SPI0 Flash Utilities

An Arduino ESP8266 library that supports reclaiming the use of GPIO pins 9 and 10 on ESP8266 modules that expose access to those pins. The build requires SPI Flash Mode "DIO" or "DOUT".

I expect this solution to work for the ESP-12F Module and the NodeMCU 1.0 DEV board; however, it may depend on the SPI flash chip the module manufacturer selected. The few I have are working. I assume board vendors that expose GPIO pins 9 and 10 have made wise choices with their flash chip selection and allow the pin functions /WP and /HOLD to be disabled. I am cautiously optimistic about this solution. Confidence will only come with experience.

Various operations were generalized and placed in ModeDIO_ReclaimGPIOs.h, SpiFlashUtils.h, and SpiFlashUtilsQE.h. This library extensively uses experimental::SPI0Command in the Arduino ESP8266 core, which allows our initialization code to stay out of IRAM.

This library requires the Arduino ESP8266 GIT commit c2f136515a396be1101b261fe7b71e137aef0dce or the current GIT from Arduino ESP8266. Use Release 3.2.3 or better when it becomes available.

Not all flash memory is compatible with this feature. When successful, the SPI flash memory will ignore the flash memory pins connected to GPIO9 and GPIO10. That leaves each circuit free to be reassigned as a GPIO.

Generally speaking, boards that support SPI Mode: "QIO" should also work with SPI Mode: "DIO" combined with reclaim_GPIO_9_10(). Boards that support, at best, SPI Mode: "DIO" may also work with reclaim_GPIO_9_10().

Quick Start

If this quick approach doesn't work, come back and continue reading.

First, I suggest running the Analyze Sketch on your board. While it is in the examples folder, you shouldn't need to make any changes. It is a ready-to-go tool. It will indicate if the library has built-in support for your board. If not, it may provide code to add to your sketch as a new module. Copy/paste the code fragment to a new file, like CustomVendor.ino. The Analyze Sketch may spew a lot of text. To find areas of concern, focus on the lines starting with *.

Next, look at the example function Outline to see the basic things you need to add to your project to use GPIO9 and GPIO10.

For an optional test to build confidence, use the Blinky example sketch to show that this method works for your ESP8266 board/flash. Connect an LED in series with a current limiting resistor to the 3.3V rail and GPIO10. The LED will light when GPIO10 is driven LOW. Repeat with another LED and resistor to GPIO9. The Blinky sketch will alternately power each LED every 1 second.

Pin Name Clarifications and Terms:

• GPIO9, /HOLD, Flash SD3, Espressif SD_D2, and IO₃ all refer to the same connection, but from different perspectives.

• GPIO10, /WP, Flash SD2, Espressif SD_D3, and IO₂ all refer to the same connection, but from different perspectives.

• The ESP8266 mux controls whether the SPI0 controller has the pin connection or if the GPIO has the pin. You alter this assignment when you call pinMode. For example, pinMode(9, SPECIAL) assigns the connection to the SPI0 controller. Where pinMode(9, OUTPUT) connects to a GPIO output function or pinMode(9, INPUT) connects to a GPIO input function.

• Quad Enable, QE, is a bit in the Flash Status Register. With the QE bit set, pin functions /WP and /HOLD are often disabled. This bit allows the Flash to reuse those pins for SPI Modes "QIO" and "QOUT" without creating conflicts.

• SRP1 and SRP0 refer to Status Register Protect-1 and Status Register Protect-0. Not all flash memories have these bits. When they do, the pair SRP1:SRP0 (0:0) may implement an ignore /WP function.

• SR1, SR2, and SR3 refer to 8-bit SPI Status Registers 1, 2, and 3.

• Usage of S6, S9, and S15 refers to a QE bit placement in the SPI Flash Status Registers. SR1 bits 0 - 7 are S0 - S7. SR2 bits 0 - 7 are S8 - S15.

A Guide to making GPIO9 and GPIO10 work

What is the hardware and NONOS SDK expecting?

Espressif's SPI Flash Modes provides a good background knowledge of supported Flash Modes, and the FAQ covers when/why they don't work.

