The 8155 timer consists of two 8-bit registers.

1. 8-bit LSB and 8-bit MSB.
2. In these 16 bits, 14 bits are used for counter and two bit for mode selection.
3. The counter is a 14 bit down counter. It can operate in 4 different modes of operation.



Timer MSB:
M2
M1
T13
T12
T11
T10
T9
T8
Timer LSB:
T7
T6
T5
T4
T3
T2
T1
T0
  

We can select mode using two bits M2 and M1.
M2 M1
00(Mode 0)- Single Square Wave
01(Mode 1)- Square Wave
10(Mode 2)- Single Pulse on TC(terminal count)
11(Mode 3)- Pulse every TC

 Mode 0: In this mode, timer gives only one cycle of square wave, the output remains high for 1/2 count and remain s low for 1/2 count. If count is odd it remains high for (n+1)/2 and low for (n-1)/2. Where n is count value. Wave width depends on two factor: one is Input clock pulse frequency, and the other is count loaded in counter.

http://www.thesatya.com/images/8155m0.png

Mode 1: This mode is similar to single square wave in operation but the when counter becomes zero, the count value is automatically reloaded. Thus it provides continuous square wave.

http://www.thesatya.com/images/8155m1.png


Mode 2: This mode gives a single clock pulse as a output of the end of the count.The output is high normally, but it becomes low for 1 clock pulse and again it will become high and remain high.

Mode 3: This mode is similar to mode 2 but when the counter becomes zero the count value is automatically reloaded. Thus it provides continuous pulses.

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