Timer Mode Register

"TMOD"

Bit Name Timer Description
_____________________________________________________________
7 Gate 1 Gate bit; when set, timer only runs while INT-bar is high. This bit is used in conjunction with interrupts and will be dealt with later.
6 C/T-bar 1 Counter/timer select bit; when set timer is an event counter, when cleared timer is an interval timer.
5 M1 1 Mode bit 1
4 M0 1 Mode bit 0
3 Gate 0 Gate bit; when set, timer only runs while INT-bar is high.
2 C/T-bar 0 Counter/timer select bit; when set timer is an event counter, when cleared timer is an interval timer.
1 M1 0 Mode bit 1
0 M0 0 Mode bit 0



The functions of the 8-bits of TMOD are described in the above table. The top four bits are for timer 1 and the bottom four bits have the exact same function but for timer 0.


The Gate bits are used in conjunction with interrupts.
As mentioned above, the timers can be used for counting external events or for timing intervals. If you wish the timer to be an event counter you set the corresponding C/T-bar bit. Similarly, if you wish it to be an interval timer you reset the corresponding C/T-bar bit.

There are two mode bits (M1 and M0) for each timer. The table below describes their function.

M1,M0,Mode Description
0 0 0 13-bit timer mode (this mode exists simply to keep the 8051 backwards compatible with its predecessor, the 8048, which had a 13-bit timer)

0 1 1 16-bit timer mode
1 0 2 8-bit auto-reload mode
1 1 3 Split timer mode