Texas Instruments MSC1210 User Manual

Page 79

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Using Timers to Measure Time

8-5

Timers

It is apparent that the maximum value a timer may have is 65,535 because there
are only two bytes devoted to the value of each timer. If a timer contains the value
65,535 and is subsequently incremented, it will reset—or overflow—back to 0.

8.3.3

TMOD SFR

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:

TIMER 1

TIMER 0

Reset Value

SFR 89H

GATE

C/T

GATE

C/T

00H

GATE (bit 7)—Timer 1 Gate Control. This bit enables/disables the ability of
Timer 1 to increment.

0: Timer 1 will clock when TR1 = 1, regardless of the state of pin INT1.

1: Timer 1 will clock only when TR1 = 1 and pin INT1 = 1.

C/T (bit 6)—Timer 1 Counter/Timer Select.

0: Timer is incremented by internal clocks.

1: Timer is incremented by pulses on pin T1 when TR1 (TCON.6, SFR 88

) is 1.

M1, M0 (bits 5-4)—Timer 1 Mode Select. These bits select the operating
mode of Timer 1.

Mode

Mode 0: 8-bit counter with 5-bit prescale.

Mode 1: 16 bits.

Mode 2: 8-bit counter with auto-reload.

Mode 3: Timer 1 is halted, but holds its count.

GATE (bit 3)—Timer 0 Gate Control. This bit enables/disables the ability of
Timer 0 to increment.

0: Timer 0 will clock when TR0 = 1, regardless of the state of pin INT0 (software
control).

1: Timer 0 will clock only when TR0 = 1 and pin INT0 = 1 (hardware control).

C/T (bit 2)—Timer 0 Counter/Timer Select.

0: Timer is incremented by internal clocks.

1: Timer is incremented by pulses on pin T0 when TR0 (TCON.4, SFR 88H) is 1.

M1, M0 (bits 1-0) Timer 0 Mode Select. These bits select the operating mode
of Timer 0.