Transcript 8051 timers and interrupts

8051 Timers
• Since this is a microcontroller it mainly finds itself
in embedded devices
• Quite often embedded devices need to synchronize
events
• The way this is done is via time
– We could do it by inserting a loop or a bunch of NOP commands
• Is this accurate?
– A better way to do it is to use timers
• Basically an internal stop watch
• The 8051 has two timers
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Timers and Interrupts
8051 implementation
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Using the timers
• Controlled through two special function
registers (SFR)
– TMOD – configure the timers
– TCON – control the timers
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TMOD
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Timer Modes
• Modes 1 and 2 are the most common
– Mode 1: 16-bit timer, sets overflow when
complete
– Mode 2: 8-bit timer, reloads and sets overflow
when complete
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TCON
• Bits 5 and 7 are the overflow bits for each timer and are set
by hardware
• Bits 4 and 6 turn the timers on/off
• More on bits 0-3 later
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Timer control
• Setting bits TR0 and TR1 (setb TR0 and setb TR1
instructions) start the timers
• Clearing bits TR0 and TR1 (clr TR0 and clr TR1
instructions) stop the timers
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Reading the timers
• If the timers are stopped, reading is trivial
mov R6, TL0
mov R7, TH0
• If the timers are running, it gets tricky
try:
mov A, TH0
mov R6, TL0
cjne A, TH0, try
mov R7, A
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We don’t want TH0 to roll
over while we are reading TL0
Programming the timer (part 1)
; set timer 0 to 8-bit mode
mov TMOD, #0x02
; start value
mov TL0, #0x00
; reset value when
; the timer hits 0xFF
mov TH0, #0x00
; start timer 0
setb TR0
; kill some time
nop
nop
nop
nop
nop
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Programming the timer (part 2)
; stop timer 0
clr TR0
tryagain:
; read the high byte of the timer
mov A, TH0
; read the low byte of the timer
mov R6, TL0
; read the high byte again to make sure it didn't change
; while we were reading the low byte
cjne A, TH0, tryagain
; keep the high byte if it didn't change
mov R7, A
; the full 16-bit timer is now in R7/R6
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8051 Interrupts
• A signal to let the CPU know that
something out of the ordinary flow of
instructions has occurred
• Various sources of interrupts in the 8051
– Two external
– Two internal (timers)
– I/O (serial port)
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Set up by the TCON register
• The bits we didn’t talk about previously
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When an interrupt occurs
• Current instruction is allowed to complete
• Program counter (PC register) is saved on
the stack (SP)
• Address of the interrupt service routine
(ISR) is loaded to the program counter
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Interrupt service routine
• Also known as an interrupt vector
– The 8051 has six of them
– Usually the ISR is nothing more than a JMP to the actual
subroutine (due to lack of space between interrupt vectors)
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Enabling/Disabling interrupts
• To use an interrupt you must first enable it
– They are disabled by default (power up)
– Enabling is a 2 step process
• First enable all interrupts – setb EA
• Then enable individual interrupts – setb ET0
• See next slide for why 2 steps…
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Why two steps?
• Because it makes turning them on and off very
easy
• Consider a critical section of code (more than 1
instruction)
– You don’t want it to be interrupted
– but you don’t want to turn off a bunch of interrupts
knowing you’ll have to turn them back on (that takes
time)
– Using clr EA and setb EA does the trick in the shortest
time possible
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There is a little bit more…
• Since the interrupt vectors are in low address
memory (see table) you can’t put your main
program there
– Address 0 is the default program memory address
• The 8051 assembler provides a solution
org address
– Provides a way to force code to a particular address –
it’s not an instruction, but a directive
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Programming interrupts (part 1)
• Setting up the ISR
; org tells the assembler where to place the code
org 0
; start at the main program label
jmp main
; timer 0 ISR is at address 0x000B
org 0x000B
; obviously not a good ISR but it makes the point
timer0:
jmp timer0
reti
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Programming interrupts (part 1)
• The main program (almost the same as before)
; start the main program at address
0x0030
org 0x0030
main:
; set timer 0 to 8-bit mode
mov TMOD, #0x02
; start value
mov TL0, #0xFC
; reset value when
; the timer hits 0xFF
mov TH0, #0x00
; enable interrupts
setb EA
setb ET0
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Programming interrupts (part 2)
• more main program (same as before)
; start timer 0
setb TR0
; kill some time
nop
nop
nop
nop
nop
; stop timer 0
;clr TR0
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Programming interrupts (part 3)
• more main program (same as before)
tryagain:
; read the high byte of the timer
mov A, TH0
; read the low byte of the timer
mov R6, TL0
; read the high byte again to make sure it didn't change
; while we were reading the low byte
cjne A, TH0, tryagain
; keep the high byte if it didn't change
mov R7, A
; the full 16-bit timer is now in R7/R6
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