SUART.bas and NMEA.bas

[gramo]

I've used the LS20031 GPS module for a few projects, and have recently stumbled onto an error that took some time to pin down.

When using the software uart module in the following program it works fine (note that the NMEA commands are commented out of the main program loop);

Code: Select all

// device and clock
Device = 18F2620
Clock = 10


// import modules...
Include "NMEA.bas"
Include "Usart.bas"
Include "SUART.bas"


' local variables
Dim NMEAItem As TNMEA
Dim Sentence As String


' program start
USART.SetBaudrate(br38400)

UART.SetTX(PORTB.0)
UART.SetMode(umTrue)
UART.SetBaudrate(sbr9600)

While True
    'If NMEA.GetItem(NMEAItem) And NMEAItem.Valid Then
        Sentence = "XXXXXXXXXXXXXXXXXXXXXXX"
        UART.Write("[Test]",Sentence,13,10)
    'endif    
Wend

And I get this output on the PICKIT 2 UART tool (the correct, 'expected' output);


However, with this code the software uart will not function correctly (NMEA commands now included in the main program loop);

Code: Select all

// device and clock
Device = 18F2620
Clock = 10


// import modules...
Include "NMEA.bas"
Include "Usart.bas"
Include "SUART.bas"

' local variables
Dim NMEAItem As TNMEA
Dim Sentence As String


' program start
USART.SetBaudrate(br38400)

UART.SetTX(PORTB.0)
UART.SetMode(umTrue)
UART.SetBaudrate(sbr9600)

While True
    If NMEA.GetItem(NMEAItem) And NMEAItem.Valid Then
        Sentence = "XXXXXXXXXXXXXXXXXXXXXXX"
        UART.Write("[Test]",Sentence,13,10)
    endif    
Wend

The result (note that some of the data is missing every second line);


I've stripped as much code as possible to simplify the bug, any assistance would be greatly appreciated!

[gramo]

I suspect it was a timing clash with the NMEA interrupt and the software delays in the SUART.bas library. My PCB design ended up with a quick fix - a hardware switch to change the TX output from the PIC to either my PICKit 2 or GPS manually.

Edit: A thought after going through the NMEA.bas library - perhaps manually disabling the USART interrupt while performing other timing crucial tasks would be a solution. Untested though, and perhaps there is a better solution

[ohararp]

gramo, this is interesting. I am looking at some 10 Hz gps modules and I am not sure my software routines are currently fast enough to handle these modules. I will be looking at using the NMEA.bas file here shortly and will report back to you on how things go.

[RadioT]

Is is not possible to use a hardware EUSART? Then you have a buffer. Is the EUSART in use all the time by another app? If not, maybe wire up an XOR circuit to route a couple of gates to the input to the EUSART depending on the functionality needed? Then you run the input much more independently and eliminate the operations spelled out in the software USART routines.

Ohararp, 10 hz sounds interesting but the bit rate must be like 56K +!

-Tom