AVR Freaks

Folks,  I'd appreciate a little help with my project.  This is my first attempt to use the ADC on the ATtiny85.  Eventually, the project will evolve, but at the moment, it is a stripped-to-the-bones attempt to get a result from the ADC.

 
My sketch is below.  The intention of the sketch is to get a voltage from a pot and light one of three LEDs, depending on whether the pot is in the lower third, middle- or upper third of its travel.  It appears to work in AS7 (by writing the "pot" value into the ASC register after the conversion), but when I put it on a chip, it lights one LED, and moving the pot does not cause it to light a different LED.  If I power it off, turn the pot, then power it on, the correct LED comes on, but that one stays on, and turning the pot does not change the LED on the fly.

The sketch is below. 

There are a few lines to set up the ADC.  These poke individual bits so I could play with them individually to see what they do.

The intention is ...

Set up the ADC

convert:  ; get pot reading into R16
call light_led
call delay
rjmp convert

light_led:
This function works.  Given a value in R16, it discovers what third of (0...255) R16 is in, and lights the appropriate LED.

delay:
This function also works, and delays about a second.

.org 0x0

 rjmp main                   ; Reset - Address 0
 reti                        ; INT0
 reti                        ; Pin Change Interrupt Request 0
 reti                        ; Timer/Counter1 Compare Match A
 reti                        ; Timer/Counter1 Overflow
 reti                        ; Timer/Counter0 Overflow
 reti                        ; EEPROM Ready
 reti                        ; Analog Comparator
 reti                        ; ADC Conversion Complete
 reti                        ; Timer/Counter1 Compare Match B
 reti ; TCC0_cmpA_handler    ; Timer/Counter0 Compare Match A
 reti                        ; Timer/Counter0 Compare Match B
 reti                        ; Watchdog Time-out
 reti                        ; USI START
 reti                        ; USI Overflow

;========================
; ISRs go here
;========================

main:
; Load stack register
LDI R16, HIGH(RAMEND) ; Upper byte
OUT SPH,R16 ; to stack pointer
LDI R16, LOW(RAMEND) ; Lower byte
OUT SPL,R16 ; to stack pointer

;===================================================
; initialise ADC   PB3-Red, PB4-Yellow, PB1-Green
;                   _____
;           PB5 [1]|*    |[8] VCC
;    (Red)  PB3 [2]|     |[7] PB2 (Pot in)
; (Yellow)  PB4 [3]|     |[6] PB1 (Green)
;           GND [4]|_____|[5] PB0 (to FET)
;
;===================================================
; ATTINY 85 at 1MHz
;===================================================

; set direction reg
ldi R16,(1<<PB0)|(1<<PB1)|(1<<PB3)|(1<<PB4) ; PB3-Red, PB4-Yellow, PB1-Green, PB0-FET
out DDRB,R16  ; PB0 drives pulses to LED via FET

sbi DIDR0,ADC1D    ; ADC1 (PB2) is analog, not digital
sbi ADCSRA, ADEN   ; ADC Enable
sbi ADCSRA, ADPS1  ;\_ Prescaler to div_8 for 125 khz sampling
sbi ADCSRA, ADPS0  ;/
sbi ADMUX, ADLAR   ; all msb's will be in ADCH
sbi ADMUX, MUX0    ; single-ended on ADC2 (PB4 - pin 3)

convert:
; get pot reading into R16  

sbi ADCSRA,ADSC     ; START conversion. I see bit-6 get set in ADCSRA. NOTE: the Cycle Counter = 11
cbi ADCSRA,ADIF     ; this does not clear ADIF in AS7, so I guess it is controlled by the hardware.
                    ; I have not discovered how to clear it, so this is probably the crux of my problem.

; Wait for ADSC to go low again.
wait_adif:        ; NOTE: The datasheet says that at the end of conversion,
sbic ADCSRA,ADSC  ;       ADSC will go false again.  It does in AS7, but when I test it with sbic,
rjmp wait_adif    ; the rjmp is not skipped, so I deduce that ADSC was not, in fact, false.
                  ; If I run the program down to the nop on the next line, I find that ADIF has gone true.
                  ; I experimented with waiting for ADIF to go true, but the result is the same as testing ADSC false.
                  ; That is also the reason why I manually reset ADIF a few lines above.

; copy the pot reading to R16
nop
in R16,adch
rcall light_led
rjmp convert
;===================================================================

light_led:   ; PB3-Red, PB4-Yellow, PB1-Green
cpi R16,86
brlo red
cpi R16,171
brlo yellow
green:
ldi R17,(1<<PB1)
out PORTB,R17   ; Green
ret  ;rjmp convert
yellow:
ldi R17,(1<<PB4)
out PORTB,R17   ; Yellow
ret   ; rjmp convert
red:
ldi R17,(1<<PB3)
out PORTB,R17   ; Red
ret
;===================================================================

delay:
ldi R20,249
ldi R21,249
ldi R22,6
d2:
dec R20
brne d2
ldi R20,249
dec R21
brne d2
ldi R21,249
dec R22
brne d2
ret

There's obviously something about the ADC I have not understood.  Can somebody please explain what I'm missing?