TMOD register

As we know there are 2 timer registers in 8051. Timer 0 and timer 1. Both of these registers use the same register called TMOD to set various timer operation modes.

TMOD is an 8 bit register, in which lower 4 bits are for Timer 0 and upper 4 bits are for Timer 1.

See table below. MSB ---- LSB

GATE	C/T	M1	M0	GATE	C/T	M1	M0
TIMER 1				TIMER 0

GATE	Gating control when set. The timer/counter is enabled only while the INTx pin is high and TRx control pin is set. When cleared, the timer is enabled whenever TRx control pin is set.
C/T	Timer or counter; 0 or  clear for timer operation.(connected to input from internal system clock). 1 or set for counter operation(connected to input from Tx input pin).
M1	Mode bit 1
M0	Mode bit 0

What the heck is M1 and M0??

M0 and M1 select the timer mode in TMOD. 4 modes as mode 0, mode 1, mode 2, mode 3. See table.

M1	M0	MODE	OPERATING MODE
0	0	MODE 0	13 bit timer mode 8-bit timer/counter THx with TLx as 5 bit prescaler
0	1	MODE 1	16 bit timer mode. 16 bit timer/counter THx and TLx are cascaded with no prescaler.
1	0	MODE 2	8 bit auto reload. 8 bit auto reload timer/counter  THx holds a value that’s to be reloaded in TLx each time it overflows.
1	1	MODE 3	Split timer mode

C/T (clock/timer) in TMOD

•  This bit in TMOD register is used to know whether it is used as a delay generator or as an event counter. 
•  C/T = 0, then it is used as a timer for time delay generation. The clock source for the time delay is the crystal frequency of 8051. Timer gets pulses from the crystal.
• C/T = 1, then it is used as a counter and gets its pulses from outside the 8051. That is from pins T0 and T1 belonging to port 3. In case of timer 0, when C/T = 1 pin 3.4 provides the clock pulse and counter counts up for each clock pulse coming from that pin. Similarly for timer 1, when C/T = 1 pin 3.5 provides the clock pulse and counter counts up for each clock pulse coming from that pin.

GATE – IN-- TMOD

• One of the bit of TMOD is GATE bit. What does this GATE thingy do or whats its purpose on this heavenly earth and in our pretty little microcontroller??
• Every timer has means of starting and stopping. This can be achieved either by software or by hardware or by both. 8051 gives us facility of both.
• 8051 the start and stop of timer is controlled by way of software by TR(timer start) bits TR0 and TR1, using instructions :
• SETB TR1
• CLR TR1
• For timer 1 and for timer 0:
• SETB TR0
• CLR TR0.
• The software start and stop is achieved when GATE = 0 in TMOD register.
• When GATE = 1 in TMOD register it means hardware control.  Its preferable to use GATE = 0 for most of the time.

Values of TMOD to operate as timers in following mode.

Mode 1 timer 1  0001 0000 = 10h

Mode 2 timer 0, mode 2 timer 1 0010 0010 = 22h

 

Note whatever the frequency of crystal is timer will always function at 1/12 th of the crystal frequency.