Hi all

I've been lurking for a while and I have been working on a hobby project
for the past year (when i have had time) essentially learning whatever i
can when I have spare time.

Working on projects like this are the only way i find that i REALLY
learn, however I think i am now truly stuck and quite possibly out of my
depth and that is one of those thing i hate admitting.

Basically I am working on producing a daughter board with a 16f628-A pic
on it to replace an ageing 21 year old Ferranti ULA chip that is
pre-programmed for working in an instrumentation pod of a Sinclair C5
electric car. The problems associated with this is that I am working to
the constraints of the original electronics that are there. I understand
what the inputs and outputs of this chip are (which helps!!!)

I am going to sell these to Sinclair enthusiasts when I finish, only
because when the original chip malfunctions the C5 cant be driven. I
will never make a profit with them because of the time and effort taken
so far but in reality that is not the reason I am working on them. It is
primarily just a project that forces me to learn but also because so
many people over the years have made promises that they are working on a
replacement chip for it and none have ever come to anything I dont want
this project to fail and I am so close.

The choice of 16f628 was primarily because of the I/O count ADC and the
size of the chip in comparison to the ULA which is a 28pin DIL (Giving
me a little room for manoeuvring within the original chip outline)

I am about 60% of the way there. My daughter board works in a basic
fashion - it operates the power control to the main power relay-
illuminates LED's as appropriate and and i am just now trying to get
some of the other functions to work.

The First problem i have is really as follows.
There is a 5v supply feeding a thermistor that is as shown below which
detects the temperature of the motor across a std voltage divider .


5v------- 10k ntc thermistor------------------------- ferranti dil
socket input 1
| |
| |
| 100ohm
10nf |
| |
0v-----------1k ohm------------------------------------------ferranti
dil socket input 2

It has a thermistor and voltage divider on 2 pins of the chip and what i
am having problems with is understanding how to get pic to accept the
voltage across these two pins.

The operation of the system is that when the 10k thermistor reaches
around 1000 ohms (in practice this is actually 750ohm )the input across
the two is about 0.3 - 0.5v and the chip sets off an audible alarm i
know it probably has a simple answer but my brain is now in a rut.

What I was going to do is take the analogue voltage and display that on
the LED bargraph as an approximate temp display. This display is
presently being utilised by the ULA as a current meter for the motor, I
simply thought that this display could be utilised as a temp display
whenever the motor is not being used as well as utilising the audible
output that is presently there.
I hope this makes some sense.

Its also funny how looking at this and talking about it on email has
made me come up a couple of thoughts but my head hurts.

Thankyou




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