Electronic circuit having electrically isolated digital and analog circuitry

In order to reduce noise in a controlling circuit including digital circuitry and analog circuitry constructed together on a single CMOS integrated circuit, a regulator is constructed as part of the integrated circuit. A power supply is directly connected to the digital circuitry, and a voltage output of the regulator is supplied to the analog circuitry. The output state of the regulator may be controlled by a switch. Degradation of the performance characteristics of the analog circuitry while the digital circuitry is being operated can be prevented by electrically isolating the power supply to the digital circuitry from the power supply to the analog circuitry. Since the regulator is built into the integrated circuit, the number of externally provided components is thereby reduced, further contributing to the downsizing of an electronic device such as a camera. Furthermore, even when the power supply of the digital circuitry fluctuates greatly, the effects on the analog circuitry are minimized.

BACKGROUND OF THE INVENTION 
The present invention generally relates to an electronic control circuit 
and more particularly relates to an integrated circuit having digital and 
analog circuitry integrated thereon which are electrically isolated by a 
voltage regulator. 
Conventionally, in an electronic circuit for controlling the operation of a 
device such as a camera, digital integrated circuitry such as a 
calculating circuit, and analog integrated circuitry for performing light 
metering and distance measuring functions, for example, have not been 
combined in a single integrated circuit, but have instead been formed in 
different integrated circuits. This is due to the fact that digital and 
analog circuitry, although capable of being integrated, are fundamentally 
incompatible in terms of their operating characteristics, power 
requirements and noise tolerance. Because of this incompatibility, 
interface circuitry permitting communication between digital circuitry and 
analog circuitry is usually repaired. Consequently, a large space is 
needed to accommodate the circuitry, which is extremely disadvantageous in 
a camera, for example, wherein compactness is of primary importance. 
In this connection, it has been proposed in Laid-Open Patent Application 
No. Hei 1-258459 that digital circuitry, such as a microcomputer for 
processing digital signals, and analog circuitry, such as a photometer 
circuit, for processing analog signals, can be constructed within a single 
chip using CMOS integrated circuit technology and that a power supply for 
the respective digital and analog circuitry can be commonly shared along 
with the same power supply for powering the peripheral circuitry, such as 
a light source, a shutter, a motor, and the like. 
In this proposed integrated circuit, in order to shield the analog 
circuitry from electromagnetic noise produced by the digital circuitry and 
peripheral circuitry, the digital circuitry and control lines therefor are 
physically located away from the analog circuitry. In this manner, the 
performance of the analog circuitry is not deteriorated due to noise 
caused by the digital circuitry. An electrolytic capacitor is connected 
outside the integrated circuitry to suppress spurious variation in the 
power supply voltage supplied to the analog circuitry. 
Even when such countermeasures have been taken to eliminate noise as 
described above, they have not been completely successful. For instance, 
in cases where the analog circuitry must process signals having a very 
small amplitude or when the power supply fluctuates due to the operation 
of the digital circuitry, the performance of the analog circuitry 
nevertheless deteriorates. Furthermore, when an electrolytic capacitor and 
a regulator are externally connected, additional mounting space is 
required and this is also disadvantageous in terms of space. 
SUMMARY OF THE INVENTION 
In order to overcome the aforementioned drawbacks and to maintain high 
performance of the analog circuitry even when the power supply voltage 
fluctuates due to the operation of digital circuitry, in the present 
invention a regulator circuit is formed in an integrated circuit having 
commonly integrated digital and analog circuitry, and the power supplies 
to the digital circuitry and the analog circuitry are thereby isolated. 
A switch means may be provided for turning ON or OFF the output of the 
regulator such that when the power supply of the digital circuitry 
fluctuates greatly (although within permissible limits for digital 
circuitry), the supply of the power to the analog circuitry is 
electrically disconnected such that the analog circuitry becomes immune to 
such power source fluctuation. During these times, the analog circuitry is 
powered by stored energy, for example, by a capacitor connected between 
the regulator output and the analog circuitry. The resulting effect of 
noise on the analog circuitry is thereby minimized. 
