waiting for the start delay to expire. The state machine has a chain of 22 states for transferring an 11-bit data frame, the state transitions occurring after the clock from the keyboard or mouse changes polarity. A time-out error state is entered if a timer indicates that a 32 ms period of time has elapsed. Both transmit and receive operations may time-out with the same delay, and enter the same error state. Finally, after the 11-bit chain is completed, and a stop bit sent or received, the state machine terminates for a transmit, or waits for the end of the stop bit if reading from the keyboard. The state machine may be modified to transfer data with a mouse pointing device as well as a keyboard by using multiplexers to select the clock and data from either the mouse of the keyboard, depending upon the transfer device.

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First Claim