The ESP8266EX BootROM expects:

• The Flash to support 16-bit Writes to Flash Status Register-1 using Flash instruction 01h. For some newer Flash parts, the datasheets describe this as legacy support.

• The Flash has a Quad Enable bit in the Status Register at S9/BIT9. AKA BIT1 in Status Register-2.

• WEL "Write Enable Latch" at BIT1 of SR1 and WIP "Write In Progress" at BIT0 of SR1.

The NONOS SDK is prepared to handle:

• The above items for BootROM.

• GigaDevice GD25Q32 and GD25Q128 support only 8-bit Write SR1 and SR2; however, they have the QE bit at S9. While Espressif's NONOS_SDK has Quad support for GigaDevice, it is unavailable in the Arduino ESP8266 core, freeing some code to the available IRAM pool. (See NONOS SDK Release Version 2.1.0 - Release Notes: System.5)

• ISSI/PMC Flash with the QE/S6 BIT6 in SR1

• Macronix Flash with the QE/S6 BIT6 in SR1

Based on the SPI Flash Mode in the Firmware Image Header, the BootROM tries to set or clear the non-volatile QE bit in the SPI Flash Status Register-2, using QE=1 for QIO & QOUT and QE=0 for DIO and DOUT. Some ESP8266 module providers do not pair the ESP8266EX with a 100% compatible SPI Flash Chip. And the BootROM's attempt to update the QE bit fails. While some Flash Chips are QIO capable, they are incompatible because they do not support 16-bit status register-1 writes (or don't have a QE bit at S9). When the Status Register's WEL bit is left set after a Write Status Register instruction, this is a good indicator that the 16-bit transfer failed.

I suspect that ESP8266 Modules with silkscreen marks of DIO on the back of the antenna area may have Flash chips that technically support QIO but are incompatible with the ESP8266's BootROM code for QIO. However, they work fine using DIO.

The Winbond W25Q128JVSIQ (and others) supports the 8-bit and legacy 16-bit writes to Status Register-1. Note older parts with only legacy 16-bit Write Status Register-1 are still in the supply chain and shipping. And, for GigaDevice GD25Q32 (and other vendors) that only support discrete 8-bit Write Status Registers (1 and 2), the BootROM initialization will fail to change the current mode.

Theory of Operation

For quad-data mode on an 8-pin SPI Flash, two pins will function as either /WP (Write Protect) and /HOLD or as two data pins SD2 and SD3. When QE is clear, the pin functions /WP and /HOLD are enabled. If the /HOLD pin is held LOW, the flash is put on hold; its output lines float. If /HOLD pin is LOW for too long, a WDT Reset will occur. With the QE set, the pin functions /WP and /HOLD are disabled. Only with a quad flash instruction do those physical pins become SD2 and SD3. Otherwise, those pins float, and the flash ignores them.

By selecting SPI Flash Mode: "DIO" for our Sketch build, we have the SPI0 controller setup for DIO operations. Thus, the quad flash instructions are never issued. In this ideal case, we set the volatile copy of the QE bit before calling pinMode() to configure GPIO9 and GPIO10 for the sketch. By setting the QE bit in the Flash Status Register, we disable the pin functions /WP and /HOLD so that no crash will occur from reassigning ESP8266's mux pin from /HOLD to GPIO9 or /WP to GPIO10. The SPI Flash memory pins for /WP and /HOLD are still connected; however, they stay in the Float state because the SPI0 controller is not configured to use Quad instructions.

The ideal case described above is typical for an ESP8266 module that works with all available SPI Flash Modes, QIO, QOUT, DIO, and DOUT. While we don't use QIO or QOUT mode, a device that functions with those options is more likely to support turning off pin functions /WP and /HOLD in a way we are familiar with.

For some flash devices to ignore the /WP pin, SRP1 and SRP0 must also be set to 0. SRP1:SRP0 are often in the zero states after boot with SPI Flash Mode: "DIO", which may explain why some people experience GPIO10 working and GPIO9 crashing. I find some datasheets confusing on the requirements of QE, SRP1, and SRP0 to meet our goal, and the requirements will also differ between manufacturers.