In a preferred embodiment of the present invention, both the digital 
circuitry and the analog circuitry are formed in a single CMOS integrated 
circuit, and a voltage regulator is provided therein. The power supply 
terminals are directly connected to the digital circuitry, and the power 
supply as regulated by the voltage regulator is supplied to the analog 
circuitry.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Details of a preferred embodiment of the present invention will now be 
explained in accordance with FIG. 1. In this embodiment, the inventive 
circuit is described as a controlling circuit for an electronic camera. It 
should be appreciated, however, that the present invention may be used in 
any electronic device in which analog and digital circuitry are used. 
A power cell 1 serves as a power supply for the camera, and the cell 1 may 
comprise, for instance, a lithium cell with a nominal voltage Ve of 3 V. 
Any type of power cell may be used, such as a copper-zinc or 
magnesium-type cell. A booster circuit 10 is connected to both terminals 
of the cell 1. The booster circuit 10 is constructed of a coil 11, a 
transistor 12, a diode 13, and a capacitor 14, the combination of which 
constitutes a known chopper-type booster circuit. In the booster circuit 
10 of this embodiment, a comparator 15, an oscillator 16, a resistor 18, 
and a resistor 19 are also included. These components serve to detect the 
boosted voltage of the booster circuit 10 and start or stop the voltage 
boosting operation. A variable output voltage Vs is output by the booster 
circuit 10 and input to an IC 20. 
The IC 20 is a single chip CMOS integrated circuit and contains a 
microprocessor or microcomputer (CPU) 21, a voltage regulator 22, a 
distance measuring circuit 23, and a photometer circuit 24. The distance 
measuring circuit 23 controls a transistor 25 to drive a near infrared 
light-emitting element ("IRED") 26 at a constant current. The distance 
measuring circuit 23 also converts a light signal received by a 
unidimensional semiconductor light-receiving element 27, commonly known as 
a position sensitive device ("PSD"), into distance information and outputs 
this distance information to the microcomputer 21. A photosensor, for 
example, a cadmium sulphide photosensor ("CdS") 28 is connected to the 
photometer circuit 24. The photosensor 28 measures the brightness level of 
the subject being photographed and converts the brightness level into a 
brightness signal and outputs the brightness signal to the microcomputer 
21. 
A shutter drive circuit 30 is driven by the voltage Vs of the booster 
circuit 10 and controls a conventional stepping motor-type shutter 31 in 
accordance with a control signal from the microcomputer 21 to open and 
close blade sectors (not shown) of the shutter 31 for exposing a 
photographic film (not shown). A DC motor drive circuit 40 is powered by 
the voltage Ve of the power cell 1 and controls a DC motor 41 in 
accordance with a control signal from the microcomputer 21 to rotate a 
sprocket 42 for advancing the film. 
The operation of this circuit will now be described. When the power supply 
of the camera is turned ON, the power cell 1 supplies power to the 
microcomputer 21 via the coil 11 and the diode 13. When the voltage 
applied to the microcomputer 21 exceeds a predetermined voltage VO (for 
example, 2.4 V), the microcomputer 21 becomes activated. The microcomputer 
21 first outputs a reference voltage V1 to the comparator 15. The 
comparator 15 compares the reference voltage V1 with a voltage Vd produced 
by dividing the voltage Vs by means of a voltage divider consisting of the 
resistor 18 and the resistor 19 and outputs a high ("H") level signal when 
the voltage V1 is higher than the voltage Vd. 
The output of the comparator 15 is input to an AND gate 17 along with the 
output of the oscillator 16, and the oscillator 16 drives the transistor 
12 upon the condition that V1&gt;Vd. That is, when the voltage Vd is lower 
than the reference voltage V1 output by the microcomputer 21, the output 
of the oscillator 16 is input to the transistor 12 and voltage boosting is 
performed by the boosting circuit 10 such that the voltage Vs is boosted 
to the level of the reference voltage V1. When the voltage Vd is higher 
than the voltage V1, sufficiently high voltages are being applied to the 
respective circuits, and since the comparator 15 outputs a low ("L") level 
signal, no voltage boosting is carried out by the boosting circuit 10. 
