Sequential prophet 5 - Potentiometer Multiplexer (Pot Mux); And Analog to Digital Conversion (Adc)

POT MUX/AOC
2-14 POTENTIOMETER MULTIPLEXER (POT MUX)
AND ANALOC-TO-DICITAL CONVERSION (ADC)
The POT MUX and ADC allow the computer to read and therefore record knob settings for a patch.
They are only active in MANUAL and EDIT mode. During PRESET, data for knob settings is read from
the NV PRGM RAM. Four pots are not involved in this stage: VOL, and P-BND, W-MOD, and MTUN
(which are in the COMMON ANALOG circuitry). The remaining 24. all having black knobs, connect
to the POT MUX.
The POT MUX and ADC can be thought of as the reciprocal of the DAC and CV DMUX. The POT MUX
sequentially connects each pot wiper to the ADC. which provides a 7-bit (0 -127) representation of its
setting. Addresses specifying the POTs to be sampled are latched off of the DATA BUS by U210, when
clocked with CSA from the OUT PORT DCOD. The ST CONV signal, latched elsewhere in the
computer, tells the ADC to start converting VMUX. Inside the ADC a binary counter increments until
an external comparator finds its analog equivalent equal to VMUX. The comparison stops the
counter,and sends END CONV to the interrupt (-INT) pin of the CPU. The IN PORT DCOD issues CSF,
driving the counter value on to the bus for recording into the NV PRGM RAM. The POT MUX selects
the next pot, and conversion again begins with ST CONV.
Since the ADC counter increments until it matches VMUX. the conversion time for pots with higher
settings will be longer than for those with lower settings. (See Waveforms 2-1, below.) Under inter¬
rupt control, the loop is not delayed by changing conversion times. The CPU will continue to execute
instructions throughout the conversion period.
2-18