8051 microcontroller lcd countdown timer - c

Question

Currently i faced some problem on my lcd screen. I'm try to do a countdown timer but when i set Hour = 0, Min = 1, the sec hit 0 and my hour turn to some unknown character and min = 59, sec = 59. I'm i missing out something?
void Timer1(void) interrupt 3
{
TF1 = 0;
TH1 = 0xB1;
TL1 = 0XE0;
cd_msec--;
if(cd_msec == 0)
{
cd_msec = 99;
cd_sec--;
}
if(cd_sec == 0)
{
cd_sec = 59;
cd_min--;
}
if(cd_min == 0)
{
cd_min = 59;
cd_hour--;
}
if(cd_hour == 0)
{
cd_hour = 0;
}
if(cd_hour == 0 && cd_min == 0)
{
cd_hour = 0;
cd_min = 0;
}
if(cd_hour == 0 && cd_min == 0 && cd_sec == 0)
{
cd_hour = 0;
cd_min = 0;
cd_sec = 0;
cd_msec = 0;
}
}

Answer 1

I agree with #nielsen that the logic is wrong. You may consider following approach to update all the variables properly at every millisecond tick.
Also, I have assigned milli_sec to 999 considering that you will manage to get a 16 bit variable for it.
if (milli_sec > 0)
{
milli_sec--
} else {
if (second > 0) {
milli_sec = 999;
second--;
} else {
if (minute > 0) {
milli_sec = 999;
second = 59
minute--
} else {
if (hour > 0) {
milli_sec = 999;
second = 59;
minute = 59;
hour--
}
else {
//COUNTDOWN REACHED TO 0
//hour,minute,second,milli_second is zero
}
}
}
}

Answer 2

Your logic is pretty wrong.
Assume the pre-condition:
cd_msec=1; cd_sec=1; cd_min=5;
When the code executes, you'll get:
cd_msec=99; cd_sec=59; cd_min=4;
So a single tick changed the countdown more than 1 sec.
Remember that zero is a valid value! I'll suggest that you rewrite the code so that you check for zero before decrementing.
Something like:
if (cd_msec > 0) {
cd_msec--;
}
else
{
if (cd_sec > 0) {
cd_sec--;
cd_msec = 99; // Assummes 10ms ticks
}
else
{
// Handle case with both cd_msec and cd_sec being zero
// ...
// ....
}
}

Answer 3

You have 0H : 1M : 0S.
You check seconds, seconds is zero. You set minutes = 0.
You check minutes, it is now 0. So you subtract 1 from hours. Hours is already zero. So it wraps around to maybe ~65k.
IMHO it would be better to have only msecs and convert to hours:minutes:seconds only when you update the display (if you need).