Hold GC1109 FEM power during deep sleep for LNA RX wake

[weebl2000]

Summary

• Use rtc_gpio_hold_en to latch PA_POWER (LDO) and PA_EN (CSD) HIGH during deep sleep, keeping the GC1109 LNA powered for wake-on-packet RX
• Skip these pins in the blanket RTC hold release on wake (initDeepSleep)
• Release them in SX126xInterface::init() after setting GPIO registers HIGH first, avoiding a power glitch
• Add 1ms LDO startup delay on cold boot before driving CSD/CPS, per GC1109 power-on sequence requirement (TLV75733P tSTR ~550us)
• Applies to Heltec V4 and Heltec Wireless Tracker V2

Background

Previously, the GC1109 FEM power pins used weak internal pull-ups (gpio_pullup_en) during deep sleep, which is less reliable than explicitly latching the pad state. Additionally, SX126xInterface::sleep() sets PA_EN LOW before enableLoraInterrupt() re-enables it, creating a window where the GC1109 could lose power.

The blanket rtc_gpio_hold_dis loop in initDeepSleep() also released these pins before SX126xInterface::init() had a chance to set them HIGH, causing a brief power glitch on every deep sleep wake.

Without this fix, the GC1109 LNA could be unpowered during sleep, degrading RX wake sensitivity by ~17dB.

The TLV75733P LDO has a startup time (tSTR) of ~550us. On cold boot, PA_EN (CSD) was being driven HIGH only ~12.5us after PA_POWER, before the LDO output had stabilised. The GC1109 datasheet requires "VBAT must be prior to CSD/CPS/CTX for the power on sequence". On deep sleep wake the LDO is held on via RTC latch, so no delay is needed.

Trade-off: ~1.2mA additional deep sleep current for significantly improved wake-on-packet range.

Reference: meshcore-dev/MeshCore#1600
Reference: #9029 (comments by @compumike, @ascendr, @lindorffs)

Test plan

• Flash Heltec V4, enable ROUTER role (triggers deep sleep with LoRa wake)
• Verify wake-on-packet works at expected range
• Verify TX still works correctly after wake
• Measure deep sleep current (~1.2mA extra expected with LNA active)
• Verify no regression on cold boot (1ms delay is negligible)
• Confirm normal RX/TX operation after cold boot