Revision as of 15:10, 2 January 2014

In this tutorial we will see 8051 timers. We will use the 8051 timers to generate a precise delay of 1 sec.

8051 timers/counters

The 8051 has 2 timers/counters.

They can be used to generate precise timing, i.e., we can measure time between events. The unit is then called timer.
It can also be used to count external events, known as counter.
Timer 1 is also used for generating baud rate in serial communication, which we will discuss in the next tutorial

Timer Operation

The Timer 0 is a 16 bit registers as shown. This can be accessed as 2 eight bit registers TL0 and TL1. Same applies to Timer 1.
The 8051 timer and counter is the same unit, but in this tutorial we will discuss only the timer unit to simplify the discussion.

fig 1:Basic Timer

Timer Tick

Fig 1, shows the basic 8051 timer unit. The registers TCON and TMOD affect the timer operation. The clock frequency is divided by 12 and used by the timer unit. Thus if a 11.0592MHz external crystal is used, the timer uses a frequency of 921KHz. Thus timer increments every 1.085μ seconds.

The C/Ṫ = 0 bit of TMOD register selects operation of Timer/counter unit as timer.
The TR bit of TCON register is used to start the timer.

Timer Registers

Timer Register T0/T1

Table. 1: Timer Register
T0
TH0								TL0
D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0

TMOD Register

The TMOD Register specifies the operational mode of the two timers. The higher nibble is used for Timer 1 and Lower for the timer 0 as shown below.

Table 3. Timer Modes
M1	M0	Operation
0	0	13 bit Timer
0	1	16 bit Timer
1	0	8 bit Auto Reload
1	1	Split Mode

Table 2. TMOD Register
TMOD
D7	D6	D5	D4	D3	D2	D1	D0
Gate	C/T	M1	M0	Gate	C/T	M1	M0
Timer1				Timer 0

fig 2:Mode 0

fig 3:Mode 1

fig 4:Mode 2

TCON Register

Table 4. TCON Register
TCON
D7	D6	D5	D4	D3	D2	D1	D0
TF1	TR1	TF0	TR0	IE1	IT1	IE0	IT0
Timer1		Timer 0		Timer Interrupts

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