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8254
Contador/temporizador de 16 bits
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Pinagem
3 contadores / temporizadores de 16 bits (0, 1 e 2)GATE, CLK, OUTBus dados (D0..7)Linhas controlo: /cs, /wr, /rd2 linhas endereços (A0 e A1)
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Diagrama de blocos
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Linhas de endereços e controlo
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Palavra de configuração
SC1, SC0 – seleccionam o contador (0, 1 ou 2)RW1, RW0 – Leitura/EscritaM2, M1, M0 – Modo de funcionamentoBCD – Tipo de contagem
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Selecção do contador
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Modo de escrita/leitura
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Modo de funcionamento
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Contagem BCD
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Modos de funcionamento
Mode 0: Interrupt on Terminal CountMode 1: Hardware Retriggerable One-ShotMode 2: Rate GeneratorMode 3: Square Wave ModeMode 4: Software Triggered ModeMode 5: Hardware Triggered Strobe(Retriggerable)
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Sequencia típica de programação
Escrever palavra de controlo em (A0=A1=1)Tipicamente RW0=1 e RW1=1, para contagem inicial a 16 bits (primeiro byte é o menos significativo, e segundo byte o mais significativo)
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Modo 3: Gerador Onda Quadrada
É periódicoRepete-se infinitamenteGate=1 habilita a contagemGate=0 inibe a contagem
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Altifalante no PC
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Sequência de programação