[otbn] Document the idle_o signal
As Sri pointed out, I've just added some DV code that checks the
idle_o signal does what I think it should do, but this isn't actually
specced anywhere. Fix that!
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/doc/_index.md b/hw/ip/otbn/doc/_index.md
index 6a8e37a..f345c07 100644
--- a/hw/ip/otbn/doc/_index.md
+++ b/hw/ip/otbn/doc/_index.md
@@ -507,6 +507,12 @@
If OTBN was running, this value will also be reflected in the {{< regref "ERR_BITS" >}} register.
A fatal alert can only be cleared by resetting OTBN through the `rst_ni` line.
+### Idle
+
+OTBN exposes a single-bit `idle_o` signal, intended to be used by the clock manager to clock-gate the block when it is not in use.
+This signal is high when OTBN is not running.
+The cycle after a write to {{< regref "CMD.start" >}}, the signal goes low.
+This remains low until the end of the operation (either from an [`ECALL`]({{< relref "isa#ecall" >}}) or an error, at which point it goes high again.
# Running applications on OTBN
