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AN449 Measure resistance and capacitance without an A / D

Author: Don Sherman December 1993

MICROCONTROLLER PRODUCTS
AN449 Measure resistance and capacitance without an A / D
Author: Don Sherman December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
Author: Don Sherman
AN449
1.0 VCC
Time in Seconds
SU00438
Figure 2.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
Table 1.
C GATED TIMER INPUT LM324
1N914 LM324 DRIVER
SU00439
Figure 3.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
Table 2.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
87C751
OC DRIVER HC
87C751
DRIVER 0 74HC05 OC HC
VCC C0 R1
GATED TIMER INPUT
HC DRIVER 1
74HC05 OC HC C1
PROGRAM A2D82
GATED TIMER INPUT 74HC05 OC HC
(a) Single Resistor Conversion
74HC05 OC HC
(b) Dual Resistor Conversion
SU00440
Figure 4. family derivatives (i.e. SDA, SCL, Vpp on the 87C751). With these open drain outputs, the open drain buffer can be eliminated as shown in Figure 5.
OPEN DRAIN C
SU00441
GATED TIMER INPUT
PROGRAM A2DS02
Figure 5. The previous circuits will work with just about any type of microcontroller port I / O. Some microcontrollers can be programmed to have an open drain output configuration and some port outputs are only available as open drain. Examples of the latter type are port 0 of the 87C51 family and I2C and Vpp pins on some of the 87C51
In Figure 6, the timer runs from the release of the capacitor hold down to the detection of a low level on the Software Timer Input port pin. The heavy dark lines labeled "Time" indicate the timing periods for each of the four capacitor timing circuits. The detection of a logic high on the one capacitor which is integrating current causes the Nor gate to go low. The software timer halts when the logic low is detected and then the driver transistor is turned on to discharge the capacitor. As soon as the capacitor voltage goes below the threshold for logic one, the Nor gate returns to a high logic level as shown on the "Input" section of Figure 6.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
87C51
PORT 0.3 C PORT 0.2 VCC PORT 0.1 R 74HC4002 DUAL 4-INPUT NOR
PORT 0.0 C
GATED SOFTWARE TIMER INPUT
C0 INPUT
TIME C0 C1 C2 C3
SU00442
Figure 6. In Figure 7 we have the simplest embodiment of the RC timer. This configuration will not allow the use of the Gated Timer, since the Gated Timer input pin is not open drain. The software counter technique is used, where a counter is decremented and the input is checked for a logic high (see Figure 8). When the counter is stopped, the value is inverted to give a count-up appearing output. Ground bounce on the microcontroller caused by the clock distribution is probably the main source of the noise with this configuration. The result is an 8 bit answer, but it is only repeatable to within 5 bits of accuracy.
OPEN DRAIN C
PROGRAM A2DS01 Figure 7.
SU00443
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
ADLOOP: JB DJNZ
SDA, OVER
OVER:
SU00444
Figure 8.
87C751
DRIVER D 74HC00 GATED TIMER INPUT E 74HC00
F 74HC00
A PROGRAM A2DS06 B
SU00445
Figure 9.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
80C51
DRIVER 7 C7 DRIVER 6 DRIVER 5 DRIVER 4 74HC30 8 IN NAND
DRIVER 3 VCC DRIVER 2 DRIVER 1 DRIVER 0 C0 GATED TIMER INPUT R0
SU00446
Figure 10.
OPEN DRAIN
TEST RESULTS
GATED TIMER INPUT
PROGRAM A2DS04
SU00447
Figure 11.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
74HC4017
80C51
Q7 C7 Q6 DRIVER CP0 Q5 Q4 RR Q3 RESET Q2 CR CP1 Q0 C0 Q1 R0 VCC 74HC30 8 IN NAND
COUNTER INPUT
DRIVER
RESET
COUNTER INPUT "C0" "C1" "C2"
SU00448
Figure 12.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
74HC138 80C51
Y7 DRIVER 2 DRIVER 1 DRIVER 0 VCC A2 Y6 A1 Y5 A0 Y4 C7
74HC30 8 IN NAND
Y3 VCC Y2 Y1 Y0 C0 R0
COUNTER INPUT
SU00449
Figure 13.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
87C750
C0 DRIVER 0 R0 VCC
C1 DRIVER 1 R1 PROGRAM A2D85
GATED TIMER INPUT
DRIVER 0
DRIVER 1
GATED TIMER INPUT
SU00450
Figure 14.
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
80C51 ASSEMBLY LANGUAGE PROGRAMS
One of the programs has been listed below to show the section for the statistical data gathering. Note that the actual A to D code is quite short. The other programs which were used are available on the Philips Bulletin Board System, BBS. The BBS phone number and the data sheets for the 80C51 family products are available in the Philips IC20 "80C51-Based 8-Bit Microcontrollers" data book.
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
December 1993
Philips Semiconductors
Application note
Measure resistance and capacitance without an A / D
AN449
December 1993
Philips Semiconductors Microcontroller Products
Application note
Measure resistance and capacitance without an A / D
AN449