intel_alm: Documentation improvements

techlibs/intel_alm/common/dff_sim.v

@@ -1,3 +1,47 @@
+// The four D flip-flops (DFFs) in a Cyclone V/10GX Adaptive Logic Module (ALM)
+// act as one-bit memory cells that can be placed very flexibly (wherever there's
+// an ALM); each flop is represented by a MISTRAL_FF cell.
+//
+// The flops in these chips are rather flexible in some ways, but in practice
+// quite crippled by FPGA standards.
+//
+// What the flops can do
+// ---------------------
+// The core flop acts as a single-bit memory that initialises to zero at chip
+// reset. It takes in data on the rising edge of CLK if ENA is high,
+// and outputs it to Q. The ENA (clock enable) pin can therefore be used to
+// capture the input only if a condition is true.
+//
+// The data itself is zero if SCLR (synchronous clear) is high, else it comes
+// from SDATA (synchronous data) if SLOAD (synchronous load) is high, or DATAIN
+// if SLOAD is low.
+//
+// If ACLR (asynchronous clear) is low then Q is forced to zero, regardless of
+// the synchronous inputs or CLK edge. This is most often used for an FPGA-wide
+// power-on reset.
+//
+// An asynchronous set that sets Q to one can be emulated by inverting the input
+// and output of the flop, resulting in ACLR forcing Q to zero, which then gets
+// inverted to produce one. Likewise, logic can operate on the falling edge of
+// CLK if CLK is inverted before being passed as an input.
+//
+// What the flops *can't* do
+// -------------------------
+// The trickiest part of the above capabilities is the lack of configurable
+// initialisation state. For example, it isn't possible to implement a flop with
+// asynchronous clear that initialises to one, because the hardware initialises
+// to zero. Likewise, you can't emulate a flop with asynchronous set that
+// initialises to zero, because the inverters mean the flop initialises to one.
+//
+// If the input design requires one of these cells (which appears to be rare
+// in practice) then synth_intel_alm will fail to synthesize the design where
+// other Yosys synthesis scripts might succeed.
+//
+// This stands in notable contrast to e.g. Xilinx flip-flops, which have
+// configurable initialisation state and native synchronous/asynchronous
+// set/clear (although not at the same time), which means they can generally
+// implement a much wider variety of logic.
+
 // DATAIN: synchronous data input
 // CLK: clock input (positive edge)
 // ACLR: asynchronous clear (negative-true)