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Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 254
KEYPAD DECODER
Each key on the front panel communicates with a decoder IC via a separate analog line. When a key is
depressed the decoder chip notices the change of state of the associated signal; latches and holds the state of
all eight lines (in effect creating an 8-bit data word); alerts the key-depress-detect circuit (a flip-flop IC); translates
the 8-bit word into serial data and; sends this to the I
2
C interface chip.
KEY-DEPRESS-DETECT CIRCUIT
This circuit flips the state of one of the inputs to the I
2
C interface chip causing it to send an interrupt signal to the
CPU