This application note presents programming techniques for implementing a real time clock that keeps a 16-bit milliseconds count, and has the option for full time clock capabilities, including seconds, minutes, hours, and days. The routine takes advantage of the SX's internal interrupt feature to allow background operation of the clock as a virtual peripheral.
This firmware module requires no external circuitry (other than an oscillator crystal) and is quite straight forward. There are three options for code assembly, controlled by the clock_type parameter. If clock_type=0, the clock only counts milliseconds. A value of clock_type=1 adds seconds, minutes and hours, and a value of clock_type=2 allows the days to be counted as well.
The timing constant for each millisecond ‘tick’ is determined as follows:
msec tick timing = osc. freq. / (1000 msec/sec * prescaler * mode) where mode=1 (turbo) or =4 (normal)
So, for a crystal frequency of 50 MHz, in turbo mode, with a prescaler of 1, the msec tick timing constant is:
msec tick timing = 50 x 106 / (1000 * 1 * 1) = 50 x 103
By comparing the number of elapsed instructions with the millisecond tick timing, the code decides when one millisecond has passed and increments the msc_lo and msec_hi counters accordingly. In the same way, if selected, the code checks the corresponding count and ticks off each second, minute, hour, day, etc.
The time clock’s accuracy is dependent upon the accuracy of the oscillator used, which for crystals is usually extremely good. For oscillators, especially slower ones, that do not have a frequency in kHz that is divisible by an integer, the accuracy starts being affected by the msec count timing algorithm, and it should be adjusted or left out accordingly1.
Ideally the circuit and program will provide a method for the user to enter the time and date, etc., otherwise it is a relative time count in reference to the last time the circuit was turned on or reset.
If the need for processor power between timed events is minimal, the routine could be modified and set up in conjunction with the watchdog timer instead of the internal RTCC interrupt where the SX is put in sleep mode between watchdog time-outs. This allows for a tremendous savings in power consumption.
With some additional programming, day-of-the-week, month, and even year counts could be added, the month count being somewhat more involved, for obvious reasons.
;****************************************************************************** ; Software Time Clock ; ; ; Length: 28/50/56 bytes (depending upon clock type, +1 for bank select) ; Author: Craig Webb ; Written: 98/8/17 ; ; This program implements a software time clock virtual peripheral ; that keeps a 16 bit count of elapsed time in milliseconds. ; The option is available to include seconds, minutes, hours and even ; days to this clock if desired. ; The code takes advantage of the SX's internal RTCC-driven interrupt ; to operate in the background while the main program loop is executing. ; ;****************************************************************************** ; ;****** Assembler directives ; ; uses: SX28AC, 2 pages of program memory, 8 banks of RAM, high speed osc. ; operating in turbo mode, with 8-level stack & extended option reg. ; DEVICE pins28,pages2,banks8,oschs DEVICE turbo,stackx,optionx ID 'TimeClck' ;program ID label RESET reset_entry ;set reset/boot address ; ;******************************* Program Variables *************************** ; ;****** Program Parameters ; ;clock_type = 0 ;16 bit msec count only clock_type = 1 ;include sec, min, hours ;clock_type = 2 ;include day counter ; ;****** Program constants ; tick_lo = 80 ;50000 = msec instruction count tick_hi = 195 ; for 50MHz, turbo, prescaler=1 ; int_period = 163 ;period between interrupts ; mspersec_hi = 1000/256 ;msec per second hi count mspersec_lo = 1000-(mspersec_hi*256) ;msec per second lo count ; ;****** Register definitions ; org 8 ;start of program registers main = $ ;main bank ; temp ds 1 ;temporary storage ; org 010H ;bank0 variables clock EQU $ ;clock bank ; time_base_lo DS 1 ;time base delay (low byte) time_base_hi DS 1 ;time base delay (high byte) msec_lo DS 1 ;millisecond count (low) msec_hi DS 1 ;millisecond count (high) IF clock_type>0 ;do we want sec, min, hours? seconds DS 1 ;seconds count minutes DS 1 ;minutes count hours DS 1 ;hours count ENDIF IF clock_type>1 ;do we want day count? days DS 1 ;days count ENDIF ; ;*************************** INTERRUPT VECTOR ****************************** ; ; Note: The interrupt code must always originate at 0h. ; A jump vector is not needed if there is no program data that needs ; to be accessed by the IREAD instruction, or if it can all fit into ; the lower half of page 0 with the interrupt routine. ; ORG 0 ;interrupt always at 0h ; JMP interrupt ;interrupt vector ; ;**************************** INTERRUPT