Two main steps for using this library

1. Run the "Analyze" example on your module or development board. It will evaluate if the SPI flash memory on your board supports turning off pin functions /WP and /HOLD. The "Analyze" sketch will also confirm if library built-in support is present and suggest copy/paste code to handle your flash memory. If needed, copy and paste the suggested code to the file CustomVendor.ino in your sketch folder.

2. Call reclaim_GPIO_9_10() from your Sketch startup code, either preinit() or setup(). Perform additional setup as needed.

See example Sketches for more details.

Four common QE flash cases

Much of this library's many lines of code run the Analyze tool. Once you know what is needed to support recovering GPIO pins 9 and 10, your Sketch requires very little additional code to support this. When your flash memory is not in the built-in QE handler, you add a handler function spi_flash_vendor_cases(). The Analyze tool will usually print a sample module that implements this function.

The function spi_flash_vendor_cases() shown in the examples below is part of the reclaim_GPIO_9_10() call chain. See the Outline example sketch for reclaim_GPIO_9_10() use and placement.

1. set_S6_QE_bit__8_bit_sr1_write

For flash memory with QE or WPDis bit at BIT6:

bool set_S6_QE_bit__8_bit_sr1_write(bool non_volatile);

Example EON flash EN25Q32C, CustomVendor.ino:

#include <ModeDIO_ReclaimGPIOs.h>

extern  "C"
bool spi_flash_vendor_cases(uint32_t device) {
  using namespace experimental;
  bool success = false;

  if (0x301Cu == (device & 0xFFFFu)) {
    success = set_S6_QE_bit__8_bit_sr1_write(non_volatile_bit);
  }

  if (! success) {
    // then try built-in support
    success = __spi_flash_vendor_cases(device);
  }
  return success;
}

2. set_S9_QE_bit__16_bit_sr1_write

For flash memory with QE bit at BIT9 and support 16-bit Status Register-1 writes which will also include boards that support the build option SPI Mode: "QIO":

bool set_S9_QE_bit__16_bit_sr1_write(bool non_volatile);

Example Winbond flash 25Q32FVSIG, CustomVendor.ino:

#include <ModeDIO_ReclaimGPIOs.h>

extern "C"
bool spi_flash_vendor_cases(uint32_t device) {
  using namespace experimental;
  bool success = false;

  if (0x40EFu == (device & 0xFFFFu)) {
    success = set_S9_QE_bit__16_bit_sr1_write(volatile_bit);
  }

  if (! success) {
    // then try built-in support
    success = __spi_flash_vendor_cases(device);
  }
  return success;
}

3. set_S9_QE_bit__8_bit_sr2_write

For flash memory with QE bit at BIT9 and support only 8-bit Status Register writes:

bool set_S9_QE_bit__8_bit_sr2_write(bool non_volatile);

Example GigaDevice flash GD25Q32C, CustomVendor.ino:

#include <ModeDIO_ReclaimGPIOs.h>

extern "C"
bool spi_flash_vendor_cases(uint32_t device) {
  using namespace experimental;
  bool success = false;

  if (0x40C8u == (device & 0xFFFFu)) {
    success = set_S9_QE_bit__8_bit_sr2_write(volatile_bit);
  }

  if (! success) {
    // then try built-in support
    success = __spi_flash_vendor_cases(device);
  }
  return success;
}

4. success = true;

Flash memory that does not implement either pin function WP or HOLD, no special handling is required.

Example:

#include <ModeDIO_ReclaimGPIOs.h>

extern "C"
bool spi_flash_vendor_cases(uint32_t device) {
  using namespace experimental;
  bool success = false;

  // TODO find a vendor that does this. EON omits /HOLD.
  if (0x????u == (device & 0xFFFFu)) {
    success = true;
  }

  if (! success) {
    // then try built-in support
    success = __spi_flash_vendor_cases(device);
  }
  return success;
}

For set_ ... _write(bool non_volatile) styled functions

If the flash memory supports volatile Status Register bits, use volatile_bit for the non_volatile argument. Otherwise, use non_volatile_bit. A concern with using the non_volatile_bit is wear on the flash. For the cases where the BootROM cannot change the QE bit, it remains set or clear. Often observed with QE/S6 or the flash only supports 8-bit Status Register writes. The BootROM can only handle QE/S9 and 16-bit Status Register-1 writes.

