Introduction
In the 90s I worked as an electronics design engineer for a small company that supplied rechargeable batteries, battery packs, power supplies and chargers.
The client base included individuals, small and large companies, the hobby industry, the emergency lighting industry, mobile phone providers, the Ministry of Defence, Post Office, and Prison Services.
One particular client in the hobby industry commissioned us to build a multifunction charger that could charge, discharge, and perform load testing on NiCd (nickel–cadmium) battery packs.
Design
I had built a number of charging systems around the bq2003PN IC, a fast charge IC with with high-speed switching power control circuitry (PWM). It detects a negative delta voltage (-ΔV) in the battery’s charge curve to terminates fast charge:
To control the charge circuity and allow the user to set a charge / discharge cycle and view the status on the front panel I chose the Microchip PIC16F84 microcontroller. These were very popular back then, specifically the ones that use flash memory, which allows the program to be reprogrammed. We take this for granted now with the Arduino and other devices, but back then it was a brilliant concept and allowed for easy iterative development.
The power fast charging circuit consists of a 555 driving a MOSFET via PNP / NPN transistors in a standard buck configuration. The PWM signal is provided by the bq2003PN. Discharge is via an LM317 in constant current mode. The LEDs are latched using 74HCT574 D-type tristate flip flops.
Firmware
The PIC16C84 firmware was written in Assembly language, and is listed below:
PIC16C84 Microcontroller Assembly code:
; uP Controlled -dV Fast Charger - Copyright © Russell Carter 01/1998
;
List P=16C84,F=inhx8m
;
tmr0 equ 01
pcl equ 02
status equ 03
porta equ 05
portb equ 06
eedata equ 08
eeadr equ 09
intcon equ 0B
options equ 81
trisa equ 85
trisb equ 86
eecon1 equ 88
eecon2 equ 89
count_1 equ 0C ; Second divider
count_2 equ 0D ; Second divider
lt_latch equ 0E ; IC3 : D7 - D0
sc_latch equ 0F ; IC4 : D7 - D0
del_1 equ 10 ; Delay reg 1
del_2 equ 11 ; Delay reg 2
flags equ 12 ; Flag register
sec equ 13 ; Seconds register
min equ 14 ; Minutes register
table equ 15 ; Next table read location
del_mult equ 16 ; Delay Multiplier
;
org 0
goto init_ports ; Start
nop
nop
nop
goto service_int ; Interrupt Vector
init_ports
movlw b'00001000' ; RA3 - CCMD held high
movwf porta
clrf portb ; Clear Port B
bsf status,5
clrf trisb ; Port B set for outputs
movlw b'00010010' ; RA1, RA4 Set for inputs
movwf trisa
bcf status,5
init_timer
movlw .250
movwf count_1 ; 2ms Interrupt x 250 = 500ms
movlw .2
movwf count_2 ; 500ms x 2 = 1000ms = 1 second
movlw .60
movwf sec ; 1s x 60 = 1min
movlw .15
movwf min ; 15min timeout for 100mAH @ 400mA discharge
movlw .10
movwf del_mult ; Multiply delay by 10
init_table
movlw .36 ; Last value on the table
movwf table
led_test
movlw 0XFF
movwf sc_latch
movwf lt_latch ; Set all LED's
call latch_write
call med_delay
call med_delay
call med_delay
clrf lt_latch
movlw b'10010000'
movwf sc_latch ; Set Idle and Charge Only LED's
call latch_write
key_input
bsf status,5
bcf options,7 ; Enable weak pullups on Port B
movlw b'11000000'
movwf trisb ; Set RA7 & RA6 inputs