CODE ******************************* ; ; Note: Care should be taken to see that any very timing sensitive routines ; (such as adcs, etc.) are placed before other peripherals or code ; which may have varying execution rates (like the software clock, for ; example). ; interrupt ;beginning of interrupt code ; ;****** Virtual Peripheral: Time Clock ; ; This routine maintains a real-time clock count (in msec) and allows processing ; of routines which only need to be run once every millisecond. ; ; Input variable(s) : time_base_lo,time_base_hi,msec_lo,msec_hi ; seconds, minutes, hours, days ; Output variable(s) : msec_lo,msec_hi ; seconds, minutes, hours, days ; Variable(s) affected : time_base_lo,time_base_hi,msec_lo,msec_hi ; seconds, minutes, hours, days ; Flag(s) affected : ; Size : 17/39/45 bytes (depending upon clock type) ; + 1 if bank select needed ; Timing (turbo) : [99.9% of time] 14 cycles ; [0.1% of time] 17/39/45 cycles (or less) ; + 1 if bank select needed ; ; BANK clock ;select clock register bank MOV W,#int_period ;load period between interrupts ADD time_base_lo,W ;add it to time base SNC ;skip ahead if no underflow INC time_base_hi ;yes overflow, adjust high byte MOV W,#tick_hi ;check for 1 msec click MOV W,time_base_hi-W ;Is high byte above or equal? MOV W,#tick_lo ;load instr. count low byte SNZ ;If hi byte equal, skip ahead MOV W,time_base_lo-W ;check low byte vs. time base SC ;skip ahead if low JMP :done_clock ;If not, end clock routine :got_tick CLR time_base_hi ;Yes, adjust time_base reg.'s SUB time_base_lo,#tick_lo ; leaving time remainder INCSZ msec_lo ;And adjust msec count DEC msec_hi ; making sure to adjust high INC msec_hi ; byte as necessary IF clock_type>0 ;do we want sec, min, hours? MOV W,#mspersec_hi ;check for 1000 msec (1 sec tick) MOV W,msec_hi-W ;Is high byte above or equal? MOV W,#mspersec_lo ;load #1000 low byte SNZ ;If hi byte equal, skip ahead MOV W,msec_lo-W ;check low byte vs. msec count SC ;skip ahead if low JMP :done_clock ;If not, end clock routine INC seconds ;increment seconds count CLR msec_lo ;clear msec counters CLR msec_hi ; MOV W,#60 ;60 seconds per minute MOV W,seconds-W ;are we at minute tick yet JNZ :done_clock ;if not, jump INC minutes ;increment minutes count CLR seconds ;clear seconds count MOV W,#60 ;60 minutes/hour MOV W,minutes-W ;are we at hour tick yet? JNZ :done_clock ;if not, jump INC hours ;increment hours count CLR minutes ;clear minutes count ENDIF ;<if> we wanted sec, min, hours IF clock_type>1 ;do we want to count days? MOV W,#24 ;24 hours per day MOV W,hours-W ;are we at midnight? JNZ :done_clock ;if not, jump INC days ;increment days count CLR hours ;clear hours count ENDIF ;<if> we wanted day count :done_clock ; done_int mov w,#-int_period ;interrupt every 'int_period' clocks retiw ;exit interrupt ; ;****** End of interrupt sequence ; ;************************** RESET ENTRY POINT ***************************** ; reset_entry ; PAGE start ;Set page bits and then ; JMP start ; jump to start of code ; ;********************* ;* Main Program Code * ;********************* ; start ; mov !rb,#%00001111 ;Set RB in/out directions CLR FSR ;reset all ram starting at 08h :zero_ram SB FSR.4 ;are we on low half of bank? SETB FSR.3 ;If so, don't touch regs 0-7 CLR IND ;clear using indirect addressing IJNZ FSR,:zero_ram ;repeat until done MOV !OPTION,#%10011111 ;enable rtcc interrupt ; ; Main loop ; :loop ; BANK Clock ;set default bank ; ; <main program code goes here> ; JMP :loop ;back to main loop ; ;*************** END ;End of program code
Code:
In the clock, UART, etc. VP's, the assumption is that the processor is running at 50 Mhz. The constants that are used are: int_period = 163 ;period between interrupts ; tick_lo = 80 ;50000 = msec instruction count tick_hi = 195 ; for 50MHz, turbo, prescaler=1 To convert these for use at 4 Mhz, change the constants to: int_period = 104 ;period between interrupts ; tick_lo = 160 ;4000 = msec instruction count tick_hi = 15 ; for 4MHz, turbo, prescaler=1 That's all you need for the clock/timer VP's. For the UART VP, here are the settings for 2400 and 1200 BPS. (at 4Mhz you can't go higher than 2400, and I didn't bother to figure out anything lower than 1200) ; *** 2400 baud (at 50MHZ) ;baud_bit = 7 ;for 2400 baud ;start_delay = 128+64+1 ; " " " ;int_period = 163 ; " " " ; ; *** 2400 baud (at 4MHZ) baud_bit = 4 ;for 2400 baud start_delay = 16+8+1 ; ' ' ' int_period = 104 ; ' ' ' ; ; *** 1200 baud (at 4MHZ) ;baud_bit = 5 ;for 1200 baud ;start_delay = 32+16+1 ; ' ' ' ;int_period = 104 ; ' ' ' I have tested the 2400 baud settings and it works. I've not tested the 1200 baud settings but the math follows and they should work. Note that the time allowed in the ISR is significantly shorter with these settings. If you need more time in the ISR you could use 204 for int_period and adjust everything else accordingly. William Phelps