MODES OF 8254 6 DIFFERENT MODES
MODE 0 : Interrupt on Terminal Count
• In this mode, initially the OUT is low.
• Once the count Is loaded in the register, the counter is decremented every cycle and when the count reaches zero, the OUT goes high. This can be used as an interrupt
• The OUT remains high until a new count or a command word is loaded.
MODE 1: Hardware – Retriggerable One-Shot
• In this mode, the OUT is initially high
• When the Gate is triggered, the OUT goes low, and at the end of the count, the OUT goes high again, thus generating a one-shot pulse
MODE 2 : Rate Generator
• This mode is used to generate a pulse equal to the clock period at a given interval.
• When a count is loaded, the OUT stays high until the count reaches 1, and then the OUT goes low for one clock period
• The count is reloaded automatically, and the pulse is generated continuously.
• The count = 1 is illegal in this mode.
MODE 3: Square-Wave Generator
} In this mode, when a count is loaded, the OUT is high.
} The count is decremented by two at every clock cycle, and when it reaches zero, the OUT goes low, and the count is reloaded again.
} This is repeated continuously; thus a continuous square wave with period equal to the period of the count is generated
◦ The frequency of the square wave is equal to the frequency of the clock divided by the count
MODE 4: Software Triggered Strobe
• In this mode, the OUT is initially high; it goes low for one clock period at the end of the count.
• The count must be reloaded for subsequent outputs.
MODE 5:Hardware Triggered Strobe
Thus mode is similar to Mode 4, except that it is triggered by the rising pulse at the gate
• Initially, the OUT is low, and when the Gate pulse is triggered from low to high, the count begins.
• At the end of the count, the OUT goes low for one clock period.
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