ELE22MIC Lecture 9

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ELE22MIC Lecture 9

• ASll Examples
• 16 Bit Counters
• Buffalo Jump Table
• Interrupt processing (IRQ/RTI)
• Stack Frame Base Pointer
• Wired OR

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AS11 Example (1)-16 bit counters
16 Bit counter problems loop_counter rmb 2
declare 2 bytes for 16 bit counter then
execute code as follows dec_count dec loop_coun
ter only one byte is decremented
!!! rts How can we fix this problem?
One way is to use a 16 bit register as
follows dec_count ldd FFFF we can loop
65535 times std loop_counter lots_of_loops js
r do_something do something pshy save the
y index register value ldy loop_counter load
value of loop_counter -gt Y dey decrement y
decrement loop_counter sty loop_counter store
value of y-gt loop_counter puly recover saved
y index register value bne loop_again if
(loop_counter ltgt 0), loop again rts
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68HC11 Parallel I/O Control (1)
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AS11 Example (2) - Bit IO (1)
The 68HC11 has single Bit Set Bit Clear
instructions set or clear individual bits at
the selected memory location. The instruction
format is BSET/BCLR MemoryAddress Mask A 1
in the Mask indicates this bit should be
Set/Cleared A 0 in the Mask indicates that
the bit will not be changed. I.e. BSET
bitwise-ORs the Mask onto memory BCLR
bitwise-ANDs the inverse of the Mask onto
memory We can declare equates as follows to
access them Bit0 EQU 00000001
1 Bit1 EQU 00000010 2 Bit2 EQU 00000100
4 Bit3 EQU 00001000 8 Bit4 EQU 00010000
10 Bit5 EQU 00100000 20 Bit6 EQU 01000000
40 Bit7 EQU 10000000 80 AllBits EQU 11
111111 FF LowNyyble EQU 00001111
0F HighNyyble EQU 11110000
F0 LoopMax EQU 100 IOREG EQU 1000 start
address of Configuration Registers PORTB EQU 4
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AS11 Example (2) - Bit IO (2)
FlashLeds Connect LEDs to PortB LDAA LoopMax
A 100 Our loop counter LDX IOREG X
1000 BCLR PORTB, X AllBits PortB
00000000 LOOPAGAIN BSET PORTB, X BIT0
PortB 00000001 BSET PORTB, X BIT1 PortB
00000011 BSET PORTB, X BIT2 PortB
00000111 BSET PORTB, X BIT3 PortB
00001111 BSET PORTB, X BIT4 PortB
00011111 BSET PORTB, X BIT5 PortB
00111111 BSET PORTB, X BIT6 PortB
01111111 BSET PORTB, X BIT7 PortB
11111111 jsr wait_a_bit BCLR PORTB, X
LowNybble PortB 11110000 BCLR PORTB, X
HighNybble PortB 00000000 jsr wait_a_bit BSE
T PORTB, X HighNybble PortB 11110000 BCLR POR
TB, X HighNybble PortB 00000000 DECA BNE
LOOPAGAIN loop 100 times RTS
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Wired OR - IRQ
A Wired-NOR gate is formed by connecting
open-collector device interrupt lines
together. Normal gates would fight causing high
current drain and possibly damage the gate.
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IRQ Processing (1)
Any device requiring attention activates an
interrupt service routine simply by asserting its
interrupt output. The interrupt causes the CPU to
1. Complete the current instruction - delay
interrupt servicing until the current
instruction completes (delay of from 1 (ABA) -
41 (IDIV) clock cycles) 2. Push all registers -
PC, X, Y, AccA, AccB CCR 3. Mask interrupts by
setting the I bit in CCR 3. Fetch the interrupt
vector 4. Jump to the address fetched from the
int. vector
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IRQ Processing (2)
The Interrupt Service Routine (ISR) must then
poll all devices connected to the IRQ pin and ask
each device in turn Did you interrupt
me? Upon finding a device requiring service,
perform the appropriate Input/Output/Processing
and reset the devices interrupt request (so that
upon return the cpu is not immediately
interrupted again). Upon successful servicing,
the ISR must exit without altering the previously
