8051 timer selection

niranjan23 · Oct 16, 2013

I want to know

which timer is selected when we Set TMOD=00H, that mean all the 8 bits are zero's?

And TMOD=00H at this condition which timer will take 8051 default?

And if I want to select Timer 1 in mode 0 or Timer 0 in mode 0 is that TMOD=00H is right??!! Or what will happen in this case while selection of timer in any mode??

& What 8051 CPU will take decision???

embpic · Oct 16, 2013

if you choose TMOD as 00 then no timer will get select and if want to select timer0 then TMOD should be 0x01 and for timer1 then 0x10.
i think you should refer this
https://www.google.co.in/url?sa=t&r...MbjIkjZnn6P1SX3DW-kzR_g&bvm=bv.54176721,d.bmk

bigdogguru · Oct 16, 2013

The TMOD or Timer Mode Special Function Register (SFR) simply determines the mode of either Timer0 or Timer1 when enabled/started.

The purpose of the TCON or Timer Control SFR is to enable/start or disable/stop either Timer0 or Timer1.

Therefore without enabling/starting Timer0 or Timer1 by setting either bits TR0 or TR1 respectfully in TCON SFR, the current value in TMOD SFR is irrelevant.

Concerning Timer Mode 0 operation:

Reference: **broken link removed**

13-bit Time Mode (mode 0)

Timer mode "0" is a 13-bit timer. This is a relic that was kept around in the 8051 to maintain compatability with its predecesor, the 8048. Generally the 13-bit timer mode is not used in new development.

When the timer is in 13-bit mode, TLx will count from 0 to 31. When TLx is incremented from 31, it will "reset" to 0 and increment THx. Thus, effectively, only 13 bits of the two timer bytes are being used: bits 0-4 of TLx and bits 0-7 of THx. This also means, in essence, the timer can only contain 8192 values. If you set a 13-bit timer to 0, it will overflow back to zero 8192 machine cycles later.

Again, there is very little reason to use this mode and it is only mentioned so you wont be surprised if you ever end up analyzing archaeic code which has been passed down through the generations (a generation in a programming shop is often on the order of about 3 or 4 months).

Additional 8051 Timer Information:

Programming Timers on 8051

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8051 timer selection

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