Working with numbers larger than 255

kingneb

I want to read a nibble (4 bits on the ra channel)·off the RA·channel on the sx28.· I want to multiply that nibble value by 100.· I want to also subtract that value from 1000.· I know there is a trick for this type of operation.· how do you do it?

Thanks,
kingneb·

Chris Savage

Hello,

·· There are plenty of Math routines on www.sxlist.com for doing up to 32-bit math I believe.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

kingneb

You can do it by using two bytes to store the 16 bit value.· The arethmetic is not hard at all.· What I want to do is this.

1.· Read a nibble

2.· Multiply that nibble by 100 (this is for pulse width modulation, calculating the high delay and the low delay) to get the high duration.

3.· Subtract the nibble by 1000 to get the low duration

Here is the Picbasic code:

high_dur var word
low_dur var word
 
here:
 
high_dur = ra * 100 'multiply the ra channel by 100 to get high duration
low_dur = 1000 - high_dur 'get low duration
 
high rb.0
pause_us high_dur
low rb.0
pause_us low_dur
 
goto here

Thanks, kingneb

Paul Baker

Im gathering that the nibble is BCD because your equations produce a negative value for values > 10. I think there is a simpler method of doing the delays than figuring out the number of cycles for high and low periods. Since you are only dealing in multiples of 100 cycles, why not have a delay of 100 cycles (actually a little less to account for the overhead of doing this method) then iteratively loop for how many units of 100 cycle delays?

for the high duration you iterate ra 100 cycle delays,
for the low duration you iterate 10-ra 100 cycle delays (by doing 'mov w,#10' followed by 'sub w,ra')

though you should probably have ra buffered in the event that ra is changed externally between the use of ra when calculating the high duration and the low duration.

Use Gunther's·link in your other post to generate the code for the 100 cycle delay.

The nice thing about this method is you keep all the math as 8 bit values, reducing computational overhead.

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·1+1=10

Post Edited (Paul Baker) : 10/8/2005 2:18:00 PM GMT

kingneb

The nibble is read off PC a parallel port. There are·11 posible settings (speeds) for the PWM. Zero means off and Ten means full speed. This is the code generated for a 100 cycle delay. I am puzzled because I cannot see how to calculate the values that go into a loop like this.

d1 DS 1

;98 cycles
mov w, #$19
mov d1, w
Delay_0:
decsz d1
jmp Delay_0

;2 cycles
nop
nop

Post Edited (kingneb) : 10/9/2005 12:18:18 AM GMT