documentation...

examples/pal_reg.v

@@ -20,16 +20,37 @@
 /*
  * This example shows how to use Icarus Verilog to generate PLD output.
  * The design is intended to fit into a 22v10 in a PLCC package, with
- * pin assignments locked down by design.
+ * pin assignments locked down by design. The command to compile this
+ * into a jedec file is;
+ *
+ *   iverilog -tpal -fpart=generic-22v10-plcc -opal_reg.jed pal_reg.v
+ *
+ * The output file name (passed through the -o<file> switch) can be
+ * any file you desire. If the compilation and fittin all succeed, the
+ * output file will be a JEDEC file that you can take to your favorite
+ * PROM programmer to program the part.
+ *
+ * This source demonstrates some important principles of synthesizing
+ * a design for a PLD, including how to specify synchronous logic, and
+ * how to assign signals to pins. The pin assignment in particular is
+ * part specific, and must be right for the fitting to succeed.
  */
 
+
 /*
  * The register module is an 8 bit register that copies the input to
- * the output registers on the rising edge of the clk input. The output
- * drivers are controled by a single active low output enable.
+ * the output registers on the rising edge of the clk input. The
+ * always statement creates a simple d-type flip-flop that is loaded
+ * on the rising edge of the clock.
  *
- * This module contains all the logic of the device, but includes nothing
- * that has anything to do with the real hardware.
+ * The output drivers are controlled by a single active low output
+ * enable. I used bufif0 devices in this example, but the exact same
+ * thing can be achived with a continuous assignment like so:
+ *
+ *   assign out = oe? 8'hzz : Q;
+ *
+ * Many people prefer the expression form. It is true that it does
+ * seem to express the intent a bit more clearly.
  */
 module register (out, val, clk, oe);
 
@@ -53,6 +74,13 @@ endmodule
  * the device. We use this to lock down the pin assignments of the
  * synthesized result. The pin number assignments are for a 22v10 in
  * a PLCC package.
+ *
+ * Note that this module has no logic in it. It is a convention I use
+ * that I put all the functionality in a seperate module (seen above)
+ * and isolate the Icarus Verilog specific $attribute madness into a
+ * top-level module. The advantage of this style is that the entire
+ * module can be `ifdef'ed out when doing simulation and you don't
+ * need to worry that functionality will be affected.
  */
 module pal;
 

tgt-pal/fit_reg.c

@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: fit_reg.c,v 1.1 2000/12/09 03:42:52 steve Exp $"
+#ident "$Id: fit_reg.c,v 1.2 2000/12/09 05:40:42 steve Exp $"
 #endif
 
 # include  <ivl_target.h>
@@ -25,6 +25,13 @@
 # include  <assert.h>
 # include  "priv.h"
 
+/*
+ * The fit_registers function scans all the scopes for flip-flop
+ * devices to be assigned to macrocells. First look to see if the
+ * device is connected to a PAD directly or through a bufif device. If
+ * not, then just pick a free macrocell and drop it there.
+ */
+
 static int scan_ff_q(ivl_lpm_ff_t ff, unsigned q);
 
 int fit_registers(ivl_scope_t scope)
@@ -33,12 +40,17 @@ int fit_registers(ivl_scope_t scope)
       unsigned idx;
       unsigned lpms;
 
+	/* Scan child scopes first... */
       rc = ivl_scope_children(scope, fit_registers);
       if (rc != 0)
 	    return rc;
 
-      lpms = ivl_scope_lpms(scope);
 
+	/* Scan the current scope for flip-flop devices. Pass the
+	   devices we find to the scan_ff_q function to assign to a
+	   macrocell. */
+
+      lpms = ivl_scope_lpms(scope);
       for (idx = 0 ;  idx < lpms ;  idx += 1) {
 	    ivl_lpm_t lpm = ivl_scope_lpm(scope, idx);
 	    ivl_lpm_ff_t ff;
@@ -60,6 +72,10 @@ int fit_registers(ivl_scope_t scope)
       return 0;
 }
 
+/*
+ * This is the part that actually assigns the single bit of a single
+ * flip-flop to a single macrocell.
+ */
 int scan_ff_q(ivl_lpm_ff_t ff, unsigned q)
 {
       unsigned idx;
@@ -93,7 +109,7 @@ int scan_ff_q(ivl_lpm_ff_t ff, unsigned q)
 	    }
       }
 
-	/* There is no poin connection, so try setting this to an
+	/* There is no pin connection, so try setting this to an
 	   unbound sop cell. We know that a sop is unbound if there
 	   are no enables, nexus or ff devices connected to it. */
 
@@ -120,6 +136,9 @@ int scan_ff_q(ivl_lpm_ff_t ff, unsigned q)
 
 /*
  * $Log: fit_reg.c,v $
+ * Revision 1.2  2000/12/09 05:40:42  steve
+ *  documentation...
+ *
  * Revision 1.1  2000/12/09 03:42:52  steve
  *  Stuff registers into macrocells.
  *

tgt-pal/imain.c

@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT) && !defined(macintosh)
-#ident "$Id: imain.c,v 1.2 2000/12/09 03:42:52 steve Exp $"
+#ident "$Id: imain.c,v 1.3 2000/12/09 05:40:42 steve Exp $"
 #endif
 
 /*
@@ -67,8 +67,18 @@ int target_design(ivl_design_t des)
 	   key. Given the part type, try to open the pal description
 	   so that we can figure out the device. */
       part = ivl_design_flag(des, "part");
-      assert(part);
+      if (part == 0) {
+	    fprintf(stderr, "error: part must be specified. Specify a\n");
+	    fprintf(stderr, "     : type with the -fpart=<type> option.\n");
+	    return -1;
+      }
+
       pal = pal_alloc(part);
+      if (pal == 0) {
+	    fprintf(stderr, "error: %s is not a valid part type.\n", part);
+	    return -1;
+      }
+
       assert(pal);
 
       pins = pal_pins(pal);
@@ -132,6 +142,9 @@ DECLARE_CYGWIN_DLL(DllMain);
 
 /*
  * $Log: imain.c,v $
+ * Revision 1.3  2000/12/09 05:40:42  steve
+ *  documentation...
+ *
  * Revision 1.2  2000/12/09 03:42:52  steve
  *  Stuff registers into macrocells.
  *