I nearly forgot: some more text about the innards of the 8701 is
hereNow for the oscillator section.
Recommended reading: Fairchild Application Note AN-340 about CMOS oscillators.
Attachment:
AN-340.pdf [146.5 KiB]
Downloaded 220 times
Two interesting areas on the chip:
First picture: (South to Nord)
A dead superbuffer, which seems to be a leftover from a previous revision of the silicon.
Two superbuffers and a FET working as a resistor in parallel.
Attachment:
osc1.png [ 15.96 KiB | Viewed 5757 times ]
Second picture: (South to Nord)
Two pads for connecting the external crystal, Input and output.
ESD protection for these pads.
An oddly shaped resistor in the Crystal output signal before it goes to the output pad,
and some part of the big capacitor connected to the Crystal input pad.
Attachment:
osc2.png [ 31.21 KiB | Viewed 5757 times ]
Note, that ESD protection for the output pad is connected to GND instead of GND_Osc to simplify the chip layout.
About that oddly shaped resistor in the oscillator output
(PolySilicon "snake" nailed to a metal trace with vias):
I think the designers wanted to be able to increase the resistance without changing the layout by just removing vias, starting from West to East.
Attachment:
8701_oscillator.png [ 245.36 KiB | Viewed 5757 times ]