Hi, I've just got this GPS module:
http://www.amazon.co.uk/gp/product/B00H28RUSS?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00
I'm using TinyGPSPlus to communicate with it. This works fine, the test programmes run well.
However, I'm making a GPS speedometer/accelerometer/compass thing for my car with it, displaying things on 4 7 segment displays. I can drive the display happily and I can read data from the GPS module, so long as I display it through serial monitor. I can't run both at the same time!
It's quite impossible to describe or explain what's happening with the display, so I'll upload a video. The way I drive the display is I load the first digit into the LED driver's shift register (these drivers sink current btw), then I enable the outputs of the driver at the same time as supplying power to the first 7 segment display. Then I switch to the second display, etc. All I can deduce from what happens is that the programme is periodically halted, and I know it's due to the communication with the GPS module.
This is my code. I apologise in advance, I'm not a programmer 
#include <math.h>
#include <TinyGPS++.h>
#include <SoftwareSerial.h>
static const byte RXPin = 0, TXPin = 1;
static const int GPSBaud = 9600;
// The TinyGPS++ object
TinyGPSPlus gps;
// The serial connection to the GPS device
SoftwareSerial ss(RXPin, TXPin);
// Outputs
byte sdi = 8;
byte clk = 9;
byte oe = 7;
byte digits[4] = {4,5,2,3};
// Inputs
byte but = 6;
byte index = 0;
byte index2 = 0;
byte mode = 0;
String modes[5] = {"SPEd", "ACEL", "ALt", "InCL", "dIr"};
byte butpressed = 0;
byte dec = -1;
byte segments[8] = {0,0,0,0,0,0,0,0};
float numbers[50];// 50
float number = 0.0;
char vals[10];
String str = "0000";
// the setup routine runs once when you press reset:
void setup() {
ss.begin(GPSBaud);
analogReference(EXTERNAL);
pinMode(clk, OUTPUT);
pinMode(sdi, OUTPUT);
pinMode(oe, OUTPUT);
pinMode(digits[0], OUTPUT);
pinMode(digits[1], OUTPUT);
pinMode(digits[2], OUTPUT);
pinMode(digits[3], OUTPUT);
pinMode(but, INPUT);
}
// the loop routine runs over and over again forever:
void loop() {
index++;
index2++;
digitalWrite(digits[index], LOW);
digitalWrite(digits[(index+1)%4], HIGH);
numbers[index2] = valuek();
number = mean();
smartDelay(0);
getStr();
if (dec != -1 && index >= dec) {
displayFigure(str[index+1]);
}
else {
displayFigure(str[index]);
}
if (index == 3) {
index = -1;
dec = -1;
}
if (index2 == 49) { //49
index2 = -1;
}
}
float mean() {
float sum = 0;
for (byte x = 0; x < 50; x++) {
sum += numbers[x];
}
return sum/50.0;
}
String splitString(String txt) {
byte i = txt.indexOf('.');
if (i == -1 or i > 3) {
dec = -1;
return txt;
}
else {
dec = i;
return txt;
}
}
void buttonTest() {
if (digitalRead(but) == LOW) {
if (butpressed == 0) {
mode++;
}
if (mode == 5) {
mode = 0;
}
str = modes[mode];
butpressed = 1;
}
else {
butpressed = 0;
}
}
void getStr() {
switch (mode) {
case 0:
dtostrf(number, 4, 2, vals);
str = vals;
buttonTest();
str = splitString(str);
break;
case 1:
dtostrf(number, 4, 1, vals);
str = vals;
buttonTest();
str = splitString(str);
break;
case 2:
dtostrf(number, 4, 2, vals);
str = vals;
buttonTest();
str = splitString(str);
break;
case 3:
dtostrf(number, 4, 0, vals);
str = vals;
buttonTest();
str = splitString(str);
break;
case 4:
dtostrf(number, 4, 2, vals);
str = vals;
buttonTest();
str = splitString(str);
break;
}
}
float valuek() {
float x = ((((analogRead(A0)*3.29)/1024.0)-1.64)/0.333333)*9.80665;
float y = ((((analogRead(A1)*3.29)/1024.0)-1.62)/0.3326)*9.80665;
float z = ((((analogRead(A2)*3.29)/1024.0)-1.64)/0.351)*9.80665;
switch (mode) {
case 0:
return gps.speed.mph();
break;
case 1:
return sqrt(sq(x) + sq(y));
break;
case 2:
return gps.altitude.meters();
break;
case 3:
if (z <= 9.80665) {
return acos(z/9.80665)*180/M_PI;
}
else {
return 0;
}
break;
case 4:
return 0.0;
break;
}
}
static void smartDelay(unsigned long ms)
{
unsigned long start = millis();
do
{
while (ss.available())
gps.encode(ss.read());
} while (millis() - start < ms);
}
void displayFigure(char fig) {
switch (fig) {
case '0':
segments[1] = 1;
segments[2] = 1;
segments[3] = 1;
segments[5] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case '1':
segments[1] = 1;
segments[7] = 1;
break;
case '2':
segments[0] = 1;
segments[2] = 1;
segments[3] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case '3':
segments[0] = 1;
segments[2] = 1;
segments[3] = 1;
segments[5] = 1;
segments[6] = 1;
break;
case '4':
segments[0] = 1;
segments[1] = 1;
segments[3] = 1;
segments[5] = 1;
break;
case '5':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[5] = 1;
segments[6] = 1;
break;
case '6':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[5] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case '7':
segments[2] = 1;
segments[3] = 1;
segments[5] = 1;
break;
case '8':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[3] = 1;
segments[5] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case '9':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[3] = 1;
segments[5] = 1;
segments[6] = 1;
break;
case 'S':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[5] = 1;
segments[6] = 1;
break;
case 'P':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[3] = 1;
segments[7] = 1;
break;
case 'E':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case 'd':
segments[0] = 1;
segments[3] = 1;
segments[5] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case 'A':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[3] = 1;
segments[5] = 1;
segments[7] = 1;
break;
case 'C':
segments[1] = 1;
segments[2] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case 'L':
segments[1] = 1;
segments[6] = 1;
segments[7] = 1;
break;
case 'I':
segments[1] = 1;
segments[7] = 1;
break;
case 't':
segments[0] = 1;
segments[1] = 1;
segments[7] = 1;
break;
case 'n':
segments[0] = 1;
segments[5] = 1;
segments[7] = 1;
break;
case 'r':
segments[0] = 1;
segments[7] = 1;
break;
case '-':
segments[0] = 1;
break;
case 'o':
segments[0] = 1;
segments[1] = 1;
segments[2] = 1;
segments[3] = 1;
break;
}
if (index == dec-1){
segments[4] = 1;
}
digitalWrite(oe, HIGH);
for (byte x = 0;x < 8; x++){
digitalWrite(sdi, segments[x]);
segments[x] = 0;
digitalWrite(clk, HIGH);
digitalWrite(clk, LOW);
}
digitalWrite(oe, LOW);
}
I was wondering if my problem has anything to do with the smartDelay() function. I just copied it from the TinyGPS example code because it made it work, but I don't really understand what it's doing.
Thank you very much in advance for any help you can give me 
(The video is to come)