Reading and fixing Chinese scales

[BillTodd]

Cheers Irving, that looks promising :)

[irving2008]

Had a bit of time spare today so knocked up this bread board...



and a few tens of lines of C code later...



Many thanks to 'bogstandard' who rummaged in his box'o bits and found me a couple of cables for the scales which saved me some hassle in connecting.

This is running off a remote 1.5V supply generated using a 555 and a -ve charge pump (nit in pic), generating -6v for the RS232 into the VFD display and -1.45v for the scales and the -ve side of the comparators (uing 311 but will replace with a single quad unit) using two diodes as the reference. The PIC is a 16F877 but again will find something smaller for the final box as really dont need a 40pin device - 14 input pins (6 for the scales and 8 for a keypad), 1 UART so probably a 20pin device will do the job.

Bill/Robin, I know you guys have both done something along these lines already, how did you force the scales into fast read mode (currently I do this by hand) and, more importantly, how did you know when they got there and weren't in some 'hold' mode?

[BillTodd]

This is running off a remote 1.5V supply generated using a 555 and a -ve charge pump

Can't you use a spare o/p pin on the PIC instead of the 555?

and a few tens of lines of C code later...

I s C fast enough to read the clock pulses or are you using in-line assembler ? (ISTR the guy on cnczone couldn't get it to work in C)

Bill/Robin, I know you guys have both done something along these lines already, how did you force the scales into fast read mode (currently I do this by hand) and, more importantly, how did you know when they got there and weren't in some 'hold' mode?

If you look at the PIC code I posted on the CNCzone site, you'll see the sequence I used (I don't have it to hand ATM).

Essentially, I measure the data repeat rate from the scale to determine the current mode then apply pulses to change to the desired mode. The clk/data pulse sequence varies from scale to scale so I also test the data type to determine the type of scale.

Hold mode is a PITA because i can see no way to test the scale in in that Mode. In the end, I just used a reliable 'get back to normal' sequence then output the fast mode sequence blindly.

I have a one scale here that requires one extra pulse to get it to fast mode from hold, yet no way to determine the that this is a different type of scale from within the PIC. I will either have to program a special chip just for this scale or change the PIC code so it stores the sequence in EEPROM , that way I can change it from the PC program.

[edit]
Here you go:

;Zero vernier display 
Zero        movf    rxbuff+1,w
        andlw    0xDF
        xorwf    IDbyte,w    ;test correct axis 
        btfss    status,z
        return

        call    zeronorm
        btfss    Mode,Fast    ;if it was in fast mode then setfast
        return
        goto    setFast

;set zero normal mode        
ZeroNorm    call    measure    ;measure checks the clock to determine fast/slow mode
        btfss    Mode,Fflg
        goto    tstslow
pdlp        call    PlsData        ;pulse data until slow
        call    measure
        btfsc    Mode,Fflg    ;test slow        
        goto    pdlp
        call     PlsClk        ;pulse clk (zero switch)
        return

;measure the vernier reading speed (300mS slow,20mS fast)
measure        bcf    CMCON,CIS    ;switch comp to clk input
        call    synlp        ;wait for clock low >1mS
        bcf    Mode,Fflg
;if clock occurs within ~25mS then Fast flag will be set
        movlw    25
        movwf    vdata        ;borrow vdata as counter
m2lp        btfsc    CMCON,COUT
        bsf    Mode,Fflg    ;
        decfsz    vdata+3,f
        goto    m2lp    
        decfsz    vdata,f
        goto    m2lp
        return
Tstslow        call     PlsClk        ;pulse clk
        call    measure        
        btfsc    Mode,Fflg    ;test slow    
        goto    pdlp        ;if fast, pulse data until slow    
        return            ;normal zero

[irving2008]

Its easily fast enough in C whan you're running a 20Mhz clock - this scale is running at 74KHz clock rate or 13.5uS.. the actual C routine to sample the clock, determine the falling edge, read the data line and shift it into a 32bit register is 49 assembler instructions (7 lines of C) which takes 9.8uS. It may be that on another scale it will need optimising... although this compiler is pretty good but being C it tends to use temporary variables in RAM (mainly becuase I haven't yet worked out how to force it to use registers as variables)

 
//now in lead up to second data burst 
unsigned char count=24;
long data = 0;
while(count>0)
{
data >>= 1; //prepare for next bit
while(CK==1); //wait for clock to go low
if(DT==1) //sample data
 data |= 0x00800000;
while(CK==0); //wait for clock to go hi
count--;
}
//do stuff with data...

doubt I could read three scales on one PIC tho in C and I want to do that so the real one will use a 20pin device 18F1220 at 40Mhz...