Adc Pic16f877a

[derrick826]

Alright nigel, thanks

Nigel i want to clarify somethings which i particularly kinda blur. how do i maintain at least 100 samples/ cycle for a specific frequency of sinewave (ie 50k hertz sinewave) besides increasing my baudrate?

 

[Nigel Goodwin]

Alright nigel, thanks

Nigel i want to clarify somethings which i particularly kinda blur. how do i maintain at least 100 samples/ cycle for a specific frequency of sinewave (ie 50k hertz sinewave) besides increasing my baudrate?

For a 100 samples of a 50KHz sinewave you would need 5,000,000 samples a second, and to transfer 10,000,000 bytes per second.

But you seem to have suddenly jumped from 50Hz to 50KHz, which makes everything VERY, VERY different.

Perhaps you might try mentioning what you're actually trying to do, rather than keep randomly changing the rules?.

 

[derrick826]

oops that will be a typo , ok let's say its 50khz, and i need 10,000,000 bytes per second... that's mean i need a really really large baud rate right?

if for a really high baud rate, what PIC would you recommend that can sample at least 100 samples/second and TX the datas to the pc via the serial cable throguh rs232?

 

[Nigel Goodwin]

oops that will be a typo , ok let's say its 50khz, and i need 10,000,000 bytes per second... that's mean i need a really really large baud rate right?

if for a really high baud rate, what PIC would you recommend that can sample at least 100 samples/second and TX the datas to the pc via the serial cable throguh rs232?

Like I said above, 9600 baud will send 480 ten bit samples per second, so that easily meets your requirement. Any PIC with A2D will do it, although one with a hardware USART would make it slightly simpler, but even a software UART would work fine.

Check my tutorial 11.4 at which does pretty well what you're looking for?, although it will require speeding up for your use.

 

[derrick826]

Hi nigel,

i found out that when i input a 190hertz frequency of the sinewave, it seems that the data displayed in the hyper terminal is very limited, only 4 samples/cycle. it's worst when i input 5k hertz sinewave

Is there a way i can have more samples on the higher frequency of sinewave of about 5k hertz. My fixed maximum sinewave frequency that i have to plot is 5k hertz

 

[Nigel Goodwin]

I've explained above how to calculate how fast you need to send the data, to sample 5KHz you need more than 10,000 samples per second, and you MUST, MUST, have a sharp 5KHz low-pass filter before the A2D. Transferring 10,000 two byte samples would require a 200,000 baud serial link. Dropping to 8 bit samples would only require 100,000 baud so would be possible at 115Kbaud.

 

[derrick826]

i was quite confused with these calculations as well:

200,000 baud rate = 200,000 bits/sec

since i'm sending (1 sample= 2 bytes), 1 byte got 10bits, 2 bytes = 20bits

200,000bits/sec divide by 20 bits = 10,000 of 2 bytes samples,

since i'm sampling 5khz sinewave ,

so 10,000 divided by 5khz = 2 samples/cycle ?

kinda confused cause eric taught me this calculations

 

[Nigel Goodwin]

i was quite confused with these calculations as well:

200,000 baud rate = 200,000 bits/sec

since i'm sending (1 sample= 2 bytes), 1 byte got 10bits, 2 bytes = 20bits

200,000bits/sec divide by 20 bits = 10,000 of 2 bytes samples,

Yes, just as I said.

since i'm sampling 5khz sinewave ,

so 10,000 divided by 5khz = 2 samples/cycle ?

kinda confused cause eric taught me this calculations

In what way are you confused? - the minimum sampling rate is double the highest frequency, and preferably more (CD is just over 2.2 times maximum frequency) - and like I said, it's essential to lowpass filter the input.

 

[derrick826]

oh i mean about the 10,000 divided bu 5khz = 2 samples/cycle

meaning in one cycle , there is only 2 samples? meaning in one cycle, 2 point is being sampled from the 5khz sinewave?

 

[ericgibbs]

i was quite confused with these calculations as well:

200,000 baud rate = 200,000 bits/sec

since i'm sending (1 sample= 2 bytes), 1 byte got 10bits, 2 bytes = 20bits

200,000bits/sec divide by 20 bits = 10,000 of 2 bytes samples,

since i'm sampling 5khz sinewave ,

so 10,000 divided by 5khz = 2 samples/cycle ?

kinda confused cause eric taught me this calculations

Hi derrick,
Thats not quite accurate.
I explained in my PM,the baud rates needed for ASCII transfer, assuming that you had 4 ascii data bytes and 2 for CRLF.

Also we were discussing a 50HZ sinewave.

