imxrt: hold EVKB Ethernet PHY in reset while tracing, 50 MHz trace pin

Fresh bring-up pass on mimxrt1170_evkb: holding the 100M RTL8201 in reset
(ENET_RST_B = GPIO_LPSR_04) stops its RMII lines driving against the
shared trace pads and doubles the clean trace-pin rate to 50 MHz
(100 MHz CSTRACE root; 133 MHz root is marginal, stock 132 corrupts).
Validated 3x 8 s TinyUSB captures at 11.46M fetches. D1-D3 remain silent
in every configuration - the welded R1882-R1884 are electrically open;
reflow is the remaining step to width 4. Board notes gain JP4 (must be
shorted for an external probe on J58).

2 files changed, 25 insertions, 10 deletions

diff --git a/.claude/skills/etm-trace/boards.md b/.claude/skills/etm-trace/boards.md
index dd628ea80..ea6c6181e 100644
--- a/.claude/skills/etm-trace/boards.md
+++ b/.claude/skills/etm-trace/boards.md
@@ -23,7 +23,7 @@ reference.
 | ea4088_quickstart  | 120 MHz              | 120 MHz               | 4     | 0 (unset) | J7 (fully wired)              | —                                           |
 | nrf52840dk         | 64 MHz               | 16 MHz (hw cap)       | 4     | 0 (unset) | solder P25, SW7 → Alt         | —                                           |
 | nrf5340dk (M33)    | 64 MHz               | 16 MHz (TAD, forced)  | 4     | +3 ns   | mount P25; cut SB27/SB28      | —                                           |
-| mimxrt1170_evkb    | 996 MHz              | 25 MHz (root/2)       | 1     | 0       | populate 0 Ω R1881-R1886; J58 | verify/reflow R1882-R1884 → width 4         |
+| mimxrt1170_evkb    | 996 MHz              | 50 MHz (root/2)       | 1     | 0       | weld 0 Ω R1881-R1886; JP4 shorted; J58 (populated) | reflow R1882-R1884 (D1-D3 open) → width 4 |
 | ra6m5_ek (M33)     | 200 MHz              | 25 MHz (TRCLK/4 /2)   | 4     | 0 (unset) | J9 closed; native J20 trace | —                                           |
 | ra8m1_ek (M85)     | 480 MHz              | 60 MHz (TRCLK/4 /2)   | 4     | 0 (unset) | J9 closed + Table 7 jumpers | —                                           |
 | raspberry_pi_pico2 (RP2350 M33) | 48 MHz  | 24 MHz (clk_sys/2)    | 4     | 0 (unset) | fly-wire GPIO1-5 → MIPI20 (map in jdebug) | 72-80 MHz per seating (re-qualify); >80 needs V3 probe + trace board |
@@ -66,13 +66,19 @@ Board caveats (beyond the table):
   builds force the TAD port to 16 MHz (SystemInit's 64 MHz is marginal).
 - **ea4088_quickstart**: FS enumeration ends < 100 ms — for `--isr` use
   `--duration-ms 150`. Boot-ROM address warning (0x1FFF1FF0) is normal.
-- **mimxrt1170_evkb**: width 4 fails (D1-D3 path under investigation).
-  `trace_etm_init` fixes the JTAG_nTRST/DMIC_DATA1 pad, pins CSTRACE to
-  50 MHz (stock 132 MHz corrupts — the 100M PHY drives the CLK net) and
-  enables the CM7 platform trace-funnel port, which J-Link doesn't program:
-  without it everything reads register-perfect yet zero data arrives.
-  FlexSPI apps: ROM bootloader must set SP/PC — the committed reset/download
-  hooks handle this. Startup-burst overflow at 996 MHz is normal.
+- **mimxrt1170_evkb**: width 1 only — D1-D3 are stone silent at any config
+  (pinmux register-perfect, PHY quieted, funnel enabled): the welded 0402s
+  R1882/R1883/R1884 are electrically open — reflow to unlock width 4.
+  `trace_etm_init` fixes the JTAG_nTRST/DMIC_DATA1 pad, holds the 100M
+  RTL8201 PHY in reset (ENET_RST_B = GPIO_LPSR_04 — its RMII lines share
+  the trace pads; with it quiet the CLK line runs a 100 MHz root/50 MHz
+  pin, 2x the pre-lever rate; 133 MHz root is marginal, stock 132 corrupts)
+  and enables the CM7 platform trace-funnel port, which J-Link doesn't
+  program: without it everything reads register-perfect yet zero data
+  arrives. JP4 must be shorted (disables MCU-Link SWD) for the external
+  J-Trace on J58. FlexSPI apps: ROM bootloader must set SP/PC — the
+  committed reset/download hooks handle this. Startup-burst overflow at
+  996 MHz is normal. No Ethernet (100M) while tracing.
 - **ra6m5_ek**: TRCKCR div-2 (100 MHz TRCLK = 50 MHz pin, the chip max) is
   unusable on this board — swept widths 1/4 across -2..+4 ns, all dead;
   TRACE_ETM builds use div-4 (25 MHz pin). The TCLK pin runs TRCLK/2. 50 MHz SWD TIF
diff --git a/hw/bsp/imxrt/family.c b/hw/bsp/imxrt/family.c
index e3ef6233c..1b1a1f1d8 100644
--- a/hw/bsp/imxrt/family.c
+++ b/hw/bsp/imxrt/family.c
@@ -131,6 +131,13 @@ static void trace_etm_init(void) {
   // breaks ETM trace - switch the pad to GPIO (MIMXRT1170-EVKB HUG 3.2)
   IOMUXC_SetPinMux(IOMUXC_GPIO_LPSR_10_GPIO12_IO10, 0U);
 
