TMOD register is special function register in 8051 microcontroller. The function of TMOD register is to select the modes of operation of timer/counter. The modes being Mode0 in which only 13bits are used for timer/counter, Mode1 in which all the 16bits are used. In Mode2 and in Mode3 only 8 bits are used. the selection of modes is done is setting the first two bits of the TMOD register for timer0 and fifth, sixth bits for timer1.

TMOD Register (Timer Mode) This register selects mode of the timers T0 and T1. As illustrated in the following picture, the lower 4 bits (bit0 - bit3) refer to the timer 0, while the higher 4 bits (bit4 - bit7) refer to the timer 1. Bits of this register have the following purpose: * GATE1 starts and stops Timer 1 by means of a signal provided to the pin INT1 (P3.3): o 1 - Timer 1 operates only if the bit INT1 is set o 0 - Timer 1 operates regardless of the state of the bit INT 1 * C/T1 selects which pulses are to be counted up by the timer/counter 1: o 1 - Timer counts pulses provided to the pin T1 (P3.5) o 0 - Timer counts pulses from internal oscillator * T1M1,T1M0 These two bits selects the Timer 1 operating mode. http://www.mikroe.com/en/books/8051book/ch2/ http://www.faqs.org/faqs/microcontroller-faq/8051/ http://www.edsim51.com/8051Notes/8051/timers.html

SFR: TMOD (Timer Mode). The TMOD SFR is used to control the mode of operation of both timers. Each bit of the SFR gives the microcontroller specific information concerning how to run a timer. The high four bits (bits 4 through 7) relate to Timer 1 whereas the low four bits (bits 0 through 3) perform the exact same functions, but for timer 0. The individual bits of TMOD have the following functions: TMOD (89h) SFR Bit Name Explanation of Function Timer 7 GATE1 When this bit is set the timer will only run when INT1 (P3.3) is high. When this bit is clear the timer will run regardless of the state of INT1. 1 6 C/T1 When this bit is set the timer will count events on T1 (P3.5). When this bit is clear the timer will be incremented every machine cycle. 1 5 T1M1 Timer mode bit (see below) 1 4 T1M0 Timer mode bit (see below) 1 3 GATE0 When this bit is set the timer will only run when INT0 (P3.2) is high. When this bit is clear the timer will run regardless of the state of INT0. 0 2 C/T0 When this bit is set the timer will count events on T0 (P3.4). When this bit is clear the timer will be incremented every machine cycle. 0 1 T0M1 Timer mode bit (see below) 0 0 T0M0 Timer mode bit (see below) 0 As you can see in the above chart, four bits (two for each timer) are used to specify a mode of operation. The modes of operation are: TxM1 TxM0 Timer Mode Description of Mode 0 0 0 13-bit Timer. 0 1 1 16-bit Timer 1 0 2 8-bit auto-reload 1 1 3 Split timer mode 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). 16-bit Time Mode (mode 1) Timer mode "1" is a 16-bit timer. This is a very commonly used mode. It functions just like 13-bit mode except that all 16 bits are used. TLx is incremented from 0 to 255. When TLx is incremented from 255, it resets to 0 and causes THx to be incremented by 1. Since this is a full 16-bit timer, the timer may contain up to 65536 distinct values. If you set a 16-bit timer to 0, it will overflow back to 0 after 65,536 machine cycles. 8-bit Time Mode (mode 2) Timer mode "2" is an 8-bit auto-reload mode. What is that, you may ask? Simple. When a timer is in mode 2, THx holds the "reload value" and TLx is the timer itself. Thus, TLx starts counting up. When TLx reaches 255 and is subsequently incremented, instead of resetting to 0 (as in the case of modes 0 and 1), it will be reset to the value stored in THx. For example, lets say TH0 holds the value FDh and TL0 holds the value FEh. If we were to watch the values of TH0 and TL0 for a few machine cycles this is what wed see: Machine Cycle TH0 Value TL0 Value 1 FDh FEh 2 FDh FFh 3 FDh FDh 4 FDh FEh 5 FDh FFh 6 FDh FDh 7 FDh FEh As you can see, the value of TH0 never changed. In fact, when you use mode 2 you almost always set THx to a known value and TLx is the SFR that is constantly incremented. Whats the benefit of auto-reload mode? Perhaps you want the timer to always have a value from 200 to 255. If you use mode 0 or 1, youd have to check in code to see if the timer had overflowed and, if so, reset the timer to 200. This takes precious instructions of execution time to check the value and/or to reload it. When you use mode 2 the microcontroller takes care of this for you. Once youve configured a timer in mode 2 you dont have to worry about checking to see if the timer has overflowed nor do you have to worry about resetting the value--the microcontroller hardware will do it all for you. The auto-reload mode is very commonly used for establishing a baud rate which we will talk more about in the Serial Communications chapter. Split Timer Mode (mode 3) Timer mode "3" is a split-timer mode. When Timer 0 is placed in mode 3, it essentially becomes two separate 8-bit timers. That is to say, Timer 0 is TL0 and Timer 1 is TH0. Both timers count from 0 to 255 and overflow back to 0. All the bits that are related to Timer 1 will now be tied to TH0. While Timer 0 is in split mode, the real Timer 1 (i.e. TH1 and TL1) can be put into modes 0, 1 or 2 normally--however, you may not start or stop the real timer 1 since the bits that do that are now linked to TH0. The real timer 1, in this case, will be incremented every machine cycle no matter what. The only real use I can see of using split timer mode is if you need to have two separate timers and, additionally, a baud rate generator. In such case you can use the real Timer 1 as a baud rate generator and use TH0/TL0 as two separate timers.