This program implements a simple two-digit voltmeter, utilizing an

　　; AT89Cx051 microcontroller, two resistors, a capacitor and two HP5082-7300

　　; decimal LED displays. The code is compatible with both the AT89C1051 and

　　; AT89C2051 when operating with a 12 MHz clock. Code modifications may be

　　; required for operation at a different clock rate.

　　; The voltmeter application demonstrates the RC analog-to-digital conversion

　　; method. The microcontroller uses an output pin, which swings from ground to

　　; Vcc, to alternately charge and discharge the capacitor through a resistor.

　　; Utilizing the internal comparator, the microcontroller measures the time

　　; required for the voltage on the capacitor to match the unknown voltage and

　　; uses the result as an index into a table. The table contains display values

　　; corresponding to the capacitor voltage. Each display value is encoded in

　　; one byte as two BCD digits, which are displayed as volts and tenths of a

　　; volt on the two decimal displays. There is no software hysteresis, so the

　　; display may oscillate at a transition voltage.

　　; The conversion routine, ADC, is general purpose, returning the entry in

　　; the table which corresponds to the measured time. The table contents may

　　; be modified to any data format required by an application.
The NOP instructions in the conversion routine are used to delay the first

　　; sample in the charge and discharge portions of the measurement cycle.

　　; The amount of delay has an effect on measurement accuracy, and the number

　　; of NOPs may be adjusted (slightly) for best results.

　　;

　　; SCOUNT is defined as the minimum number of samples which must be taken to

　　; guarantee that the voltage on the capacitor has reached Vcc/2. TCHARGE and

　　; TDISCHARGE are defined as the minimum time required for the voltage on the

　　; capacitor to approach within 1/2 delta V of Vcc and ground, respectively.

　　; The minimum conversion time is approximately TCHARGE + TDISCHARGE, or

　　; six milliseconds. The maximum conversion time is approximately TCHARGE +

　　; TDISCHARGE + 2 * (5 microseconds * SCOUNT), or seven milliseconds.

　　;

　　; For additional information refer to the application note.

　　SCOUNT　　　　EQU　　79　　　　; minimum samples to reach Vcc/2
　　TCHARGE　　　　EQU　　3　　　　; cap charge time, in milliseconds
　　TDISCHARGE　　EQU　　3　　　　; cap discharge time, in milliseconds
　　DSEG AT 0020H
　　ORG　　0020H　　　　; stack origin
　　stack:　　　　DS　　0020H　　　　; stack depth
　　CSEG
　　ORG　　0000H　　　　; power on/reset vector
　　jmp　　on_reset
　　ORG　　0003H　　　　; external interrupt 0 vector
　　reti　　　　　　; undefined
　　ORG　　000BH　　　　; timer 0 overflow vector
　　reti　　　　　　; undefined
　　ORG　　0013H　　　　; external interrupt 1 vector
　　reti　　　　　　; undefined
　　ORG　　001BH　　　　; timer 1 overflow vector
　　reti　　　　　　; undefined
　　ORG　　0023H　　　　; serial I/O interrupt vector
　　reti　　　　　　; undefined
　　ORG　　0040H　　　　; begin constant data space
　　$INCLUDE(vtable.asm)　　　　　　; get lookup table
　　ORG　　00E0H　　　　; begin code space
　　USING　　0　　　　; register bank zero
　　on_reset:
　　mov　　sp, #(stack-1)　　; initialize stack pointer
　　mov　　IE, #0　　　　; deactivate all interrupts
　　mov　　p1, #0　　　　; write zeros to displays
　　mov　　a, #0ffh　　; deactivate output ports
　　mov　　p1, a　　　　;
　　mov　　p3, a　　　　;
　　clr　　p3.7　　　　; discharge capacitor
　　mov　　a, #TDISCHARGE　　; wait
　　call　　delay_ms　　;
　　loop:
　　call　　adc　　　　; convert
　　call　　vshow　　　　; display voltage
　　sjmp　　loop　　　　; again
　　ADC:

　　; Convert analog-to-digital.

