[Oberon] FPGA Oberon - Reset Button

[Paul Reed]

Hi Tomas,

> I wish to install reset switch, which pin header / button is the reset?
...
> Connector J8
>
> 5V GND GPIO0 GPIO1 GPIO2 GPIO3 GPIO4 GPIO5 GPIO6 GPIO7
>
> 3.3V GND BTN0 BTN1 BTN2 BTN3 N/C N/C N/C N/C

On the Spartan 3 board btn3 is marked "User Reset" but since you have (I
believe) a Pepino you wouldn't necessarily know that. :)

The relevant line in Prof. Wirth's Verilog is in RISC5Top.v, near the bottom:

...
always @(posedge clk)
begin
  rst <= ((cnt1[4:0] == 0) & limit) ? ~btn[3] : rst;
...

Note that this reset merely aborts the current Oberon command and returns
to the main loop, like the Interrupt key on the Ceres.

There isn't an I/O signal which does a cold reset of the Oberon system in
the way a PC reset button works, since the Spartan 3 board has a
pushbutton called "Prog" which reloads the FPGA configuration (forcing a
complete cold start of the hardware).

>From the Pepino schematic it looks like Magnus has the Spartan 6 FPGA PROG
pin just pulled high (signal PROGRAM_B) by resistor R32.  You could try
grounding the other side of the resistor to get the same effect as the
PROG button - but take care not to destroy your board obviously!

There is a pushbutton on the Pepino, marked "BUTTON" on the schematic, but
you'd have to change the Verilog and reprogram the board if you wanted to
use that as btn3.

HTH
Paul