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USER’S GUIDE
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88
Timers
The microprocessor disables timer activity (excluding
the Watchdog) and serial port communication on a re-
set. Therefore, each timer must be setup and enabled
as part of the reset routine. The serial port mode must
also be initialized if used. This is covered in detail in the
User’s Guide section on Timers and Serial I/O respec-
tively. Shown here is an example of Timer and Serial
Port setup. In this example, Timer 0 is set up to generate
a 10 ms interrupt. Timer 1 is setup to generate 9600
baud for the serial port. The serial port is set up for
asynchronous communication with a PC (mode 1). A
crystal frequency of 11.0592 MHz is assumed.
ORG 00h
SJMP Start
Org 30h
Start :
SETB PS ;Set Serial Port Interrupt to High Priority
SETB ES ;Enable Serial Port Interrupt
SETB ET0 ;Enable Timer 0 Interrupt
MOV TMOD, #00100001b ;Select Timer 1 mode 2 – 8 bit auto–reload,
; Timer 0 mode 1 – 16 bit manual reload
MOV TH1, #0FDh ;Setup 9600 baud
MOV TL1, #00h ; ” ”
MOV TH0, #0DBh ;Select a 10 ms count. 9216 counts = 10 ms
MOV TL0, #0FFh ; 9216d counts = 2400h counts (FFFFh–2400h =
; DBFFh)
; Timer 0 ISR must reload DBFFh manually
MOV SCON, #01010011b ;Select Serial Port mode 1,
; TXD and RXD interrupts active
MOV TCON, #01010000b ;Enable the operation of both Timers
SETB EA ;Globally enable interrupts
Protection
The microprocessor provides protection from transients
through a built in power–fail/power–on reset and Watch-
dog Timer. Each of these functions should be initialized
by the user as part of the reset routine. The following
code demonstrates the set up for a user that will support
the Watchdog function.
TA EQU 0C7h
ORG 00h
SJMP Start
Org 30h
Start :
MOV TA, #0AAh ;Timed
MOV TA, #55h ; Access
ORL IP, #80h ;Set RWT to restart the Watchdog Timer
MOV TA, #0AAh ;Timed
MOV TA, #55h ; Access
ORL PCON, #44h ;Set POR (PCON.6) bit for power on reset detect
; and enable Watchdog Timer by setting EWT (PCON.2)