(57) A voltage reference circuit is described which is capable of providing either an
internally generated voltage having a trimming capability, or an externally generated
voltage, with the use of only two pins (2,4). The internal voltage is connected through
an interrupt circuit (T2,T3,R4,R5,R7) to an input/output terminal (2), which can
also receive an externally generated voltage. A trimming terminal (4) is used to
apply trimming voltage signals to adjust the internally generated voltage. To convert
from the internal (8,A1) to the external voltage source (20), an interrupt voltage
is applied to the trimming terminal (4) which is outside of the normal trimming voltage
range. This interrupt voltage actuates an interrupt circuit (T2,T3,R4,R5,R7) to interrupt
the connection (T1) between the internal voltage source (8,A1) and input/output terminal
(2), leaving the output terminal (2) available for the external voltage source (20).
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