Thus, the boosted voltage Vs (for example, 5.0 V) is stored in the 
capacitor 14 and is output to subsequent stages of the circuit. 
When the camera enters the exposure operation, the microcomputer 21 
commences the photographing operation and outputs a voltage V2, which is 
higher than the voltage V1, to the comparator 15, thereby increasing the 
boosted voltage Vs to the level of the voltage V2 (for example, 5.5 V) to 
enable higher power to be supplied to subsequent stages of the circuit. 
The regulator 22 is then turned ON to supply power to the distance 
measuring circuit 23 and photometer circuit 24 at a constant voltage 
despite any further variation in the level of the boosted voltage Vs. 
Next, the microcomputer 21 sends a command to the distance measuring 
circuit 23 to control the transistor 25 to pulse-drive the IRED 26 to emit 
a pulsed infrared light beam. The light reflected from the subject being 
photographed is received by the PSD 27, and the distance to the subject 
being photographed is converted into an electrical signal. When the 
light-emitting operation is completed, the microcomputer 21 ceases 
supplying the voltage V2 to the IRED 26 and the microcomputer 21 
calculates the distance to the subject being photographed from the 
distance signal. The microcomputer 21 then outputs the voltage V1 to the 
comparator 15. 
With the photometer circuit 24 so enabled, the microcomputer 21 sends a 
command to the photometer circuit 24 and converts the brightness of the 
subject being photographed from the CdS sensor 28 into a brightness 
signal. The microcomputer 21 calculates the brightness of the subject 
being photographed from this brightness signal in order to determine an 
optimum exposure time. Then, the microcomputer 21 Controls the shutter 
drive circuit 30 in accordance with an optimum exposure program selected 
in accordance with the distance and the brightness calculated by the 
above-described operation and the shutter 31 is driven to expose the 
photographic film for the calculated exposure time to perform an exposure 
operation. When the exposure is completed, the booster circuit 10 is 
controlled in the above manner and the DC motor 41 is driven to rotate the 
sprocket 42 so that the film 43 is advanced by one frame. When the 
above-explained series of photographic operations is completed, the 
microcomputer 21 turns OFF the regulator 22. 
In FIG. 2 there is shown a timing diagram of the photographing operation 
described above. Before entering into the exposure operation, the 
regulator 22 is OFF (time "a" to "b" of FIG. 2), so that the distance 
measuring circuit 23 and the photometer circuit 24 are deactivated. 
Meanwhile, the voltage Ve of the power cell is raised to a voltage Vs 
which is set at a predetermined reference voltage V1 by the microcomputer 
21 as described above, and the microcomputer 21 is then operated at the 
predetermined voltage V1. When the distance-measuring circuit 23 and the 
photometer circuit 24 are controlled by the microcomputer 21 to shift from 
OFF to ON ("b" of FIG. 2), the voltage Vs is controlled to be boosted to 
the voltage V2 during a time period T1 ("b" to "cl" of FIG. 2) and the 
regulator 22 is turned ON. At this time, both the microcomputer 21 and the 
IRED 26 are operated at the predetermined voltage V2, and both the 
distance measuring circuit 23 and the photometer circuit 24 are operated 
at the voltage Va (for instance, 4.8 V) output by the regulator 22. 
Thereafter, in order to reduce the effect of noise, the booster clock is 
stopped by the microcomputer 21, which ceases to output a voltage to the 
comparator 15, and the microcomputer 21 and the IRED 26 are operated only 
by the charge stored in the capacitor 14. 