Review and Considerations

• Out of fear of bricking a device, I avoided using a generalized handler. The library requires a specific flash match before changing Flash Status Register bits.

• Select a non-quad SPI Flash Mode: DIO or DOUT. For the Arduino IDE selection Tools->Board: "NodeMCU v1.0", DIO is internally selected.

• After BootROM and NONOS SDK init, the state of the GPIO pins 9 and 10 is equivalent to pinMode( , SPECIAL). These pins are actively driven.

• Before taking control of these pins with pinMode(), the SPI Flash Chip must be configured to ignore the /WP and /HOLD signal lines.

• After reset (or boot), the /WP and /HOLD signals are actively driven. When used as an input, insert a series resistor (suggest 300Ω minimum) to limit the virtual short circuit current, which occurs when connecting two opposing drivers. When booting, the ESP8266 must be allowed to win this conflict. Otherwise, the system will not run. The output rating for an ESP8266 GPIO pin is 12ma MAX. When calculating the current limiting resistor between the ESP and the signal source, use the source with the smaller current limit.

• Generously adding a series resistor may conflict with the I²C specification.

• When used as an output, consider that the pin will be driven high at boot time until pinMode() is called. If this poses a problem, additional logic may be needed to gate these GPIO pins. (A similar issue exists with other GPIOs 0, 1, 2, 3, and 16). A possible option lies with GPIO15. Since GPIO15 must be LOW for the system to boot, use this to create a master gate signal. At reset, GPIO15 will return to LOW.

• While not likely an issue, the capacitive load of the flash chip's /WP and /HOLD pins may need to be considered on rare occasions. Of the datasheet parameters COUT, CIN, and CL, only CIN is of interest. CIN is the capacitance contributed by a Flash pin for an input operation. The typical capacitive load of a flash input pin is 6pF. This contribution to a circuit should be negligible. However, you should include it when calculating the capacitive loading for something like I²C.

Notes, Concerns, and Possible Complications

• The BootROM expects a flash chip that supports 16-bit status register writes. If the Flash memory does not support 16-bit SR-1, this can work to our advantage.

• Not all Flash chips support 16-bit status register writes. exp. GD25Q32E

• Not all devices that support 16-bit register writes support 8-bit status register writes to status register-2. e.g. BG25Q32A and Winbond W25Q32VS

• Not all devices support a volatile copy of the Status Register. e.g. EN25Q32C

• Not all Flash chips support a 2nd status register. For example, the EN25Q32C does not have the QE bit as defined by other vendors. It does not have the /HOLD signal. And /WP is disabled by 8-bit status register-1 BIT6.

• Excess flash wear is of concern. If the flash memory only supports a non-volatile QE bit and supports 16-bit Flash Status Register-1, the non-volatile QE bit will get set when reclaim_GPIO_9_10() is called and cleared at boot by the BootROM. The bit flips back and forth at every boot cycle. When a device does not support a 16-bit Flash Status Register-1 write, the write fails, and the bit stays as is. Thus, there is no need to rewrite QE=1.

• Caution: Another reason to prefer writing to volatile copies of status register bits. Some bits in the Flash Status Register are OTP, One-Time-Programming. Setting an OTP by accident can be non-recoverable. It is strongly recommended that you identify your flash memory and review its datasheet for issues.

• I often rely on RTOS_SDK for insight when the NONOS_SDK does not provide the details needed. With info from flash_qio_mode.c, I found an issue; it shows XM25QU64A as having QE/S6, while the datasheet I downloaded says it is QE/S9, and my ESP12-F module with XMC flash is also QE/S9. There is no substitute for testing on real hardware.

• Some flash chips may need individual initialization codes.