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need to digital divide by 2.76

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Dave Koller

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I have found a ratio of 2.76 from the transmission of a Saturn SL2's VSS (Vehicle Speed Sensor NOT power terminal lol) at 230 HZ it goes to the PCM (Power Control Module ie computer) and outs 60MPH to speedometer in pulses at 83.333333 HZ. - hence 230 \ 2.76 = 83.333333... My car is now being converted to electric and I would like to get rid of the PCM to run the speedometer ... VSS will vary from 0 to under 300 HZ and I would like to divide this frequency by 2.76 - always... I know how to do this in software BUT do not want to go that route..
Generating new frequencies with decimal points is hard with embedded micros.. IS there a simple way to do this with flip-flops or programmable chips.. the decimal part has me stumped. I can use PLL and multiple by 100 then divide using 276 but there must be some way I have overlooked... Thanks.

Dave
 

crutschow

Well-Known Member
Most Helpful Member
You could using several binary rate multiplier in series such as the CD4089 Specialty Logic - Rate Multipliers/Frequency Dividers/Timers - CD4089B - TI.com. These allow you to do fractional multiplying of a digital input frequency. Each chip allows multiplication of the input frequency by 1/16 to 15/16.

By cascading them you can get arbitrarily close to your desired divide ratio (to be within the accuracy of your speedometer). For example, if you cascade two chips in the "Add" mode with the first multiply ratio of 5/16 (80/256) and the second multiply ratio of 13/256, the total multiply ratio will be 93/256 for a divide ratio of 256 ÷ 93 = 2.752, which is within 0.3% of the desired value.

For even higher accuracy you could cascade more chips. Edit: For example three chips will give a divide ratio of 4096 ÷ 1484 = 2.7601, within .004% of the desired 2.76.
 
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Dave Koller

New Member
You could using several binary rate multiplier in series such as the CD4089 Specialty Logic - Rate Multipliers/Frequency Dividers/Timers - CD4089B - TI.com. These allow you to do fractional multiplying of a digital input frequency. Each chip allows multiplication of the input frequency by 1/16 to 15/16.

By cascading them you can get arbitrarily close to your desired divide ratio (to be within the accuracy of your speedometer). For example, if you cascade two chips in the "Add" mode with the first multiply ratio of 5/16 (80/256) and the second multiply ratio of 13/256, the total multiply ratio will be 93/256 for a divide ratio of 256 ÷ 93 = 2.752, which is within 0.3% of the desired value.

For even higher accuracy you could cascade more chips.

I think my replies are being held up as a "Newbie" so I can not reply till a moderator OKs me -- but am plodding along thanks so far if this is posted!!
Dave
 
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