RF Module Qualifier, CRC?

[Bruce]

Have a look at this article. The PIC controls a Linx LR transmitter with data received from the PC serial port. It simulates the Holtek 8-bit encoder, and can control a Linx FCTN-RLY4-xxx receiver/relay, or a Linx KH2 receiver module. Both use the Holtek 8-bit decoder IC.

http://www.myamicus.co.uk/content.ph...Remote-Control

And this thread: http://www.picbasic.co.uk/forum/showthread.php?t=6581 has PBP code for decoding a Holtek 8-bit encoder.

[languer]

Those post from Bruce are very valuable. They show very simple methods to make you comms more robust.

Dhouston also has some very good information on similar subject: http://www.picbasic.co.uk/forum/showthread.php?t=6261

in lamen's terms how I can use CRC?

This is my best attempt to address this in particular (full disclosure - I am no expert). CRC is used in many places when you want to validate the contents of a packet (i.e. detect the packet to be error free). A packet can be anything: wireless data, ethernet data, bootloader data, archive data (read WinZIP). It is a very simple way of coming up with a somewhat unique identifier that tells you the packet you have has not been compromised (i.e. corrupted, changed, etc).

How effective is it? - It depends. There are as many algorithms (polynomials) as days of the year (and then some). It all depends on how "unique" you want to define your packet and how much math (read - computer resources) you want to use to come up with the CRC.

How simple is it? - Again, it depends.

Why use it? - It is pretty standard method of validating packetized data; as long as you have the computing power to do it, and select an algorithm/polynomial and stick with it.

What it does? - It gives you confidence on the validity of your packet (i.e. error detection).

What it does not do? - If your data packet is corrupted, it does not tell you how to correct it (i.e. it does not have error correction).

How you could use it? - On the transmit side, you calculate the CRC of your data packet (after selecting the algorithm you want/need) and send it along with your data packet (usually at the end). On the receive side, you receive the data packet and the CRC. Calculate a new CRC value form the data packet (not including the received CRC) and compare it to the received CRC. If they match you have a higher degree of certainty the packet is free from errors.

Good reads:
http://en.wikipedia.org/wiki/Cyclic_redundancy_check
http://www.hackersdelight.org/crc.pdf
http://www.repairfaq.org/filipg/LINK/F_crc_v3.html

[rmteo]

How effective is it? - It depends. There are as many algorithms (polynomials) as days of the year (and then some). It all depends on how "unique" you want to define your packet and how much math (read - computer resources) you want to use to come up with the CRC.

How simple is it? - Again, it depends.

Why use it? - It is pretty standard method of validating packetized data; as long as you have the computing power to do it, and select an algorithm/polynomial and stick with it.

Many higher-end devices (such as PIC24) implement it in hardware for the reasons you mention.

[Bruce]

Many higher-end devices (such as PIC24) implement it in hardware for the reasons you mention.

Yep. Unfortunately PBP just doesn't support the PIC24, PIC32, or dual-core pentiums..;o) And, for this particular application, that would be like hunting rabbits with a Sherman TANK. Honestly.

You just-do-not-need-to get-that-complex for such simple wireless communications, and it really does NOT require any PIC type beyond what PBP supports.

[rmteo]

I am aware of the limitations and shortcomings of PBP. I was agreeing with languer that implementing CRC in software is resource intensive and may not be the best solution for the OP. That is why it is implemented in hardware - for those that feel they have a need for it. But hey, rabbit hunting with a Sherman tank does sound like fun.

[ScaleRobotics]

implementing CRC in software is resource intensive and may not be the best solution for the OP.

Rmteo, is running CRC so intensive that it will not keep up with the lightning speed of 9600 baud? He will need to move up to a 16 bit chip for and new compiler for that?

I don't think so.

I heard this asked before, but do you use PBP?

[tazntex]

Thanks for all the help but I was looking at Bruce's remote example,
http://www.picbasic.co.uk/forum/showthread.php?t=12554

I thought I could also use the timer portion that could be helpful to jump out of the SERIN2 if no data is received, But ASM??? This is why I bought PicBasic Pro...

From what I understand, very little, I need to add this to my code:

DEFINE NO_CLRWDT 1 ' Watchdog timer is disabled, so we don't need to reset it
DEFINE INTHAND RESET_VT ' Interrrupt on Timer1 overflow to reset outputs in 65.5mS
' if no serial data received in this time period
SYMBOL TMR1IF = PIR1.0 ' Timer1 overflow interrupt flag (reset in int handler)
SYMBOL TMR1IE = PIE1.0 ' Timer1 interrupt enable bit

' Setup Timer1 for resets after ~65.5mS
T1CON = %00000000 ' Internal clock, 1:1 prescale, Timer1 off for now
TMR1L = 0
TMR1H = 0 ' Timer1 low & high bytes cleared
TMR1IF = 0 ' Clear Timer1 overflow flag before enabling interrupt
TMR1IE = 1 ' Enable Timer1 overflow interrupt
INTCON = %11000000 ' Global & peripheral ints enabled
chksum2 = 0 ' Clear match count
GOTO initialize ' Jump over int handler

ASM
RESET_VT
; Do interrupt stuff here
bcf T1CON,TMR1ON ; Stop Timer1
clrf TMR1L ; Clear low byte
clrf TMR1H ; Clear high byte
bcf PIR1,TMR1IF ; Clear Timer1 interrupt flag bit

DON'T KNOW ABOUT THE NEXT TWO LINES, SO I COMMENTED THOSE OUT. I BEEN READY THE DATASHEET FOR THE 16F628A, AND AT BEST GUESS TO CLEAR A SINGLE BIT ON PORTA, WHAT I SEEN IN THE DATASHEET REFERENCING WOULD BE PORTA, 1 OR PORT??1. DOESN'T COMPILE, HOW COULD I CLEAR PARTICULAR BITS HERE FOR PORTA AND PORTB?

;clrf portb ; Clear outputs on button release (or timeout)
;clrf porta ; Clear outputs on button release (or timeout)
bsf T1CON,TMR1ON ; Re-enable Timer1 before exiting interrupt handler
retfie ; Return from the interrupt
ENDASM

And then here is my serial in:

' Fire up Timer1 before entry to serial input routine
T1CON.0 = 1

' at 4MHz Timer1 overflows in 65536 * 1uS (~65.5mS) if no Synch byte
' and serial data arrive on time. SERIN2 timeout & label options
' are useless with a noisy RF receiver output - as noise continually
' resets the timeout period causing it to hang forever.

' Wait for Synch byte, then get new inbound data & checksum

SERIN2 serpin,16468,[wait(254),address1,address2,address3,address4,_
mydata1,mydata2,mydata3,mydata4,mydata5,mydata6]

T1CON.0 = 0 ' Stop Timer1 once we've received data
TMR1L = 0 ' Clear low byte
TMR1H = 0 ' Clear high byte

I compiled this with the rest of my code and it seems to be working fine, but am waiting for the gremlins to pop out.

Thanks a million for all the recommendations already.