Sunday, 28 July 2013

Adding a crystal and capacitors the the HP-20b, HP-30b or WP-34s

Here is my experience of mounting the capacitors and crystal in the HP-30b or WP-34s.

The tools I used were simple:

Choosing the crystal and capacitor:
 Pretty much any 32.768 kHz crystal of the right dimensions will do. Usually you will find them under "watch crystals". I used the Abracon AB26T:

Choosing the capacitors:
For information on attaching a crystal and capacitors to the AT91SAM7L processor used in the HP-20b/30b see this application note page 9:

Equivalent circuit:

Total capacitative load on the crystal is the 2 external capacitors in series (as far as the crystal is concerned, the ground connects C1 & C2 in series to itself) + the capacitance of the processor and stray capacitance in parallel:
C= (C1*C2)/(C1+C2) + C + Cs 

C= the capacitative load on the crystal
C1 & C2 are the 2 external capacitors we are going to add
Ci = internal capacitance of the processor for the crystal pins
Cs = stray (track & PCB) capacitance

If we keep the 2 external capacitors the same value: C1 = C2 = C
C= C/2 + C+ Cs

From the Atmel app note above: Ci = 2.5pF
From the crystal data sheet: CL = 12.5 pF
Cs estimated ~1 pF
thus C = (C- (C+ Cs))*2 = 18 pF

The Atmel datasheet specifies a maximum of 20pF, so 18pF is in the ball-park. (section 35.4.3 table 35-15).

For choice of capacitor, I avoided Y5V in this application as they have a wide capacitance variation with temperature which will alter the frequency of the crystal - not what we want for a stable clock.

I would recommend X7R or NP0/C0G. I used Multicomp - MC0603N180J500CT - (MLCC, 18pF, 5%, 50V, C0G / NP0, 0603).

The size of the capacitor is 0603 (1.6 x 0.8 mm).  This is the size of a grain of sand. Order more than two as it is easy to loose them!

(Note 0603 is the imperial size SMT, in metric it would be 1608. Be careful to check when ordering!!)

OK, so now for the mounting. I did the capacitors first and the the crystal.
Here are a video I made mounting the first capacitor:
(Note, I am not a professional solderer, but a home hobbyist)

The final product:

I hope you have fun doing yours :-)

(I ordered my crystal and caps in bulk, so have about 100 spare sets. Anyone want some for postage only, see eml below. Each set  will have 1 crystal and 4 caps).

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