If you can accept an 8 bit ADC value and transmit as a binary byte then you can increase your data transfer rate to the PC.
The ADC conversion time is minute compared to the RS232 transmit time.

You said you wanted to use your PC to display the waveforms, not audio reproduction.

If you want to faithfully reproduce the waveforms as a trace on your PC then the sampling rate must be about 5 times the frequency of the waveform you are sampling. If you are talking audio reproduction a much lower sampling rate will be acceptable.

 

[derrick826]

i'm really trying hard to understand , have to forgive me.. sorry..

currently i'm trasmitting the 4 ascii data bytes and 2 for CRLF but from the calculation it seems that the more character i send, the less accurate my sampling data getting lesser.

anyway this is my current coding which can work to trasmit the converted ADC values into ASC11 and display in hyperterminal. the only thing is that a few modifications have to be made so that i can send up to 5k hertz waveform and i kinda lost.

list p=pic16f877, f=inhx8m
include <P16F877.INC>
__config _CP_OFF & _PWRTE_ON & _XT_OSC & _WDT_OFF & _BODEN_OFF & _LVP_OFF

ERRORLEVEL -302 ;Suppress bank warning




cblock h'20'
thirty
delaytime
digit0
digit1
digit2
digit3
resultlbyte
resulthbyte
count
temp
H_byte
L_byte
R0 ; RAM Assignments
R1
R2
endc



org 0x000
goto start

int org 0x004
goto int


start
call initial
CALL INITUSART
again
call delay
call startconv
call check
call result
call B2_BCD
call bcdsplit
call send
goto again

initial
MOVLW h'30'
MOVWF thirty
BCF STATUS,RP1
BSF STATUS,RP0
MOVLW b'00000001'
MOVWF TRISA
MOVLW b'00000000'
MOVWF TRISB
MOVLW b'00000000'
MOVWF TRISD
MOVLW b'10001110'
MOVWF ADCON1
BCF STATUS,RP1
BCF STATUS,RP0
MOVLW b'01000001'
MOVWF ADCON0
RETURN
INITUSART:
BSF STATUS,RP0
BCF TRISC,6 ; make rc6 an output
BSF TRISC,7 ; make rc7 as input
MOVLW .25 ; 9600,n,8,1
MOVWF SPBRG

BSF TXSTA,TXEN ; enable transmit
BSF TXSTA,BRGH ; select high baud rate

BCF STATUS,RP0
BSF RCSTA,SPEN ; enable serial port
BSF RCSTA,CREN ; enable continuous reception
BCF PIR1,RCIF ; clear rcif interrupt flag

BSF STATUS,RP0
MOVLW b'00000000'
MOVWF TRISB
BCF STATUS,RP0


RETURN

delay
MOVLW h'07'
MOVWF delaytime
run
DECFSZ delaytime,f ; 21micro second
goto run

startconv
BSF ADCON0,GO
RETURN


check
BTFSS PIR1,ADIF
goto check
; MOVLW b'00000001'
; MOVWF PORTC
BCF PIR1,ADIF
return

result
BCF STATUS,RP1
BSF STATUS,RP0
MOVF ADRESL,W
BCF STATUS,RP0
movwf PORTB
movwf L_byte
BCF STATUS,RP1
BCF STATUS,RP0
MOVF ADRESH,W
MOVWF PORTD
movwf H_byte
RETURN

B2_BCD
bcf STATUS,0 ; clear the carry bit
movlw .16
movwf count
clrf R0
clrf R1
clrf R2
loop16 rlf L_byte, f
rlf H_byte, f
rlf R2, f
rlf R1, f
rlf R0, f
;
decfsz count, f ; routine for bcd conversion
goto adjDEC
retlw 0
;
adjDEC movlw R2
movwf FSR
call adjBCD ; no bank switching, always
; ; indirect access for RAM
movlw R1
movwf FSR
call adjBCD
;
movlw R0
movwf FSR
call adjBCD
;
goto loop16
;
adjBCD movlw 3
addwf 0,W
movwf temp
btfsc temp,3 ; test if result > 7
movwf INDF
movlw 30
addwf 0,W
movwf temp
btfsc temp,7 ; test if result > 7
movwf INDF ; save as MSD
retlw 0

bcdsplit
movf R1,w ; bring the second digit!
andlw 0F0h ; mask the lower nibble.
movwf digit0 ; send to display routine variable.
swapf digit0,f ; after swaping!


movf R1,w ; bring first nibble
andlw 0Fh ; mask the upper nibble
movwf digit1 ; send to display routine variable.

movf R2,w ; bring the second digit!
andlw 0F0h ; mask the lower nibble.
movwf digit2 ; send to display routine variable.
swapf digit2,f ; after swaping!

