PORTA.4 as analog input

[kblim]

Thank you Melanie,

Thanks for your reply. I have learn something what i have missed. I will try them out and post the result later. Thanks!

Really appreciate your help, Melanie

[kblim]

Hi all,
I have tested the setting, but the PORTA.4 still pull down the signal to 0v.
Just for your info, the signal is Voltage fluctuated between 0 to .9 v.

don't know where i did it wrong......

[kblim]

Anyone can explain to me:

The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin
leakage specification.

1.What does this means?

2. How to measure the impedance for analog sources?

3.How to add or minus the analog sources?

Thanks for the answer....

[Melanie]

Let me expain what is a complex subject in simple terms.

Pretend that the Input pin of a PIC is a 10K Resistor connected between the pin and 0v (it isn't, but just for argument think of it this way). If you now feed that pin with a high-impedance source (eg +5v via a 1M Resistor), whilst you may measure +5v with a DMM on the end of the Resitor BEFORE you connect it, the instant you connect that Resistor to your PIC pin (10k), the voltage will collapse and you will only measure a few mV. Simple Ohms Law determies what has happened. Now this explaination isn't actually correct when dealing with impedance as we have to take the frequency of the signal into account, but will serve aquequately without going into a lecture on AC theory.

Please post your electrical schematic, so that we can understand your circuit. It could be something very basic that you are doing wrong, or you could be doing everything correctly and may have a defective component. It is difficult to give advice when blind.

[kblim]

Hi Melanie,
First of all thanks for your reply. I really appreciate it.

Attached is the schematic. The problem happens at PortA.4. However I already connect the incoming signal to PortB.7 and it is working.


Include "modedefs.bas"
Define OSC 8
Disable Interrupt 'to disable the interrupt bcoz the PortB.0 is in used

define INTRC_OSC, CCPMX_OFF, MCLR_OFF, LVP_OFF, PROTECT_ON
define MCLR_OFF, LVP_OFF

'@ __CONFIG _CONFIG1, _INTRC_OSC_NOCLKOUT&_CP_ALL

@ __CONFIG _CONFIG1, _CCP1_RB3

OSCCON = %01111110 ' 8 MHz, primary clock, internal RC

PIR1 = %00000000 ' clear all flags particularly A/D IF bit 6
SSPCON = %00000000
SSPBUF = %00000000
TXSTA = %00000000
RCSTA = %00000000
OPTION_REG.7 = 0 'weak pullups enable

'pin IO 76543210
TrisB = %10000100
TrisA = %01010101

cmcon =7 'disable the comparator register

ANSEL=%01010100'only the RA4/AN4, RA2/AN2, RB7/AN6 as analog input
ADCON1 = %10000000 ' right justified, AD Clock divided by 2,Vdd = Vref+, Vss = Vref-

CCP1CON = %00001100

CurrentLevelSensor con %00100000 'PortA.4/AN4
WaterLevelSensor con %00010000 'PortA.2/AN2

Start:
ADCON0 = OX2Input
gosub readadc
pauseus 60
Goto Start

'---------------read the ADC here ----------------------------------------
ReadADC:
ox2Result = 0
ADCON0.bit0 =1

'Pauseus 150 'can try 50 ~ 200us
Pause 250

'start the convertion here
ADCON0.bit2 = 1

while ADCON0.bit2 = 1

wend

ox2Result.Lowbyte = ADRESL
ox2Result.highbyte = ADRESH
return
'----------------------------

[Melanie]

So what do you have connected to J3?

In one position (NC), the Relay contact at RLY2 just shorts out the two pins at J3 and not much else.

In the other position (NO) that Relay contact cross connects RB3 (which is pulled up via internal weak pull-ups) via whatever you have plugged into J3 to RA4.

At the moment I'm still puzzled what you are trying to achieve.

[kblim]

Hi Melanie,

This is my explanation:

1.The J3.1 is the incoming signal(iV) that i want to measure, so i connect it to RA4.
2.The iV also connect to a relay at NC. At the normal operation mode, the iV will connect to J3.2 via the NC on Rly2.
3.In abnormal operation mode, the voltage signal from RB.3 will connect to J3.2 via the NO. That means the Rly2 need to be on and the iV need to be cut off from connecting to J3.2.
4.The RB.3 output signal will also based on the iV but with some amendment before PWM on RB.3.

Thanks!

[Melanie]

You're still not giving the whole picture.

OK... so one end of your signal (let's call it the positive end) is connected to 13.1, but what is the other side of your signal connected to? Is it 13.2?

If it's not connected to 13.2, then where is it connected?

What is plugged into 13.2?

What is this signal we are talking about from?

Let's have the whole thing, not a pile of sockets where we are guessing what may or may not be connected properly.