Increase PLL min VCO from 400MHz to 750MHz for improved stability across operating conditions (#869)

Co-authored-by: graham sanderson <graham.sanderson@raspberrypi.com>

Author: liamfraser

src/rp2040/hardware_regs/include/hardware/regs/pll.h

@@ -1,5 +1,5 @@
 /**
- * Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
+ * Copyright (c) 2022 Raspberry Pi (Trading) Ltd.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -17,7 +17,7 @@
 //               GENERAL CONSTRAINTS:
 //               Reference clock frequency min=5MHz, max=800MHz
 //               Feedback divider min=16, max=320
-//               VCO frequency min=400MHz, max=1600MHz
+//               VCO frequency min=750MHz, max=1600MHz
 #define PLL_CS_OFFSET _u(0x00000000)
 #define PLL_CS_BITS   _u(0x8000013f)
 #define PLL_CS_RESET  _u(0x00000001)

src/rp2040/hardware_regs/rp2040.svd

@@ -22367,7 +22367,7 @@
             GENERAL CONSTRAINTS:\n
             Reference clock frequency min=5MHz, max=800MHz\n
             Feedback divider min=16, max=320\n
-            VCO frequency min=400MHz, max=1600MHz</description>
+            VCO frequency min=750MHz, max=1600MHz</description>
           <fields>
             <field>
               <access>read-only</access>

src/rp2_common/hardware_clocks/clocks.c

@@ -148,13 +148,13 @@ void clocks_init(void) {
     /// \tag::pll_settings[]
     // Configure PLLs
     //                   REF     FBDIV VCO            POSTDIV
-    // PLL SYS: 12 / 1 = 12MHz * 125 = 1500MHZ / 6 / 2 = 125MHz
-    // PLL USB: 12 / 1 = 12MHz * 40  = 480 MHz / 5 / 2 =  48MHz
+    // PLL SYS: 12 / 1 = 12MHz * 125 = 1500MHz / 6 / 2 = 125MHz
+    // PLL USB: 12 / 1 = 12MHz * 100 = 1200MHz / 5 / 5 =  48MHz
     /// \end::pll_settings[]
 
     /// \tag::pll_init[]
     pll_init(pll_sys, 1, 1500 * MHZ, 6, 2);
-    pll_init(pll_usb, 1, 480 * MHZ, 5, 2);
+    pll_init(pll_usb, 1, 1200 * MHZ, 5, 5);
     /// \end::pll_init[]
 
     // Configure clocks

src/rp2_common/hardware_clocks/scripts/vcocalc.py

@@ -5,7 +5,7 @@
 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("--vco-max", default=1600, help="Override maximum VCO frequency. Default 1600 MHz", type=float)
-parser.add_argument("--vco-min", default=400, help="Override minimum VCO frequency. Default 400 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")
 parser.add_argument("output", help="Output frequency in MHz.", type=float)
 args = parser.parse_args()

src/rp2_common/hardware_pll/include/hardware/pll.h

@@ -31,6 +31,14 @@ typedef pll_hw_t *PLL;
 #define pll_sys pll_sys_hw
 #define pll_usb pll_usb_hw
 
+#ifndef PICO_PLL_VCO_MIN_FREQ_MHZ
+#define PICO_PLL_VCO_MIN_FREQ_MHZ 750
+#endif
+
+#ifndef PICO_PLL_VCO_MAX_FREQ_MHZ
+#define PICO_PLL_VCO_MAX_FREQ_MHZ 1600
+#endif
+
 /*! \brief Initialise specified PLL.
  *  \ingroup hardware_pll
  * \param pll pll_sys or pll_usb

src/rp2_common/hardware_pll/pll.c

@@ -13,6 +13,9 @@
 void pll_init(PLL pll, uint refdiv, uint vco_freq, uint post_div1, uint post_div2) {
     uint32_t ref_mhz = XOSC_MHZ / refdiv;
 
+    // Check vco freq is in an acceptable range
+    assert(vco_freq >= (PICO_PLL_VCO_MIN_FREQ_MHZ * MHZ) && vco_freq <= (PICO_PLL_VCO_MAX_FREQ_MHZ * MHZ));
+
     // What are we multiplying the reference clock by to get the vco freq
     // (The regs are called div, because you divide the vco output and compare it to the refclk)
     uint32_t fbdiv = vco_freq / (ref_mhz * MHZ);

src/rp2_common/pico_stdlib/stdlib.c

@@ -73,7 +73,7 @@ bool check_sys_clock_khz(uint32_t freq_khz, uint *vco_out, uint *postdiv1_out, u
     uint crystal_freq_khz = clock_get_hz(clk_ref) / 1000;
     for (uint fbdiv = 320; fbdiv >= 16; fbdiv--) {
         uint vco = fbdiv * crystal_freq_khz;
-        if (vco < 400000 || vco > 1600000) continue;
+        if (vco < PICO_PLL_VCO_MIN_FREQ_MHZ * 1000  || vco > PICO_PLL_VCO_MAX_FREQ_MHZ * 1000) continue;
         for (uint postdiv1 = 7; postdiv1 >= 1; postdiv1--) {
             for (uint postdiv2 = postdiv1; postdiv2 >= 1; postdiv2--) {
                 uint out = vco / (postdiv1 * postdiv2);