diff --git a/src/rp2_common/hardware_clocks/scripts/vcocalc.py b/src/rp2_common/hardware_clocks/scripts/vcocalc.py
index 5d143b3..69f4d02 100755
--- a/src/rp2_common/hardware_clocks/scripts/vcocalc.py
+++ b/src/rp2_common/hardware_clocks/scripts/vcocalc.py
@@ -4,6 +4,7 @@ import argparse
 
 parser = argparse.ArgumentParser(description="PLL parameter calculator")
 parser.add_argument("--input", "-i", default=12, help="Input (reference) frequency. Default 12 MHz", type=float)
+parser.add_argument("--ref-min", default=5, help="Override minimum reference frequency. Default 5 MHz", type=float)
 parser.add_argument("--vco-max", default=1600, help="Override maximum VCO frequency. Default 1600 MHz", type=float)
 parser.add_argument("--vco-min", default=750, help="Override minimum VCO frequency. Default 750 MHz", type=float)
 parser.add_argument("--low-vco", "-l", action="store_true", help="Use a lower VCO frequency when possible. This reduces power consumption, at the cost of increased jitter")
@@ -13,25 +14,32 @@ args = parser.parse_args()
 # Fixed hardware parameters
 fbdiv_range = range(16, 320 + 1)
 postdiv_range = range(1, 7 + 1)
+ref_min = 5
+refdiv_min = 1
+refdiv_max = 63
 
-best = (0, 0, 0, 0)
+refdiv_range = range(refdiv_min, max(refdiv_min, min(refdiv_max, int(args.input / args.ref_min))) + 1)
+
+best = (0, 0, 0, 0, 0)
 best_margin = args.output
 
-for fbdiv in (fbdiv_range if args.low_vco else reversed(fbdiv_range)):
-	vco = args.input * fbdiv
-	if vco < args.vco_min or vco > args.vco_max:
-		continue
-	# pd1 is inner loop so that we prefer higher ratios of pd1:pd2
-	for pd2 in postdiv_range:
-		for pd1 in postdiv_range:
-			out = vco / pd1 / pd2
-			margin = abs(out - args.output)
-			if margin < best_margin:
-				best = (out, fbdiv, pd1, pd2)
-				best_margin = margin
+for refdiv in refdiv_range:
+	for fbdiv in (fbdiv_range if args.low_vco else reversed(fbdiv_range)):
+		vco = args.input / refdiv * fbdiv
+		if vco < args.vco_min or vco > args.vco_max:
+			continue
+		# pd1 is inner loop so that we prefer higher ratios of pd1:pd2
+		for pd2 in postdiv_range:
+			for pd1 in postdiv_range:
+				out = vco / pd1 / pd2
+				margin = abs(out - args.output)
+				if margin < best_margin:
+					best = (out, fbdiv, pd1, pd2, refdiv)
+					best_margin = margin
 
 print("Requested: {} MHz".format(args.output))
 print("Achieved: {} MHz".format(best[0]))
-print("FBDIV: {} (VCO = {} MHz)".format(best[1], args.input * best[1]))
+print("REFDIV: {}".format(best[4]))
+print("FBDIV: {} (VCO = {} MHz)".format(best[1], args.input / best[4] * best[1]))
 print("PD1: {}".format(best[2]))
 print("PD2: {}".format(best[3]))