afb
Member
Hi ya - I have a problem with the 18X - anything stupid that I've overlooked perhaps or is it something you have seen/heard of before?
I have a 5k pot wired across the 5v supply, the wiper connected to leg18 (INPUT1/ADC1). Without the chip in the socket, twiddling the pot gives a nice smooth variation from 0 to 5v.
With a (brand new) chip in the socket, twiddling the pot I find the (scope) observed voltage on leg18 feels like its pegged to 0v, only swinging 0v to 5v over the last 10% or so of the pot as the low ohms at the end of the track fight whatever is holding leg 18 down. With a test program I verified that the ADC is actually reading the input voltage in a representative manner over this narrow adjustment range of the pot.
I have a second (used) 18X chip that behaves in just the same way. My only previous 18X circuit used leg18 as a digital input, not as an ADC, and it was driven directly by an output pin of an 08M, so I this may have masked a potential problem.
I once had a problem not unlike this where a pull-up needed to be *very* low before it worked, because at a later date in the project I'd unwittingly used what I thought was a spare pin as an input, whereas it had in fact already been used as an (unconnected) output for a timing delay using "pulsout" - but this was on the 8M where some pins can be assigned as IN or OUT. The 18X has dedicated INs and OUTs
Even so, I've tried a zero length program and then a three line trivial program accessing ADC1 just to be sure - no luck! I've also tried it without the programming lead attached and on a 3v supply.
On my circuit board, the other two ADCs (0 and 2) are similarly connected to pots and their inputs can be smoothly altered over the whole pot range with the chip in place, like you'd expect.
I'm out of ideas !!!! Any help would be gratefully received.
Alan
I have a 5k pot wired across the 5v supply, the wiper connected to leg18 (INPUT1/ADC1). Without the chip in the socket, twiddling the pot gives a nice smooth variation from 0 to 5v.
With a (brand new) chip in the socket, twiddling the pot I find the (scope) observed voltage on leg18 feels like its pegged to 0v, only swinging 0v to 5v over the last 10% or so of the pot as the low ohms at the end of the track fight whatever is holding leg 18 down. With a test program I verified that the ADC is actually reading the input voltage in a representative manner over this narrow adjustment range of the pot.
I have a second (used) 18X chip that behaves in just the same way. My only previous 18X circuit used leg18 as a digital input, not as an ADC, and it was driven directly by an output pin of an 08M, so I this may have masked a potential problem.
I once had a problem not unlike this where a pull-up needed to be *very* low before it worked, because at a later date in the project I'd unwittingly used what I thought was a spare pin as an input, whereas it had in fact already been used as an (unconnected) output for a timing delay using "pulsout" - but this was on the 8M where some pins can be assigned as IN or OUT. The 18X has dedicated INs and OUTs
Even so, I've tried a zero length program and then a three line trivial program accessing ADC1 just to be sure - no luck! I've also tried it without the programming lead attached and on a 3v supply.
On my circuit board, the other two ADCs (0 and 2) are similarly connected to pots and their inputs can be smoothly altered over the whole pot range with the chip in place, like you'd expect.
I'm out of ideas !!!! Any help would be gratefully received.
Alan