Android Question send timeticks (long from datetime) over BLE

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jimseng

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Hello
I clearly don't understand how to use byteconvert. I have a long derived from a datetime dialog (yesterday's post). I now need to convert it into a byte array (manager.WriteData(data() as byte)
I am confused. So my result from my date time picker for example is: 1714156680 (0x662BF488)
How do I convert that into something I can send over bluetooth. In my simple mind I would loop through each character and send it as a byte and re-construct it at the other end but surely there is "the correct" way to do it.
 
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emexes

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jimseng said:
a long ... convert it into a byte array
Click to expand...

Maybe something like:
To Bytes and Back: 
Dim T As Long = DateTime.Now
Log(T)
Log(Bit.ToHexStringLong(T))

Dim bc As ByteConverter
Dim B() As Byte = bc.LongsToBytes(Array As Long(T))

Log(B.As(List))    'works in B4J, might work in B4A

'Bytes are signed ie -128 to 127, use Bit.And if you need them unsigned 0..255 (you don't yet, but you might later)
Log(B.Length)
For I = 0 To B.Length - 1
    Log(I & TAB & B(I) & TAB & Bit.And(B(I), 0xFF))
Next

Dim ReceivedT As Long = bc.LongsFromBytes(B)(0)    'note the (0) at the end, to "convert" the single element Long Array to a Long, by selecting element 0

Log(ReceivedT)
Log(Bit.ToHexStringLong(ReceivedT))
Log output: 
Waiting for debugger to connect...
Program started.
1714165429229
18f1c38b3ed
(ArrayList) [0, 0, 1, -113, 28, 56, -77, -19]
8
0    0      0
1    0      0
2    1      1
3    -113   143
4    28     28
5    56     56
6    -77    179
7    -19    237
1714165429229
18f1c38b3ed
 
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jimseng

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Thanks. I divide my DateTime by 1000 which I believe converts it to seconds (i'm receiving it with micropython). Why does my array start with the first four elements as zeros?
1714169040 (0x662C24D0)
(ArrayList) [0, 0, 0, 0, 102, 44, 38, 116]
Apart from that , this is very helpful.
 
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emexes

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jimseng said:
I divide my DateTime by 1000 which I believe converts it to seconds
Click to expand...

Correct. Ticks are in milliseconds.

Unix time was originally a (signed) 32-bit integer of seconds since 01/01/1970. Maximum value of 31 bits is 2^31-1 ie tad over 2 billion. Divide by 3600 seconds/hour and 24 hours/day and 365 days/year, and you get maximum value of 68 years. 1970 + 68 = 2038 ie the Unix equivalent of Y2K.

B4X time is 64-bit integer of milliseconds since 01/01/1970. With similar math, we get maximum value of 292 million years, so we should be ok for a while. Plus it covers the entire period of human history backwards, whereas (signed 32-bit) Unix time only goes back to ~1902 (ie 1970 = 68) and thus my grandmother born in 1901 would be reborn in 2037.

jimseng said:
Why does my array start with the first four elements as zeros?
Click to expand...

Because dividing by 1000 shifts the number to the right (by 3 decimal digits = approximately 10 binary digits = approximately 1.25 bytes) and so the bits (value) of byte array elements 2 and 3 have moved down to elements 4 and 5. and the zero values of elements 0 and 1 have moved down to elements 2 and 3.

If you're thinking that because the first four elements are zero, that you can ignore them and just send the lower four bytes as a 32-bit Int rather than all eight bytes as a 64-bit Long, then you're back at the Unix 2038 problem. Although, yes, you could treat the four bytes as an UNSIGNED 32-bit value, which would double your 68 year maximum and push the rollover problem to your great-great-grandchildren.
 
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jimseng

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Thanks again for the explanation. I'm guessing that by 2038 I'll simply ask my fridge to program my microcontroller for me based a discussion I have with my heat-pump kettle. And I sensibly decided not to have children so I think I'll lop the first four bytes off.
I am getting the correct value via bluetooth on my Pico now. This has made me happy and I can now go to bed.
 
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