running programs state. The ISR does this simply
by executing the RTI instruction (ReTurn from
Interrupt).
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IRQ Processing (3)
The RTI instruction Pulls all the previously
saved registers from the stack, in the reverse
order that they were pushed, so that all
registers contain the previous values. Pul
CCR, AccB, AccA, Y, X, PC The last pul - Pul PC -
Pulls (pops) the program counter which
effectively transfers program execution to the
instruction immediately following the instruction
completed before the interrupt request was
accepted. Pulling the CCR re-enables the CPUs
ability to be interrupted as the I flag which was
cleared will be cleared again.
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IRQ Processing (4)
If the interrupt mask bit, I, in the CCR is
unmasked during the ISR, then the IRQ line could
cause one or more nested interrupts. This ISR
nesting procedure would normally only be
performed to provide more rapid response to a
higher priority device.
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Stack Frame (1)
In the C language, the function parameters are
normally pushed in the reverse order that they
are listed. This is to facilitate Cs format
statements for example Printf (P13d,
P24d\n, P1, P2) would compile to Push
P2 Push P1 Push FormatStatement JSR Printf
Printf knows where the format
statement will be Add SP, 6 drop the
parameters
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Stack Frame (2)
The subroutine Printf expects to find the format
statement as the first parameter on the stack. It
then parses the statement, and determines how
many more, and what type of, parameters are
required. The return address is also placed on
the stack. Also Local variables are pushed onto
the stack. During the function these variables
can be accessed on the stack frame as follows
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Frame/Base Pointer (2)
For the duration of the function execution a
frame pointer can be set up using the X or Y
registers, and the variables may be accessed
using indexed addressing P2 EQU A P1 EQU 8 Forma
tStt EQU 6 LocalVar1 EQU 2 LocalVar2
EQU 0 TSX X SP1 (Setup Frame
Ptr) LDD P1, X D P1 (Fetch
P1) STD LocalVar1, X LocalVar AccD P1
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Buffalo Routines Jump Table (1)
ORG ROMBS1F7C .WARMST JMP MAIN
warm start .BPCLR JMP BPCLR clear
breakpoint table .RPRINT JMP RPRINT
display user registers .HEXBIN JMP HEXBIN
convert ascii hex char to binary .BUFFAR JMP
BUFFARG build hex argument from
buffer .TERMAR JMP TERMARG read hex
argument from terminal .CHGBYT JMP CHGBYT
modify memory at address in x .READBU JMP
READBUFF read character from buffer .INCBUF
JMP INCBUFF increment buffer
pointer .DECBUF JMP DECBUFF decrement
buffer pointer .WSKIP JMP WSKIP find
non-whitespace char in buffer .CHKABR JMP
CHKABRT check for abort from terminal

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Buffalo Routines Jump Table (2)
ORG ROMBS1FA0 .UPCASE JMP UPCASE
convert to upper case .WCHEK JMP WCHEK
check for white space .DCHEK JMP DCHEK
check for delimeter .INIT JMP INIT
initialize i/o device .INPUT JMP INPUT
low level input routine .OUTPUT JMP OUTPUT
low level output routine .OUTLHL JMP
OUTLHLF display top 4 bits as hex
digit .OUTRHL JMP OUTRHLF display bottom
4 bits as hex digit .OUTA JMP OUTA
output ascii character in A .OUT1BY JMP
OUT1BYT display the hex value of byte at
X .OUT1BS JMP OUT1BSP out1byt followed by
space .OUT2BS JMP OUT2BSP display 2 hex
bytes at x and a space .OUTCRL JMP OUTCRLF
carriage return, line feed to terminal .OUTSTR
JMP OUTSTRG display string at X (term
with 04) .OUTST0 JMP OUTSTRG0 outstrg
with no initial carr ret .INCHAR JMP INCHAR
wait for and input a char from term .VECINT
JMP VECINIT initialize RAM vector table