Note: I’d been meaning to post about this for a while, and since the project made the MAKE blog, I figure it is high time…
After having two aftermarket stereo systems stolen from my car in less than a year’s time, I decided to give up and go back to the factory radio. The problem? It only has tape and radio inputs, and I prefer to have access to my entire MP3 collection. The traditional way to fix this is to either get an FM broadcaster or a tape adapter, but where is the fun in that? Instead, I elected to wire in an external input, and built a little circuit to let me switch between the two. Here is a photo set to document the process:
(click on the picture to view the album)
Source code after the break.
This code is written for a PIC 12F629 as the controller, largely because I had a bunch of them sitting around. If you are starting from scratch, I would suggest skipping this bit and spending the whole $4 on a part that has an open source C compiler (*cough* AVR *cough*).
; Input multiplexer for car stereo system
;
; By Matt Mets, completed in 2008
;
; This code is released into the public domain. Attribution is appreciated.
; Build configuration
PROCESSOR 12F629
include p12f629.inc
; Global Register Definitions
STATE equ 0x20 ; System state:
; 0=radio/tape, 1=external input
; (other bits are don't care)
SLEEP_CNT equ 0x30 ; For use in sleep routine
SLEEP_CNT_L equ 0x31
BTN_CNT equ 0x32 ; Countdown for button press
BTN_STATE equ 0x33 ; Read the GPIO
BTN_DOWN equ 0x34 ; Last known button state
; EEPROM Definitions
STATE_ADDR equ 0x00 ; Last known system status
; Set up the I/O ports to be in the right state
bsf STATUS,RP0 ; Bank 1
movlw 0x39 ; Set GP<2:1> as output
movwf TRISIO ; and set GP<5:3,0> as input
bcf STATUS,RP0 ; Bank 0
; Load the last-known state from the EEPROM
call LOAD_ST
; Main loop
START call CHECK_BTN ; If button pressed, do some debouncing
btfss BTN_DOWN,0
goto START ; Otherwise, repeat
movlw 0xff ; Must have x counts of button down
movfw BTN_CNT ; before toggling
BTN_LOOP call DO_SLEEP_L ; sleep for a bit
call CHECK_BTN ; If button released, return to loop
btfss BTN_DOWN,0
goto START
movlw 0x01 ; Subtract 1 from the count
subwf BTN_CNT,1
btfss STATUS,Z ; If non-zero, loop again
goto BTN_LOOP
call TOGGLE_ST ; Otherwise, toggle
BTN_LOOPE call DO_SLEEP ; Don't return until the button is
call CHECK_BTN
btfsc BTN_DOWN,0
goto BTN_LOOPE
goto START
; See if we can tell when the button has been pressed. Because it is being
; driven by a matrix controller, it is 'down' when both sides are at the same
; potential.
CHECK_BTN clrf BTN_DOWN ; Clear the button down register
movf GPIO,0 ; Capture the input state
movwf BTN_STATE
btfss BTN_STATE,3 ; Determine state of buttons
goto CHECK_LOW
CHECK_HIGH btfss BTN_STATE,4 ; Check if both buttons are high
return
goto SET_BTN
CHECK_LOW btfsc BTN_STATE,4 ; Check if both buttons are low
return
SET_BTN comf BTN_DOWN,1 ; Set the button down register
return
; Toggle the system state (then apply it)
TOGGLE_ST movlw 0x01 ; Flip the state variable
xorwf STATE,1
call WRITE_ST ; Then store it in EEPROM
; Apply the system state to the hardware
APPLY_ST btfss STATE,0 ; If the low bit is 0, set radio/tape
goto APPLY_ST1
APPLY_ST0 movlw 0x02 ; Select radio/tape input
movwf GPIO
return
APPLY_ST1 movlw 0x04 ; Select aux input
movwf GPIO
return
; Read the last value of the State register from the EEPROM
LOAD_ST bsf STATUS,RP0 ; Bank 1
movlw STATE_ADDR ; Program the address to read
movwf EEADR
bsf EECON1,RD ; Read the data from EEPROM
movf EEDATA,0 ; and load it to the state regiser
movwf STATE
bcf STATUS,RP0 ; Bank 0
call APPLY_ST ; And apply the state
return
; Write the current value of the State register to the EEPROM
WRITE_ST bsf STATUS,RP0 ; Bank 1
movf STATE,0 ; Load the state register value
movwf EEDATA
movlw STATE_ADDR ; And its address
movwf EEADR
WRITE_SEQ bsf EECON1,WREN ; Enable write
movlw 0x55 ; Special dance to unlock writes
movwf EECON2 ; (this feature prevents runaway code
movlw 0xAA ; from trashing the EEPROM)
movwf EECON2
bsf EECON1,WR ; Start the write
bcf EECON1,WREN ; Disable any future writes
bcf STATUS,RP0 ; Bank 0
return
; Sleep for some time
DO_SLEEP movlw 0xff ; Count down from 255
movwf SLEEP_CNT
DO_SLEEPI decfsz SLEEP_CNT,1 ; Decrement loop
goto DO_SLEEPI
return
; Sleep for even longer
DO_SLEEP_L movlw 0x6 ; Count down from 255
movwf SLEEP_CNT_L
DO_SLEEP_LI call DO_SLEEP
decfsz SLEEP_CNT_L,1 ; Decrement loop
goto DO_SLEEP_LI
return
END