8254 Programmable Interval Timer by vijay

Programmable Interval Timer 8253 / 8254

Programmable Interval Timer 8253 / 8254ByK. Vijay KumarAssistant Professor Dept. of ECE

WHY 8253 / 8254???

8253 vs 8254

Compatible with All Intel and Most other Microprocessors and Handles Inputs from DC to 10 MHz 8 MHz 8254 and 10 MHz 8254-2 The Intel 8254 is a counter/timer device designed to solve the common timing control problems in microcomputer system design. It provides three independent 16-bit counters, each capable of handling clock inputs up to 10 MHz Binary or BCD countingSingle a +5V Supply and Standard Temperature Range Six Programmable Counter Modes and All modes are software programmable. The 8254 is a superset of the 8253. The 8254 uses HMOS technology and comes in a 24-pin plastic or CERDIP package. Used for controlling real-time events such as real-time clock, events counter, and motor speed and direction control.

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Counters:Three Counters C1,C2 & C3Each 16 Bit Identical PresettableDown Counter Operates In BCD /HexControlled By Loading Count To Command Word RegisterOn The Fly Reading Control Logic:CS Logic 0 Enables 8254RD Logic 0 Tells Microprocessor Reads Count From 8254WR Logic 0 Tells Microprocessor Writes Count/ Command Into 8254A1,a0 Address Input Pins To Select Modes And Counters

Data Buffers:8 Bit Bidirectional D0-d7 Connected To Data Bus Of MicroprocessorIn Reads Data From PeripheralOut Writes Data To Peripheral

Control Word Register:Accepts 8 Bit Control Word Written By MicroprocessorCan Only Be Written ( Not Read)Control Word Chooses One Of The Six Modes Of Operation

CSRDWRA1A0OPERATION01000Write Counter 001001Write Counter 101010Write Counter 201011Write Control Word00100Read Counter 000101Read Counter 100110Read Counter 200111No Operation ( Tristated )011XXNo Operation ( Tristated )1XXXX8254 Not Selected

Control Word Format

Mode 0 (Interrupt On Terminal Count )Mode 1 (Programmable Monoshot )Mode 2 (Rate Generator )Mode 3 (Square Wave Generator )Mode 4 (Software Triggered Strobe )Mode 5 (Hardware Triggered Strobe )8254 Modes Of Operation

Mode 0: Interrupt On Terminal CountThe output becomes a logic 0 when the control word is written remains low even after count value loaded in counter.Counter starts decrementing after falling edge of clockThe OUT goes high upon reaching the terminal count & remains high till reloading OUT can be used as interruptWriting a count register , when previous counting is in process first byte when loaded stops the previous count, second byte when loaded starts new countGate high normal countingGate low counting terminated and current count latched till GATE goes high again

Mode 0: Interrupt On Terminal Count

Mode 1: One-shot mode. Monostable multivibratorGate input is used as trigger inputOutput remains high till the count is loaded after application of trigger, output goes low and remains low till count becomes zeroAnother count loaded, when output already low it does not disturb counting until a new trigger is applied at the gateNew counting starts after new trigger pulse

Mode 1: One-shot mode

Mode 2: Rate Generator / Divide by N Counter

When N is loaded as count after N pulses OUT goes low for only one clock cycle then, count N is reloaded OUT becomes high for N clock pulsesThe number of clock pulses between the two low pulses is equal to the count loadedGate logic 0 no countingGate logic 1 normal counting

Mode 2: Rate Generator / Divide By N Counter

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Mode 3: SQUARE WAVE RATE GENERATORWhen count N loaded is even output remains HIGH for half the count and LOW for the rest half of the countWhen count N loaded is odd output remains HIGH for (N+1)/2 and low for (N-1)/2.Repeated operation gives square wave Generates a continuous square-wave with G set to 1.If count is even, 50% duty cycle otherwise OUT is high 1 cycle longer.

Mode 3: Square Wave Generator

After mode is set output goes highWhen count is loaded counting down starts on reaching terminal count output goes low for only one clock cycle, and then again output goes HIGHThe above said low pulse can be used as a strobe for interfacing MP with peripheralsWhen GATE is LOW counting is inhibited and count is latchedIf a new count is loaded while counting, previous counting stops and new counting started in next clock cycle Mode 4: Software triggered Strobe

Mode 4: Software triggered Strobe

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This mode generates a strobe in response to the rising edge at the triggerMode is used to generate a delayed strobe in response to an externally generated signalOnce mode is programmed and counter loaded, OUT goes HIGHCounter starts counting after the rising edge of the trigger (GATE)The OUTPUT goes LOW for one clock period, when the terminal count is reachedOutput will not go LOW until the counter content becomes zero after the rising edge of any triggerGATE is used as trigger inputMode 5: Hardware triggered Strobe

Mode 5: Hardware triggered Strobe

Read OperationsThere are three possible methods for reading the counters: a simple read operation the Counter Latch Command the Read-Back CommandSimple read operation : The Counter which is selected with the A1, A0 inputs, the CLK input of the selected Counter must be inhibited by using either the GATE input or external logic. Otherwise, the count may be in the process of changing when it is read, giving an undefined result.

2. Counter Latch Command:

SC0, SC1 bits select one of the three countersTwo other bits, D5 and D4, distinguish this command from a control wordIf a counter is latched and then, some time later, latched again before the count is read, the second counter latch command is ignored. The count read will be the count at the time the first counter latch command was issued.

3. Read-back control command:

The read-back control, word is used, when it is necessary for the contents of more than one counter to be read at a same time.Count : logic 0, select one of the Counter to be latchedStatus : logic 0, Status must be latched to be read status of a counter and is accessed by a read from that counter

Status register:

Shows the state of the output pinCheck the counter is in null state (0) or notHow the counter is programmed

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