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

2. Learning Outcome #2 Bus Timing Analysis
“An ability to interface a microcontroller to various devices”
Bus Timing Analysis
9S12C Multiplexed Bus Expansion
General-Purpose I/O Ports
Interrupt Handling
Buffered I/O
Buffered, Interrupt-Driven Printer Design Example
How?

3. Interrupts the CPU to consume characters – when its ready
Objective
“Buffered, Interrupt-Driven Printer Design Example”
Interrupts the CPU to consume characters – when its ready
Produces Characters

4. Review: Printer Handshaking
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 bus
(C) Printer copies data off the data bus
(D) Printer puts a new value on the buffer
(E) Nothing
Review: Printer Handshaking
When should the interrupt occur?
(A) Strobe to low
(B) Strobe to high
(C) Busy to low
(D) Busy to high
(E) When next char does on bus 3 4
STROBE 1 5
BUSY 2
Previous Character
Next
Character
DATA

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)
CLR Q
IRQ
(to CPU) 1 D Q
Interrupt Request
(From Device)

6. Example Circuit
PIE

7. How to use PIE When buffer is empty: CPU has nothing to send
When buffer has content: CPU has stuff waiting to send
PIE=1

8. Life without PIE
Printer will continuously trigger interrupt while busy is 0
If buffer is empty – printer never becomes busy
STROBE
BUSY
Previous Character
Last Character in Buffer
DATA

9. Prof. Meyer calls this a “Digital Binky”
Life with PIE
When buffer empties: Set PIE=0
STROBE
BUSY
PIE
Prof. Meyer calls this a “Digital Binky”
Previous Character
Last Character in Buffer
DATA

11. How to use PIE When buffer is empty: CPU has nothing to send
When buffer has content: CPU has stuff waiting to send
PIE=1

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

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 ;enable IRQ
rts
Initialization Routine sends ASCII 0 to “Prime the system”

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

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

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 pout ;perform FULL check
beq pwait ;wait if PBUF full

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

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

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

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

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 pout ;POUT=(POUT+1) mod PSIZE
; STEP (7) Exit
rti

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