[Libre-soc-bugs] [Bug 517] Define JTAG pins and set up with litex for ulx3s85f

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=517

--- Comment #30 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Cole Poirier from comment #29)
> ```proposed FPGA External Pin to STLINK JTAG pin connecitons
>     pin #  | label # | FPGA IO PAD | GPIO # (n/p) | JTAG Pin # (Signal)  |
>  1         |  3.3v   |    NONE     |     3v3      |      1 (MCU VDD)     |
>  2         |  3.3v   |    NONE     |     3v3      |      2 (MCU VDD)     |

good, yes.  the power *from* the ulx3s goes *into* the IO side of the STLinkv2
so as to provide the Reference Voltage it needs relative to ulx3s's IO.

>  3         | -|(GND) |    NONE     |     GND      |      3 (JNTRST)      |

don't connect up reset, it's a driving pin (output) and it looks like you
shorted it to GND.  that would damage the STLinkv2.

we're running "soft" reset (which is done with 5 ticks of TMS set to 1 i think)

>  4         | -|(GND) |    NONE     |     GND      |      4 (GND)         |
>  5 (J1_5-) |    0    |     C11     |    gn[0]     |      5 (JTDI)        |
>  6 (J1_5+) |    0    |     B11     |    gp[0]     |      7 (JTMS)        |
>  7 (J1_7-) |    1    |     A11     |    gn[1]     |      9 (JTCK)        |
>  8 (J1_7+) |    1    |     A10     |    gp[1]     |     13 (JTDO)        |

these look good i.e. accurate n0 p0 n1 p1 to the platform request below

>  9 (J1_9-) |    2    |     B10     |    gn[2]     |       NONE           |
> 10 (J1_9+) |    2    |     A9      |    gp[2]     |       NONE           |
> ```
> 
> ```proposed nmigen in litex/florent/versa_ecp5.py
> gpio0    = self.platform.request("gpio", 0)
> gpio1    = self.platform.request("gpio", 1)
> 
> 
> jtag_tdi = gpio0.n
> jtag_tms = gpio0.p
> jtag_tck = gpio1.n
> jtag_tdo = gpio1.p
> 
> # wire the pins up to CPU JTAG
> self.comb += self.cpu.jtag_tdi.eq(jtag_tdi)
> self.comb += self.cpu.jtag_tms.eq(jtag_tms)
> self.comb += self.cpu.jtag_tdi.eq(jtag_tdi)
> self.comb += jtag_tdo.eq(self.cpu.jtag_tdo)
> ```

yeah that looks good to me

> Hi Luke, here is my plan for the physical connecting of the FPGA gpio pins
> to the STLINK JTAG pins, and the nmigen code to initialize these selected
> gpio pins as the 4 necessary JTAG pins. Can you approve this?

no i cannot.  i can help you *review* it however i am not going to "approve" it
because it's your hardware, and your responsibility to make sure the
connections are right and don't involve any short-circuits.

if i "approve" it, you might make the mistake of thinking that *i* am taking
that responsibility away from you.

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