[time-nuts] TruePosition and Arduino Progress

Chris Albertson albertson.chris at gmail.com
Sun May 21 14:32:31 EDT 2017

To add to my last message.

You CAN collect all the data then parse it like you are doing if you
were to move to an interrupt driven serial port reader.   Each
character is then read by the interrupt handler anyplace in a large
circular buffer.   The parcer then reads out of the other and of this

The phlegm with the current code is the parse ingnores serial input
and will drop data, in also ignore the PPS and will as you found drop

Typically in real time processors like your that must be interrupt
driven or they must poll MANY times laster then the data arrives

so as I wrote before, parsing the data stream one character at a tie
is in effect pooling the serial port much faster then characters
arrive.    Adding a ring buffer and interrupts guarantees yo never
miss a character and certainly you need to interrupt in the PPS to
handle the case there three s serial data and the PPS at the same

The ring buffer is like you big string except you data data onto one
end at the same time as soured data off the other.  Hopefully the ring
buffer never has much data in it as the parser should be fathers then
the serial line.  BUT if a PPS happens then the parser in interrupted
while the display updates so th ring buffer might get filled up a
little.  But the ISR terminals the parser clears the buffer.

On Sun, May 21, 2017 at 11:20 AM, Chris Albertson
<albertson.chris at gmail.com> wrote:
> The problem is that you get the ENTIRE string then parse it.  This is
> not going to work well as you found out.   Your CPU spends almost the
> entire time waiting for characters to come in slowly off the serial
> line.  You are just waiting on bits and wasting CPU cycles
> What you need to do is parse one character at a time.   I bet your
> parser reads one character at a time from the string.   Have it read
> one character at a time directly from the serial port.   (Use a state
> machine.  It will work for such a simple  job as this)
> Yes if your CPU was MUCH faster your plan could work.  But on some
> GPSes the data never has a break.   You are trying to do ALL the work
> in the break but actually most of the down time when you should be
> working is between the characters.    There is not a lot of work a
> finite state machine needs to do between characters, just move state
> based on a 'character class" table.       I you ever studied this
> formally, what you are building here is a "lexer" not a parcer.   The
> "Language" is not recursive and you never need to backtrack so it can
> be de-coded literally one character at a time.
> You DO really want the 1PPS to drive an interrupt.   Thisway you just
> continue working on the data stream and don't wait for the PPS.   When
> the PPS happens you do something QUICK. never do anything time
> consuming in the ISR or you will miss the next serial character.
> increment a seconds count and write two bytes the the LCD and exit
> On Sun, May 21, 2017 at 6:45 AM, Ben Hall <kd5byb at gmail.com> wrote:
>> Good morning all,
>> A quick update for those interested on my Arduino code development for the
>> TruePosition boards.  I've got Arduino code together than can read in the
>> serial stream, parse it, and display time, date, number of satellites, and
>> TFOM on a 2x16 LCD display.  It does not do multiple screens, handle survey,
>> or display lat/long yet.
>> What I'm having issues with is handling the 1 PPS.  Ideally, I want to use
>> the 1PPS signal to trigger the display update.  IE:
>> void loop()
>> {
>> getSerialString()  // uses serial.available to pull in the serial data
>> parser()  // this parses the data
>> wait for 1PPS tick to go high
>> if there has been a clock message, updateDispay()  // update the display
>> }
>> This works great when there is a just a clock message.  But when there is a
>> clock message, an extstatus message, and a status message, it seems like it
>> is still parsing when the 1PPS tick comes in...so it will display seconds as
>> follows:  30, 31, 33, 34, 35, 36, 37, 39, etc...
>> (If I don't wait for the 1PPS tick, it seems that my clock is one second
>> fast.  I say "seems" to be fast, as the time agrees with an NTP clock on one
>> computer, but seems a half second slow per GPSCon's time display on the
>> Z3801.  I think I need to put up the antenna and check against WWV.)
>> I've got one of those cheap little USB logic analyzers on order to figure
>> out how much time elapses between the clock, extstatus, status, and 1PPS
>> tick.  I may need something faster than an Arduino Uno to do this.
>> I'm sure there is a way to do this with an interrupt...but I couldn't make
>> that work yesterday.  More to follow.
>> thanks much and 73,
>> ben, kd5byb
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> --
> Chris Albertson
> Redondo Beach, California


Chris Albertson
Redondo Beach, California

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