Hello.
On 06/27/2018 09:47 AM, Luke Kenneth Casson Leighton wrote:
On Wed, Jun 27, 2018 at 5:23 AM, Ludwig Jaffe ludwig.jaffe@gmail.com wrote:
Hi people,
Pinmuxes are dangerous, because they sometimes mux the wrong thing like having 2 differential pins one could choose between usb or sata. If one wants both?!
that's precisely why i am not muxing USB or SATA. SATA because they require specialist IO pads with voltage reference levels way WAY different from standard GPIO.
Assume that all this hiqh-speed busses using dedicated IO cells. As we are talking about muxing, this belongs to more-or-less standard CMOS/TTL leveled signals - no differential, no MIPI (with extra small swing), no SERDES signals (USB, SATA, Gbit Ethernet, RIO,..)
basically a pinmux is good for low-speed *digital* signals up to around 200mhz, possibly 300mhz if you're really lucky.
Assume a cut-off frequency (-3dB) of roundabout 150 MHz on Pads.
Many SOCs I encountered work like this.
Many SoC and FPGA do have "configurable" GPIO IO-Banks for that.
Go for a large package with a good socket and have not too much need to mux,
Assume, that the package itself takes roughly 90% of the total chip cost. Reducing the complexity / pincount also reducing the cost. Large packages are a pain in the ass, just mostly using for having more pins for power and ground available.
also you cannot fit a 1,200 pin chip of size 40mm x 40mm onto a PCB of size 38 x 50mm.
I would suggest to use old Intel sockets as the ZIF-Sockets are in the wild and the package manufacturers also produce them.
the socket alone will cost about the same or possibly even more than the SoC, even in mass-volume quantities.
@ludwig:
The reason why we have now ball-grid arrays instead of real pins is the cut-off frequency for that old-style pins, which is quite lower than for ball-grids. Extra sockets are extra pain.
Did you know, that for high-speed memories now the internal bond-wire has to taken into account of calculating the delay (for getting the proper sampling time point) on PCBs?
i'm developing a $3 to $4 15x15mm 3W SoC, ludwig, not a $500 intel 150W completely impractical billion-transistor chip with a sale price of over $500.
Yes, an this chips has to fit together with a couple of another chips into the EOMA86 housing, known from PCMCIA cards.
Regards, Hagen Sankowski