IO Mapping

中文说明

FrameTop exposes 73 bidirectional pads:

inout wire [72:0] user_io

Fixed partition

FrameTop pad	Purpose	User design port
user_io[6:0]	7-bit design ID, sampled only during reset	Not visible
user_io[72:7]	66-bit payload for the selected design	io_in/io_out/io_oe[65:0]

The fixed conversion is io_*[n] in a user design to FrameTop.user_io[n + 7]. For example, io_out[0] drives user_io[7], and io_in[65] reads user_io[72].

Direction contract

Each payload bit uses three signals:

• io_in[n] always reads the resolved pad level;
• with io_oe[n] = 1, the design drives the pad from io_out[n];
• with io_oe[n] = 0, the design releases the pad for an external driver.

The design must assign known values to every io_out and io_oe bit. An external source must not drive an opposing level while io_oe[n] = 1, because that is electrical contention.

Design selection

The environment drives user_io[6:0] while reset = 1 and keeps it stable for at least two rising clock edges. The ID is locked after reset release; changing the pads at runtime does not switch designs. Reassert reset to select again. Design 0 is the fixed reference, and user slots range from 1 through 127.

An unregistered design ID starts no user clock and leaves all payload pads at high impedance.