movf R2,w ; Again bring the lowbyte
andlw 0Fh ; mask the upper nibble
movwf digit3 ; send it to display routine variable
return

send
MOVFW thirty
ADDWF digit0,w
CALL TXRDY
MOVFW thirty
ADDWF digit1,w
CALL TXRDY
MOVFW thirty
ADDWF digit2,w
CALL TXRDY
MOVFW thirty
ADDWF digit3,w
CALL TXRDY
MOVLW .13 ;crlf
CALL TXRDY
; MOVLW .10
; CALL TXRDY

TXRDY:
BTFSS PIR1,TXIF
GOTO TXRDY;busy,so loop
XMIT1:
MOVWF TXREG;send
RETURN


END

 

[Nigel Goodwin]

currently i'm trasmitting the 4 ascii data bytes and 2 for CRLF but from the calculation it seems that the more character i send, the less accurate my sampling data getting lesser.

If you're sending six bytes per sample then you need 600,000 baud serial transfer rate. You are quite right, and it's blindingly obvious, that the more data you send the longer it takes and the lower your sample rate needs to be.

 

[derrick826]

yea that's why i change my mind into sending 2 bytes instead. cause i have a 10-bit converted ADC values, and let the pc to do the decoding of the 2 bytes.

about the previous calculation regarding on


10,000 divided by 5khz = 2 samples/cycle

meaning in one cycle , there is only 2 samples? meaning in one cycle, 2 point is being sampled from the 5khz sinewave?

my sampling rate have to divided by the amount of the frequency of the sinewave in order for me to tell how many samples percycle are being indicated on the hyperterminal ?

May i know for the internal ADc of my PIC16F877A , what is the maximum sampling rate of the internal ADC?

Tacq+ 12Tad = 38.92us is my maximum sampling rate= 1/ 38.92us = 25.69khz ?
not sure about this calculation either

 

[Nigel Goodwin]

Perhaps you might try telling us exactly what you're trying to do?.

 

[derrick826]

i just want to know if my calculations above as well as my clarification regarding on the 10,000 divided by 5khz = 2 samples/cycle. is correct or not

 

[ericgibbs]

hi derrick,
So that we are all discussing the same problem.

Are you talking about a PC oscilloscope that can display a 5KHz waveform
accurately or audio reproduction?

 

[derrick826]

Yea close to PC oscilloscope that can display a 5khz waveform using a PIC, not only 5khz but less than that too... i'm not to worry about displaying but i want to get the samples correct for any high or low frequency of sinewave input. with these correct samples, i will get to display. but now my samples are inaccurate with high frequencies, hopefully with the correct samples which i acquired i'm able to TX to the pc with these correct datas.. something like a real time situation. that's all, no more other problems beside this.


not audio reproduction ..

 

[ericgibbs]

Yea close to PC oscilloscope that can display a 5khz waveform using a PIC, not only 5khz but less than that too... i'm not to worry about displaying but i want to get the samples correct for any high or low frequency of sinewave input. with these correct samples, i will get to display. but now my samples are inaccurate with high frequencies, hopefully with the correct samples which i acquired i'm able to TX to the pc with these correct datas.. something like a real time situation. that's all, no more other problems beside this.


not audio reproduction ..

In that case, you have to sample at least 5 times the maximum frequency of the waveform you are sampling. Its that simple.
So for a 5KHz waveform you should sample at 5000 * 5 = 25000 samples/sec!

IF you can sample at this frequency, it would mean for a 2.5KHz waveform you would be sampling at 10 samples/waveform cycle, and so on to lower frequencies. At the lower waveform frequencies you would be taking far more samples than required, eg; for say for 50HZ the sampling would be 500/cycle.
The lower frequencies are not the problem

So assume 25000smp/sec and two binary bytes/sample = 50000 bytes/sec!
As you have 10bits/byte of RS232 transmission thats 500,000 bits/sec!!!

As I have suggested before, there is software on the web, that uses the sound card in your PC. Also you could use a 8/12 bit parallel ADC connected to the PC's printer port, I have seen 10KHz PC scopes made this way.

 

[derrick826]

can you tell me more about the sound card and the software thingy? how fast can the soundcard receive data i send from my pic? i was not aware about using the soundcard to do PC scopes.. is there any links to that particular information?

 

[ericgibbs]

can you tell me more about the sound card and the software thingy? how fast can the soundcard receive data i send from my pic? i was not aware about using the soundcard to do PC scopes.. is there any links to that particular information?

hi,
Look here.
https://www.picotech.com/picoscope-oscilloscope-software.html

Google for 'PC scope software'