bcf status,5
scan
btfss portb,7
goto start
btfss portb,6 ; Determine which key is pressed
call togl_cycle
goto key_input
togl_cycle
bsf status,5
bsf options,7 ; Disable weak pullups on Port B
clrf trisb ; Set RA7 & RA6 outputs
bcf status,5
bcf sc_latch,4 ; Toggle the Cycle LED's
rrf sc_latch
bcf sc_latch,7
btfsc sc_latch,4
goto next
bsf sc_latch,4
call latch_write
call med_delay
return
next
movlw b'10010000'
movwf sc_latch
call latch_write
call med_delay
return
start
btfss portb,6 ; Test for "Old Load Test Value" request
call disp_lt
bsf status,5
bsf options,7 ; Disable weak pullups on Port B
clrf trisb ; Set RA7 & RA6 outputs
bcf status,5
btfsc sc_latch,7 ; Determine which Cycle is required
goto ch_only
btfsc sc_latch,6
goto dis_charge
goto load_test
ch_only
bcf porta,3 ; Charge Command
call short_delay
bsf porta,3
call charge ; Fast Charge
bcf sc_latch,2
bsf sc_latch,0 ; Set the Cycle Complete LED
call latch_write
goto cycle_complete
dis_charge
bsf porta,2 ; Discharge Command
call short_delay
bcf porta,2
call discharge ; Discharge
call charge ; Fast Charge
bcf sc_latch,1
bsf sc_latch,0 ; Set the Cycle Complete LED
call latch_write
goto cycle_complete
load_test
movlw .1
movwf eeadr
movlw .0
call ee_write
bcf porta,3 ; Charge Command
call short_delay
bsf porta,3
call charge ; Fast Charge
bsf sc_latch,2 ; Set Charge LED
call latch_write
call lt_delay ; Rest battery 5 min before Load Test
bsf porta,2 ; Discharge Command
call short_delay
bcf porta,2
check_1
btfss porta,4 ; Wait for Discharge signal
goto check_1
bcf sc_latch,2 ; Clear Fast Charge LED
bsf sc_latch,1 ; Set Discharge LED
call latch_write
init_interrupts
bsf status,5
movlw b'11010010' ; Set Timer 0: Internal and Prescaler 1:8
movwf options
movlw b'10100000' ; Enable Interrupts
movwf intcon
bcf status,5
wait
btfsc porta,4 ; Test for end of Discharge
goto wait ; Wait for Interrupt or end of Discharge
bsf status,5
clrf intcon ; Disable Interrupts
bcf status,5
movlw .1
movwf eeadr
movf lt_latch,0
call ee_write
call charge
bsf sc_latch,0 ; Set Complete LED
call latch_write
goto cycle_complete
charge
bcf sc_latch,1 ; Clear all Status LED's, Set Batt Low / Trickle LED
bcf sc_latch,2
bcf sc_latch,4
bsf sc_latch,3
call latch_write
check_2
btfss porta,1 ; Wait for Fast Charge signal
goto check_2
bcf sc_latch,3 ; Clear Batt Low LED, Set Fast Charge LED
bsf sc_latch,2
call latch_write
check_3
btfsc porta,1 ; Wait for Fast Charge Complete signal
goto check_3
bcf sc_latch,2 ; Clear Fast Charge LED
call latch_write
return
discharge
btfss porta,4 ; Wait for Discharge signal
goto discharge
bcf sc_latch,2 ; Clear all Status LED's, Set Discharge LED
bcf sc_latch,3
bcf sc_latch,4
bsf sc_latch,1
call latch_write
check_4
btfsc porta,4 ; Wait for end of Discharge signal
goto check_4
bcf sc_latch,1 ; Clear Discharge LED
call latch_write
return
service_int
nop
nop
movlw .07
movwf tmr0
decfsz count_1 ; Decrement counters for timing
goto ret
movlw .250
movwf count_1
decfsz count_2
goto ret
movlw .2
movwf count_2
call dec_sec
ret
movlw b'10100000'
movwf intcon
retfie
dec_sec
decfsz sec ; Decrement seconds register
return
movlw .60
movwf sec
call dec_min
return
dec_min
decfsz min ; Decrement minutes register
return
movlw .15
movwf min
btfsc flags,0
