DELAY and the 8051

Hi All

I've obtained the "The 8051 Micro Controller and Embedded Systems Using Assembly and C-2nd-Ed -", and in this manual the DELAY routine is described as below, which really confuses me a lot

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In original 8051, one machine cycle lasts 12 oscillator periods So, 1 oscilator period for a 12Mhz Crystal will be 12.0 Mhz / 12 = 1Mhz Therefore one machine cycle is is 1/1Mz = 1us (1 micro second)

For 12Mhz the delay is as follows for this example: DELAY : MOV R3 ,#200 ;= 1 Machine Cycle REPEAT: DJNZ R3,REPEAT ;= 2 Machine Cycles RET ;= 2 Machine Cycles END

Therefore the above routine equals to a delay of 403us

"#200" is equal to a delay of , [(200x2)+1+2]x1us = 403us (403 Microseconds)

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If the above is 100% correct, how will a 1 Second delay look like ?

I see a few examples showing a 1 sec delay as " MOV R1, #0FFH" - but don't seem to "reverse engineer" the "#0FFH" back to 1 Second using the above information

Can someone please help me to understand this, I don't just want to use this without knowing what I do

Kind Regards Lodewicus Maas

Reply to
Lodewicus Maas
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mov r3,#0ff mov r4,#0ff mov r5,#08 delay: djnz r3,delay 256x2uS ( 2 instructions)= approx 512uS djnz r4,delay 256xx512uS = 131 mS approx djnz r5,delay 8 x 131 mS = appex 1 sec ret sort of thing... Much better to use a timer counter set to time at say 1mS and do the 'counting ' in the RTC interrupt routine...

Reply to
TTman

The delay routine timing you posted is correct. The information about the 1 sec delay is, obviously, missing some information. The 'standard' way of making longer delays is to make a fixed delay and call it multiple times, think of nested loops, or calling a fixed delay in a loop.

Reply to
WangoTango

Hi TTman

You're completely lossing me here - Hex FF is equal to 255 - is this 0 to

255 which is in fact 256 ?:

In line 1) you set delay=512us(2us x ff), and then in line 2) you multiply the delay set in line 1) with 256 (ff), and then in line 3) you multiple the new delay value set in line 2) with 8

Do I understand this correctly ?

Can I build up a few "DELAY" routines to obtain different delay timings like DELAY_1, DELAY_2 ........ DELAY_N, everytime using its own r3,r4, .........rn value , or is there any limit to the total DELAY routines which you can declare ?

mov r3,#0ff mov r4,#0ff mov r5,#08

1) delay: djnz r3,delay 256x2uS ( 2 instructions)= approx 512uS 2) djnz r4,delay 256xx512uS = 131 mS approx 3) djnz r5,delay 8 x 131 mS = appex 1 sec

Lodewicus Maas

Reply to
Lodewicus Maas

YES....

YES, ABSOLUTELY, BUT U MAY NEED R5 AS WELL

Reply to
TTman

Thank you TTman - much appreciated

Reply to
Lodewicus Maas

Setting the register to #0ff only is 255 If you start with #00 then it will be 256

Reply to
Eddards

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his

Now that others have shown you how to do it, you should also know that you should almost never waste processor cycles in long delays, at least in "real" (as opposed to educational or hobby) applications. For example, in your case of a 1 second delay your processor could have done a million clock cycles worth of useful work.

So go ahead now and understand how to nest delays, but also understand that in real life a 1 second delay would usually be achieved in some other, more efficient manner (typically involving timers and interrupts).

Mike

Reply to
snarflemom

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