; Triggers capacitor charge/discharge and samples the comparator

　　; output at regular intervals until the comparator changes state.

　　; The sample interval is five microseconds with a 12 MHz clock.

　　; A maximum of SCOUNT samples are taken during the charge portion

　　; of the cycle and SCOUNT samples during the discharge portion of

　　; the cycle. The number of samples is used as an index into a table

　　; containing voltage equivalents. The number of table entries is

　　; twice SCOUNT. If the comparator does not switch, the last table

　　; entry is returned. Assumes that the capacitor is fully discharged

　　; on entry, and discharges the capacitor before return.

　　; The table entry is returned in A. Nothing is saved.

　　; Charge capacitor.

　　mov　　a, #0　　　　; initialize loop count
　　setb　　p3.7　　　　; begin charging　　　　1 uS
　　nop　　　　　　; padding　　　　　　1 uS
　　nop　　　　　　;　　　　　　　　1 uS
　　ad1:
　　jb　　p3.6, ad4　　; jump if comp output high　　2 uS
　　inc　　a　　　　; increment count　　　　1 uS
　　cjne　　a, #SCOUNT, ad1　　; loop until timeout　　　　2 uS
　　; Timeout.
　　; Be sure capacitor is fully charged.
　　mov　　a, #TCHARGE　　; wait
　　call　　delay_ms　　;
　　; Discharge capacitor.
　　mov　　a, #0　　　　; initialize loop count
　　clr　　p3.7　　　　; begin discharging　　　　1 uS
　　nop　　　　　　; padding　　　　　　1 uS
　　nop　　　　　　;　　　　　　　　1 uS
　　ad2:
　　jnb　　p3.6, ad3　　; jump if comp output low　　2 uS
　　inc　　a　　　　; increment count　　　　1 uS
　　cjne　　a, #SCOUNT, ad2　　; loop until timeout　　　　2 uS
　　; Timeout.
　　dec　　a　　　　; last count
　　ad3:
　　add　　a, #SCOUNT　　; use top half of table
　　; Fetch table entry.
　　ad4:
　　mov　　dptr, #VoltTable　　; pointer to base of table
　　movc　　a, @a+dptr　　　　; get voltage data
　　push　　acc　　　　　　; save data temporarily
　　; Be sure capacitor is fully discharged.
　　clr　　p3.7　　　　; begin discharging
　　mov　　a, #TDISCHARGE　　; wait
　　call　　delay_ms　　;
　　pop　　acc
　　ret

　　vshow:

; Display two BCD digits on a pair of HP5082-7300 displays.

　　; The four data lines to each display share P1.7:1.4. The units

　　; display is selected by a low on P1.3 and the tenths display by a

　　; low on P1.2. The units display is wired to light the decimal point.

　　; On entry, expects two packed BCD digits in A. Returns nothing.

　　; All registers preserved, including flags.

　　push　　psw
　　push　　acc
　　push　　acc　　　　; save digits
　　orl　　a, #00001111b　　; set unused bits
　　mov　　p1, a　　　　; write units digit
　　clr　　p1.3　　　　; strobe data into display
　　setb　　p1.3　　　　;
　　pop　　acc　　　　; restore digits
　　swap　　a　　　　; move low digit into high nybble
　　orl　　a, #00001111b　　; set unused bits
　　mov　　p1, a　　　　; write tenths digit
　　clr　　p1.2　　　　; strobe data into display
　　setb　　p1.2　　　　;
　　pop　　acc
　　pop　　psw
　　ret

　　delay_ms:

　　; Delay for approximately one millisecond times the value in A.

　　; All registers preserved, including flags.

　　push　　psw
　　push　　acc
　　push　　b
　　mov　　b, #0
　　ddm:
　　djnz　　b, $　　　　; 512 uS @ 12 MHz
　　djnz　　b, $　　　　; 512 uS @ 12 MHz
　　djnz　　acc, ddm
　　pop　　b
　　pop　　acc
　　pop　　psw
　　ret
　　END