+  // Hold the 100M Ethernet PHY (RTL8201) in reset: its RMII lines are
+  // hardwired to the trace pads and drive against the stream at speed
+  // (ENET_RST_B = GPIO_LPSR_04)
+  IOMUXC_SetPinMux(IOMUXC_GPIO_LPSR_04_GPIO12_IO04, 0U);
+  GPIO12->GDIR |= (1U << 4);
+  GPIO12->DR &= ~(1U << 4);
+
   // TRACE0-3 + TRACE_CLK on GPIO_DISP_B2_02..06, fast slew + high drive
   IOMUXC_SetPinMux(IOMUXC_GPIO_DISP_B2_02_ARM_TRACE00, 0U);
   IOMUXC_SetPinMux(IOMUXC_GPIO_DISP_B2_03_ARM_TRACE01, 0U);
@@ -143,9 +150,11 @@ static void trace_etm_init(void) {
   IOMUXC_SetPinConfig(IOMUXC_GPIO_DISP_B2_05_ARM_TRACE03, IOMUXC_SW_PAD_CTL_PAD_DSE_MASK);
   IOMUXC_SetPinConfig(IOMUXC_GPIO_DISP_B2_06_ARM_TRACE_CLK, IOMUXC_SW_PAD_CTL_PAD_DSE_MASK);
 
-  // 50 MHz CSTRACE root (OscRc400M/8) -> 25 MHz TRACE_CLK pin (= root/2)
+  // 100 MHz CSTRACE root (OscRc400M/4) -> 50 MHz TRACE_CLK pin (= root/2).
+  // With the PHY held in reset the CLK line is clean here; 133 MHz root
+  // (66 MHz pin) is marginal on this board, stock 132 MHz corrupts.
   CLOCK_SetRootClockMux(kCLOCK_Root_Cstrace, kCLOCK_CSTRACE_ClockRoot_MuxOscRc400M);
-  CLOCK_SetRootClockDiv(kCLOCK_Root_Cstrace, 8);
+  CLOCK_SetRootClockDiv(kCLOCK_Root_Cstrace, 4);
   CLOCK_EnableClock(kCLOCK_Cstrace);
 
   // Enable the CM7 slave port on the platform trace funnel (E004_3000): the