diff --git a/stmhal/boards/pllvalues.py b/stmhal/boards/pllvalues.py
new file mode 100644
index 000000000..183313f30
--- /dev/null
+++ b/stmhal/boards/pllvalues.py
@@ -0,0 +1,115 @@
+"""
+This is an auxiliary script that is used to compute valid PLL values to set
+the CPU frequency to a given value.  The algorithm here appears as C code
+for the machine.freq() function.
+"""
+
+def close_int(x):
+    return abs(x - round(x)) < 0.01
+
+# original version that requires N/M to be an integer (for simplicity)
+def compute_pll(hse, sys):
+    for P in (2, 4, 6, 8): # allowed values of P
+        Q = sys * P / 48
+        NbyM = sys * P / hse
+        # N/M and Q must be integers
+        if not (close_int(NbyM) and close_int(Q)):
+            continue
+        # VCO_OUT must be between 192MHz and 432MHz
+        if not (192 <= hse * NbyM <= 432):
+            continue
+        # compute M
+        M = int(192 // NbyM)
+        while hse > 2 * M or NbyM * M < 192:
+            M += 1
+        # VCO_IN must be between 1MHz and 2MHz (2MHz recommended)
+        if not (M <= hse):
+            continue
+        # compute N
+        N = NbyM * M
+        # N and Q are restricted
+        if not (192 <= N <= 432 and 2 <= Q <= 15):
+            continue
+        # found valid values
+        assert NbyM == N // M
+        return (M, N, P, Q)
+    # no valid values found
+    return None
+
+# improved version that doesn't require N/M to be an integer
+def compute_pll2(hse, sys):
+    for P in (2, 4, 6, 8): # allowed values of P
+        Q = sys * P / 48
+        # Q must be an integer in a set range
+        if not (close_int(Q) and 2 <= Q <= 15):
+            continue
+        NbyM = sys * P / hse
+        # VCO_OUT must be between 192MHz and 432MHz
+        if not (192 <= hse * NbyM <= 432):
+            continue
+        # compute M
+        M = 192 // NbyM # starting value
+        while hse > 2 * M or NbyM * M < 192 or not close_int(NbyM * M):
+            M += 1
+        # VCO_IN must be between 1MHz and 2MHz (2MHz recommended)
+        if not (M <= hse):
+            continue
+        # compute N
+        N = NbyM * M
+        # N must be an integer
+        if not close_int(N):
+            continue
+        # N is restricted
+        if not (192 <= N <= 432):
+            continue
+        # found valid values
+        return (M, N, P, Q)
+    # no valid values found
+    return None
+
+def verify_and_print_pll(hse, sys, pll):
+    M, N, P, Q = pll
+
+    # compute derived quantities
+    vco_in = hse / M
+    vco_out = hse * N / M
+    pllck = hse / M * N / P
+    pll48ck = hse / M * N / Q
+
+    # verify ints
+    assert close_int(M)
+    assert close_int(N)
+    assert close_int(P)
+    assert close_int(Q)
+
+    # verify range
+    assert 2 <= M <= 63
+    assert 192 <= N <= 432
+    assert P in (2, 4, 6, 8)
+    assert 2 <= Q <= 15
+    assert 1 <= vco_in <= 2
+    assert 192 <= vco_out <= 432
+
+    # print out values
+    print(out_format % (sys, M, N, P, Q, vco_in, vco_out, pllck, pll48ck))
+
+def main():
+    global out_format
+    import sys
+    if len(sys.argv) != 2:
+        print("usage: pllvalues.py <hse in MHz>")
+        sys.exit(1)
+    hse_value = int(sys.argv[1])
+    print("HSE =", hse_value, "MHz")
+    print("sys :  M      N     P     Q : VCO_IN VCO_OUT   PLLCK PLL48CK")
+    out_format = "%3u : %2u  %.1f  %.2f  %.2f :  %5.2f  %6.2f  %6.2f  %6.2f"
+    n_valid = 0
+    for sysclk in range(1, 217):
+        pll = compute_pll2(hse_value, sysclk)
+        if pll is not None:
+            n_valid += 1
+            verify_and_print_pll(hse_value, sysclk, pll)
+    print("found %u valid configurations" % n_valid)
+
+if __name__ == "__main__":
+    main()