When a predetermined time period T2 has elapsed after the boosting 
operation is ceased ("c1" to "d1" of FIG. 2), the microcomputer 21 sends a 
control signal to the distance measuring circuit 23 to pulse-drive the 
IRED 26 for a time period T3 ("d1" to "e1" of FIG. 2). Since the charge in 
the capacitor 14 is used as the load current of the distance measuring 
circuit 23 for the radiation of the IRED 26, at this time the voltage Vs 
gradually falls from the level of the voltage V2. However, since the 
voltage applied to the distance measuring circuit 23 and the photometer 
circuit 24 is controlled by the regulator 22, the stabilized voltage Va is 
supplied to the distance measuring circuit 23 and the photometer circuit 
24 throughout the entire photographing operation, whereby the effects 
caused by fluctuations in the supply voltage to these analog circuits are 
substantially reduced. 
When the light projecting operation ("c1" to "e1" of FIG. 2) is repeatedly 
performed n times, thereby completing the distance measuring process ("en" 
of FIG. 2), the microcomputer 21 outputs the voltage V1 to the comparator 
15, and the distance to the subject being photographed is calculated from 
the distance signal ("f" of FIG. 2). Then, the microcomputer 21 stands by 
for a time period T4 to permit stabilization of the voltages Vs and Vd. 
The microcomputer then controls the photometer circuit 24 to perform light 
metering over a time period T5 ("f" to "g" of FIG. 2). When the light 
metering operation is completed, the regulator 22 is turned OFF to cut off 
the supply of power to the distance measuring circuit 23 and the 
photometer circuit 24. 
A second embodiment of the present invention is shown in FIG. 3 in which a 
switch Sa is provided in the output stage of the regulator 22, separate 
from the switch for controlling the power supply of the regulator 22 
itself. This switch Sa can be controlled by a signal from the 
microcomputer 21 and is, for example, a normally closed switch which may 
comprise a properly biased transistor, a diode, or may be of mechanical 
construction. A capacitor Ca is connected to the output of the regulator 
22. In FIG. 4 there is shown an enlargement of the timing chart of FIG. 2 
from "d1" to "e1". 
Prior to driving the IRED 26, the microprocessor 21 turns OFF (opens) the 
normally closed switch Sa ("a" of FIG. 4) by sending a signal thereto. As 
a result, the output of the regulator 22 is cut off, and the power supply 
to the distance measuring circuit 23 and the photometer circuit 24 is 
supplied exclusively by the capacitor Ca. When the IRED 26 is driven by 
the Vs output of the boosting circuit 10 ("b" of FIG. 4), although the 
voltage Vs is slightly lowered, since the voltage Va across the terminals 
of the capacitor Ca is electrically independent from the voltage Vs, the 
variation in voltage Va is small and is unaffected by the driving of the 
IRED 26. The solid line "A" of FIG. 4 shows the variation in the output 
voltage Va of the regulator 22 when the switch Sa is turned OFF, and the 
broken line "B" of FIG. 4 shows the variation in the output voltage Va of 
the regulator 22 when the switch Sa is turned ON. When the irradiation by 
the IRED 26 is ended ("c" of FIG. 4), the microcomputer 21 turns ON the 
switch Sa to restart the supply of power to the distance measurement 
circuit 23 and the photometer circuit 24 ("d" of FIG. 4). The operation 
defined from "a" to "d" of FIG. 4 is repeatedly performed every time the 
IRED 26 emits light. 
Deterioration of the performance of the analog circuitry while the digital 
circuitry or peripheral circuitry is operating can be prevented by 
isolating the power supply of the digital circuitry from the power supply 
of the analog circuitry by means of a voltage regulator. Since the 
regulator is built-in to the integrated circuit, the number of externally 
provided components is reduced, further contributing to the downsizing of 
the electronic camera. 
Moreover, in the second embodiment, although the power supply of the 
digital circuit fluctuates greatly even within permissible constraints, 
the supply of power to the analog circuit unit is cut off and the effects 
on the analog circuitry are thereby further minimized.