Module 2.F. Buffered, Interrupt-Driven Printer Design Example. Tim Rogers 2017

Transcription

1 Module 2.F Buffered, Interrupt-Driven Printer Design Example Tim Rogers 2017

2 Learning Outcome #2 An ability to interface a microcontroller to various devices How? A. Bus Timing Analysis B. 9S12C Multiplexed Bus Expansion C. General-Purpose I/O Ports D. Interrupt Handling E. Buffered I/O F. Buffered, Interrupt-Driven Printer Design Example [2.F]-2

3 Objective Buffered, Interrupt-Driven Printer Design Example IN OUT Interrupts the CPU to consume characters when its ready Produces Characters [2.F]-3

4 What will happen when the interrupt occurs? (A) CPU copies a new character to the buffer (B) CPU copies next value from buffer to data busp (C) Printer copies data off the data bus (D) Printer puts a new value on the buffer (E) Nothing Review: Printer Handshaking STROBE 3 4 When should the interrupt occur? (A) Strobe to low (B) Strobe to high (C) Busy to low P (D) Busy to high (E) When next char does goes on bus BUSY DATA Previous Character Next Character Interrupt here [2.F]-4

5 What does it mean for the interrupt to be masked? Printer Interrupt Enable (PIE) Bit (from CPU) PIE has to be 1 for the interrupt to be asserted Flag Clear (From CPU) 1 CLR D Q Q IRQ (to CPU) Interrupt Request (From Device) [2.F]-5

6 Example Circuit Port X.7. PIE Port Y.2 IRQ Printer STROBE BUSY.. Port Y.3 Port X.0 Port Y.1 1 CLR D Q Q where Port X and Port Y are any available port pins [2.F]-6

7 How to use PIE When buffer is empty: CPU has nothing to send PIE=0 When buffer has content: CPU has stuff waiting to send PIE=1 [2.F]-7

8 IN OUT Life without PIE Printer will continuously trigger interrupt while busy is 0 If buffer is empty printer never becomes busy STROBE BUSY DATA Previous Character Last Character in Buffer [2.F]-8

9 IN OUT Life with PIE When buffer empties: Set PIE= STROBE BUSY PIE Prof. Meyer calls this a Digital Binky DATA Previous Character Last Character in Buffer [2.F]-9

10 [2.F]-10

11 How to use PIE When buffer is empty: CPU has nothing to send PIE=0 When buffer has content: CPU has stuff waiting to send PIE=1 [2.F]-11

12 Driver Declarations smask equ $02 ; STROBE mask imask equ $04 ; PIE enable mask cmask equ $08 ; flag clear mask psize equ 100 ; buffer size pbuf rmb psize; printer buffer pin fcb 0 ; printer IN ptr pout fcb 0 ; printer OUT ptr [2.F]-12

13 Driver Initialization Routine smask equ $02 ; printer STROBE mask imask equ $04 ; printer intr en mask cmask equ $08 ; dev flag clear mask pinit movb #$FF,ddrx movb #$0E,ddry bset porty,cmask bclr porty,cmask ;assert FLG_CLR bset porty,cmask ;to initialize bclr porty,imask ;clear PIE bit clr portx ;ASCII null bset porty,smask bclr porty,smask ;assert STROBE bset porty,smask ;to initialize cli rts ;enable IRQ [2.F]-13

14 Useful Macro Macro for incrementing B register mod BUFSIZE Example: incbmod 20 ; increments B modulo 20 incbmod MACRO incb cmpb #\1 bne $+3 clrb ENDM [2.F]-14

15 Device Driver API Application program interface that places characters in the printer buffer (the producer ) 1. Check status of printer buffer 2. If FULL, wait for space 3. Else, put character in buffer at IN 4. Increment IN modulo BUFSIZE 5. Enable ( unmask ) printer interrupts 6. Exit [2.F]-15

16 Device Driver API ; Buffered printer spooler routine ; Character passed in A register placed in PBUF ; ; STEP (1) Check PBUF status bps ldab pin incbmod psize ;(B)=(PIN+1)mod PSIZE pshb ;save on stack ; STEP (2) If FULL, wait for space pwait cmpb beq pout ;perform FULL check pwait ;wait if PBUF full [2.F]-16

17 Device Driver API ; STEP (3) Put character in PBUF at IN ldx #pbuf ldab pin staa b,x ;PBUF[PIN] = (A) ; STEP (4) Increment IN modulo PSIZE pulb ;retrieve from stack stab pin ;PIN=PIN+1 mod PSIZE ; STEP (5) Enable printer interrupts bset porty,imask ; STEP (6) Exit rts [2.F]-17

18 Device Driver ISR Interrupt service routine that outputs next character in buffer to the printer (the consumer ) 1. Check status of printer buffer 2. If EMPTY, disable ( mask ) printer interrupts and exit 3. Else, clear printer device flag 4. Output character pointed to by OUT 5. Assert printer s STROBE signal 6. Increment OUT modulo BUFSIZE 7. Exit [2.F]-18

19 Device Driver ISR ; Printer interrupt service routine ; ; STEP (1) Check PBUF status prisr ldab pout cmpb pin bne pok ;if not empty, print ; STEP (2) If buffer is empty, disable printer ; interrupts and exit bclr porty,imask rti [2.F]-19

20 Device Driver ISR ; Printer interrupt service routine ; ; STEP (3) Clear printer device flag pok bclr porty,cmask bset porty,cmask ; STEP (4) Output character pointed to by OUT ldx #pbuf ldaa b,x ;(A)=PBUF[POUT] staa portx [2.F]-20

21 Device Driver ISR ; Printer interrupt service routine ; ; STEP (5) Strobe printer bclr porty,smask bset porty,smask ; STEP (6) Increment OUT mod PSIZE incbmod psize stab ; STEP (7) Exit rti pout ;POUT=(POUT+1) mod PSIZE [2.F]-21

22 1. Will Coach need a Digital Binky to make it through this B-ball season? A. yes B. no [2.F]-22