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Hi,
I'm using a timer and external interrupt to detect the pulse, i can set the timer to run for 5 secs and loop back. But the program cannot vector to the external interrupt. This is my interrupt configurations and routine.
;******************************************** ;-----------INTERRUPTS CONFIGURATIONS-------- ;******************************************** MOV IE,#8CH MOV SWCINT,#000H MOV EIE1,#000H MOV EIE2,#000H MOV EIP1,#000H MOV EIP2,#000H MOV IP,#08H ;**************************** ;-Timer 1 Overflow Interrupt- ;**************************** T1_INT: CLR TF1 ; Clear overflow flag MOV TH1,#0D0H MOV TL1,#0A0H RETI ;********************** ;-External Interrupt 1- ;********************** EX_INT: INC 40H ; Increment 40H RETI
Does the input to the external interrupt need to be digital? Any advice would be nice... Thanks in advance.
My oscillator can only reach a minimum of 2MHz. In this case, the timer will overflow before the pulse come in. And if i take the time between 2 pulse, it will not be accurate. I could not find a 12KHz crystal which can give me 1ms ticks. I can only find 32.768KHz crystal which gives me 0.36ms ticks. Therefore, the program will run very slow and refreshes very slow.
I just thought of a method using 2MHz... Can i just run the timer when the 1st pulse comes in and let it overflow? Everytime it overflow, i increment a variable until it reaches the 2nd pulse. Than i multiply the variable with 0.39sec(Time to overflow) to give me the time. Than i 1/T = F... Than F * 60 = BPM... Just asking whether isit advisable to use the method.
Can i just run the timer when the 1st pulse comes in and let it overflow?
Yes !
That's the whole secret behind measuring long time intervals with a fast timer.
Yes, most people would count timer overflows before having to use special crystals that are not suitable for the rest of the application.
But, as I wrote in your previous thread, you really do not want to base your bmp on the time between two heart beats! The heart is not a metronome where each beat comes at exactly the same time interval. There is a bit of jitter, so you want to measure the time between a couple of heart beats to get an average.
I see... So instead of incrementing a variable until the 2nd pulse, i should increment until the 3-4 pulse?
Correct! No existing (real) heart beat monitor measures the pulse using a single heart beat interval.
I see... Thanks..
natur3
That would be one way to do it, but it would result in a very slow update rate, which might prove very irritating to the user of the device.
Instead, you can measure the beat-to-beat frequency, but output an average value of the last X measurements (the larger X, the smoother the output).
What do u mean by "output an average value of the last X measurements"? What is X measurements?
What is X measurements?
X is a positive number which is larger than 1.
So if X is 4, you take four beat-to-beat measurements and average them.
"4 beat-to-beat measurements"
You mean like from one pulse to 1 pulse, that is considerd one beat-to-beat? If i take 4 beat-to-beat, won't that a very long time?
The concept is called a running average.
You use the time between beat 5 and 1 (4 periods) to emit a bmp.
Then you use the time between beat 6 and 2 (4 periods) to emit a bmp.
Then you use the time between beat 7 and 3 (4 periods) to emit a bmp.
The first output will not be available until after you have detected 5 heart beats. Then, the display may update for every new heart beat detected.
However, since a normal person at the best has a rest pulse just under 30 and normally in the 50-70 bpm, you can ignore the concept of a running average.
If you take the time between 5 beats, a pulse of 28 bmp would give the first result after 9-11 seconds and then a new value every 9 seconds.
With a more normal pulse of 60, the first value would be available afer 4-5 seconds, with a new update every 4 seconds.
A young person at very hard load would with a pulse of 200 get a first value after 1.2-1.5 seconds, and a new update every 1.2 seconds.
I see... Many thanks for your all help...
Regards, natur3