goto dis_int
call togl_lt
return
dis_int
bsf status,5
clrf intcon ; Disable Interrupts
bcf status,5
bcf flags,0
retfie
togl_lt
movlw .0 ; Toggle to next load test value
addwf table,0
btfss status,2 ; Check for table limit
decf table
movf table,0
call table_1
movwf lt_latch
call latch_write
return
lt_delay
bsf flags,0
movlw .5
movwf min
bsf status,5
movlw b'11010010' ; Set Timer 0: Internal and Prescaler 1:8
movwf options
movlw b'10100000' ; Enable Interrupts
movwf intcon
bcf status,5
check_5
btfsc flags,0
goto check_5
return
table_1
addwf pcl
retlw b'11111111' ; Out of Range
retlw b'00001111' ; 3500 mAh
retlw b'00010111' ; 3400 mAh
retlw b'00100111' ; 3300 mAh
retlw b'01000111' ; 3200 mAh
retlw b'10000111' ; 3100 mAh
retlw b'00001011' ; 3000 mAh
retlw b'00010011' ; 2900 mAh
retlw b'00100011' ; 2800 mAh
retlw b'01000011' ; 2700 mAh
retlw b'10000011' ; 2600 mAh
retlw b'00001101' ; 2500 mAh
retlw b'00010101' ; 2400 mAh
retlw b'00100101' ; 2300 mAh
retlw b'01000101' ; 2200 mAh
retlw b'10000101' ; 2100 mAh
retlw b'00001001' ; 2000 mAh
retlw b'00010001' ; 1900 mAh
retlw b'00100001' ; 1800 mAh
retlw b'01000001' ; 1700 mAh
retlw b'10000001' ; 1600 mAh
retlw b'00001010' ; 1500 mAh
retlw b'00010010' ; 1400 mAh
retlw b'00100010' ; 1300 mAh
retlw b'01000010' ; 1200 mAh
retlw b'10000010' ; 1100 mAh
retlw b'00001100' ; 1000 mAh
retlw b'00010100' ; 900 mAh
retlw b'00100100' ; 800 mAh
retlw b'01000100' ; 700 mAh
retlw b'10000100' ; 600 mAh
retlw b'00001000' ; 500 mAh
retlw b'00010000' ; 400 mAh
retlw b'00100000' ; 300 mAh
retlw b'01000000' ; 200 mAh
retlw b'10000000' ; 100 mAh
latch_write
movf sc_latch,0
movwf portb
bsf porta,0 ; Latch Port B into IC4 - Status, Cycle
movf lt_latch,0
movwf portb
bcf porta,0 ; Latch Port B into IC3 - Load Test
return
disp_lt
bsf status,5
bsf options,7 ; Disable weak pullups on Port B
clrf trisb ; Set RA7 & RA6 outputs
bcf status,5
clrf sc_latch
clrf lt_latch
call latch_write
call ee_read ; Displays the previous value of the load test
movwf lt_latch
call latch_write
goto cycle_complete
med_delay
call short_delay
call short_delay
call short_delay
return
short_delay ; Total Delay = V1(3*V2+8)+6
movlw .244 ; Value 1
movwf del_1
loop_1
call delay_2 ; Values chosen for 100ms
decfsz del_1
goto loop_1
return
delay_2
movlw .134 ; Value 2
movwf del_2
loop_2
decfsz del_2
goto loop_2
return
ee_read
movlw .1 ; EEPROM Read
movwf eeadr
bsf status,5
bsf eecon1,0
bcf status,5
movf eedata,w
return
ee_write
movwf eedata ; EEPROM Write
bsf status,5
bsf eecon1,2
movlw 0x55
movwf eecon2
movlw 0xAA
movwf eecon2
bsf eecon1,1
bcf status,5
call wr_delay
retlw .0
wr_delay
bsf status,5 ; EEPROM Write complete detect
btfss eecon1,4
goto wr_delay
bcf eecon1,4
bcf status,5
return
cycle_complete
goto cycle_complete
endThe flowchart describes the process controlled by the PIC16C84:
Construction
Construction was simple. The PCB was glued to he top of an ABS box with the LEDs embedded within holes, to expose them through the silk screened legend on the top. Pushbuttons were mounted on the lid and the switch was mounted on the PCB. A Mains lead was brought in through the side and connected via a fuse to a transformer, which supplies AC to the PCB.
Gallery
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