[Ovmsdev] SIM808/SIM908 GPS accuracy

Edward Cheeseman cheesemanedward at gmail.com
Thu Feb 2 14:59:45 HKT 2017

Hi Mark,

I only found this out because I read the C18 user manual.
Something it has that I haven't seen before is a “long short” - a 24bit signed integer type! (also an unsigned long short too)

I’ve made a spreadsheet which was the basis for the math.
I will try with Mac gcc. Part of the fun is gcc (64bit) will have a vastly different idea of what an int is compared to C18 (8bit)!
I’ll have to change data types between the two compilers.
The “uint8_t” style declarations don’t seem to work - I haven’t found a C18 stdint.h header. I prefer these to remove this sort of ambiguity!

I’ll let you know how that step goes.


> On 2/02/2017, at 7:23 PM, Mark Webb-Johnson <mark at webb-johnson.net> wrote:
> Edwards.
> First I’ve heard of this, but it doesn’t surprise me.
> Testing this sort of stuff is not particularly easy. What we have done in the past is put in a DIAG mode command to allow this function to be called with a given string parameter, and then to print the result. Then, we can script up something to feed in a bunch of input and see what the output is like. But, that does require physical hardware.
> Perhaps also the function could just be run in a standalone C program on the mac? So long as the type sizes are the same, the results should be equivalent (perhaps with some float rounding differences in the original version).
> Regards, Mark.
>> On 2 Feb 2017, at 4:38 AM, Edward Cheeseman <cheesemanedward at gmail.com <mailto:cheesemanedward at gmail.com>> wrote:
>>> On 23/01/2017, at 8:12 PM, Edward Cheeseman wrote:
>>> I’ve started rewriting gps2latlon() to use a long long (64bit) integer multiply and divide.
>>> I see reference to SIM808 and 908 on an arduino shield, so I might have a look to see what they have done.
>> I totally overestimated the pic18F capability.
>> So it turns out the C18, and for that matter, the XC8 compilers treat long long the same as long - 32bit.

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