Reply 20 of 71, by snufkin
Ok, I think I mostly understand that now, although I'm unsure about the voltages on pin 12&13.
I'm going to assume that pin 13 connects directly to R151 (2.2k) and then R182 (10k), R183 (100k) and SW1. The other side of SW1 connects to Ground. R151 and R182 connect to a +ve supply (possibly different supplies). R183 connects to the Base of Q9. Emitter of Q9 goes to Ground.
So when SW1 is open/off then pin 13 is pulled high (but only to 2.1V?) through R182 and R151. Current also flows through R183 in to the base of Q9, so current can flow from C to E. This pulls pin 12 to Ground. So the inputs are FSEL[1..0]=01=66MHz.
When SW1 is closed/on then pin 13 is connected directly to Ground (FSEL0=0) and no current flows from B to E so Q9 is off. The Collector of Q9 connects to R184 (470k) and R152 (2.2k). They probably connect to a +ve supply, so with no current flowing through Q9 they will pull pin 12 high (FSEL1=1), so FSEL[1..0]=10=60MHz. But they only pull it up to 1.7V, which is technically below the level needed to guarantee it's read as a '1' (2V).
So there's direct control of pin 13 (FSEL0) through SW1, with the slightly trickier thing to find an easy way to control pin 12 (FSEL1). Shorting B-E of Q9 will force it off and hold FSEL1 at 1. Shorting Collector to Emitter will hold it at 0. So maybe a 3 pin header connected to Q9, middle pin on Emitter, side pins on Base and Collector. Jumper CE for 0, BE for 1, remove to give control back to SW1.
It all seems a bit more complicated than necessary. The PLL52C59-14A has internal 150k pull ups on the FSEL pins, so I'd expect they'll be there on the -14L, so in theory it'll run at 55MHz with nothing connected and only needs external pull downs. So all the extra external resistors (R182, R184, R151, R152) seem odd
Things that would be good to know though, particularly since I don't understand those input voltages:
Voltages on the vias connecting to R182, R184 and R151.
Voltages on pins 1, 8 and 14 of the clock gen (those should be the Vdd pins, may not all be the same)
There's an odd stub and a via from the Base of Q9. It can't (I think) connect to a power plane, so is there a trace on the other side of the board? I'm curious where it goes because maybe something else can control Q9.
Probably best to check that I'm right about the connections from the clock gen to SW1. So with the board off check for shorts between pin 13 and R151 / R182 / R183, and pin 12 and R152, Q9 pin 3 / R184.
[edit: I drew a circuit of how I think it's connected up, and a possible mod to give independent control for FSEL1 without having to remove any components or bend any pins (suggested method on that previous mod page for the HX2). I just stuck the supply voltage to 2V for the moment:
https://www.falstad.com/circuit/circuitjs.htm … ImvWzpioapK8EAA
[further edit: Ah, and re-reading that other mod page it looks like they already tried the pin 5 mod on the -14L and it is an OE control, so no hidden frequencies there. So it is, as they said, change the clock gen or change the crystal]