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resources/espressif__esp_hosted/docs/design_consideration.md
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resources/espressif__esp_hosted/docs/design_consideration.md
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# Design Considerations and Debugging ESP-Hosted
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**Table of Contents**
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- [1. Choosing the Correct ESP chip as Slave](#1-choosing-the-correct-esp-chip-as-slave)
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- [1.1. Using ESP chip as Hosted Master](#11-using-esp-chip-as-hosted-master)
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- [2. General Hardware Considerations](#2-general-hardware-considerations)
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- [2.1. GPIOs used for interface](#21-gpios-used-for-interface)
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- [2.2. Using SPI insted of SDIO](#22-using-spi-insted-of-sdio)
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- [2.3. Whenever possible, Use `IO_MUX` GPIOs.](#23-whenever-possible-use-io_mux-gpios)
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- [2.4. Signal Length and Noise Reduction](#24-signal-length-and-noise-reduction)
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- [3. General Debugging Guidelines](#3-general-debugging-guidelines)
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- [3.1. Add tapping points to your PCB prototype](#31-add-tapping-points-to-your-pcb-prototype)
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- [3.2. Tap out additional GPIO signals as testing points and future expansion](#32-tap-out-additional-gpio-signals-as-testing-points-and-future-expansion)
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- [3.3. Verifying Hosted Interface with Raw Throughput](#33-verifying-hosted-interface-with-raw-throughput)
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- [4. Others](#4-others)
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- [5. References](#5-references)
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There are several considerations that need to be taken into account
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when implementing ESP-Hosted for your system.
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## 1 Choosing the Correct ESP chip as Slave
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For prototyping, any ESP32 chip can be used as the slave, provided it
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has the required interface (SPI, SDIO). But when creating an actual
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product, it is important to select the proper ESP32 chip as a Hosted
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Slave.
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There are many ESP32 chips, each with difference features and
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performance capabilities. Based on your product requirements
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(interface to use, CPU, memory, power requirements, etc.), choose the
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ESP32 chip(s) that can meet your requirements.
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Use the ESP Product Selector guide to help you decide which ESP32
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chips and/or modules are suitable for your product.
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> [!NOTE]
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> See [References](#5-references) for links to the Selector Guide
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> and other links.
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## 1.1 Using ESP chip as Hosted Master
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The project defaults to using an ESP chip as the Hosted Master. This
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is to act as a reference platform and make it easier to evaluate and
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test Hosted before porting it to your MCU of choice.
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## 2 General Hardware Considerations
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### 2.1 GPIOs used for interface
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Make sure the correct GPIOs pins on the Hosted Slave and Master are
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connected together. Verify that the correct GPIOs are set-up in
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`Menuconfig` for both the Slave and Master.
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> [!NOTE]
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> In general most ESP GPIOs can be used for input and output. But on
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> the ESP32, some GPIOs can only be used for input and are not usable
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> under Hosted. Check the ESP datasheet to verify the GPIOs you select
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> can be used as a Hosted interface.
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### 2.2 Evaluate with jumpers first
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It is flexible to evaluate with jumper cables or bread board than full-fledged PCB.
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In general, SPI (Standard & Dual SPI) imposes fewer hardware requirements compared to
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SDIO. SPI is easier to prototype, and available on more ESP chips and
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MCUs compared to SDIO.
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Before going to SDIO 4 bit mode PCB, it's better to evaluate SDIO 1-Bit mode.
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Once you evaluate the solution on jumper cables, you can move to PCB solutions with same or high performance transport.
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###### Jumper cable considerations
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- Use high quality jumper cables
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- Use jumper cables as small as possible. you can cut and solder the joints if need be.
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- Use equal length jumper cables for all the connections
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- Grounds: Connect as many grounds as possible, this lowers the interference.
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- Jumper cable lengths
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- Standard SPI: At max 10cm, lower the better
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- Dual SPI: At max 10cm, lower the better
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- SDIO 1 Bit: At max 5cm, lower the better
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- Quad SPI : jumpers not supported, only PCB
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- SDIO 4 Bit: Jumpers not supported, only PCB
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### 2.3 Whenever possible, Use `IO_MUX` GPIOs.
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In general, ESP peripheral interfaces can be assigned to any available
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GPIO through a multiplexer. But some ESPs have dedicated GPIOs for
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peripherals (`IO_MUX`). These `IO_MUX` GPIOs have better timing
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characteristics and support higher frequencies. They should be use
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when possible to minimise timing and skew issues when using the
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interface for Hosted.
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> [!NOTE]
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> The SDIO interface on the ESP32 and ESP32-C6 have fixed GPIO
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> assignments and cannot be changed.
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### 2.4 Signal Length and Noise Reduction
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For best performance, a PCB with traces should be used to connect the
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Hosted Slave and Master. For prototyping, jumper cables can be used,
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but may only work at a lower `CLK` frequency.
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In general, keep the cable and PCB traces short and of the same
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length, to minimise propogation delay and clock skew:
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- for SPI, keep them to 10 cm or less
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- for SDIO, keep them to 5 cm or less
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Isolate the interface signals, expecially the `CLK` signal, from other
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signals. For PCBs, surround the signal swith a ground plane, and keep
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the `CLK` signal clean by not routing it close to other high frequency
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signals.
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For jumper cables, you can try surrounding the signals, especially the
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`CLK` signal, with grounded wires to shield them from interference.
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> [!NOTE]
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> For SDIO, external pull-up resistors (recommended value: 51 kOhms)
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> are required. Using jumper cable are **not** recommended for SDIO. You
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> may be able to get SDIO working with jumper cables by using a lower
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> `CLK` frequency and using 1-bit SDIO mode.
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> [!NOTE]
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> Also check the Hosted documentation for SPI and SDIO for more
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> information and guidelines on the interfaces.
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## 3 General Debugging Guidelines
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### 3.1 Add tapping points to your prototype
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Adding tapping points or headers to the Hosted interface signals on
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your prototype will make it easier to check whether the Hosted
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interface is working as expected.
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### 3.2 Tap out additional GPIO signals as testing points and future expansion
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Add tapping points to some unused GPIOs on both the Hosted Slave and
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Host on your prototype PCB. This can later be use for debugging or
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enhancing your own Hosted code.
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For example, add your own debugging code to the Hosted Slave and
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Master code to set a GPIO value when a condition is met. This GPIO can
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be used to light a LED or trigger a capture on an oscilloscope or
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logic analyzer, for example. This is useful for capturing rare or
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intermittent conditions while testing Hosted.
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In the future, Hosted may also offer newer transport options or more features, like controlling
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power modes on the Host and Slave. These may require additional GPIOs
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for control, so it would be good to keep some additional GPIOs
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available and accesable for future use.
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### 3.3 Verifying Hosted Interface with Raw Throughput
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ESP-Hosted has a Raw Throughput Option to test sending of data between
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the Host and Slave. This can be used to verify the hardware for signal
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errors and to check the achievable throughput of Hosted.
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> [!IMPORTANT]
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> Use this option to verify that Hosted hardware and software are
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> working as expected before involving other software layers like
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> networking.
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To enable the Raw Throughput Option on Slave, enter `Menuconfig` and
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enable **Example Configuration** ---> **Hosted Debugging** --->
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**RawTP**.
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To enable the Raw Throughput Option and set Raw Throughput direction
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on Host, enter `Menuconfig` and enable **Component config** --->
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**ESP-Hosted config** ---> **Debug Settings** ---> **RawTP**. Set
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the data transfer direction: **Host to Slave**, **Slave to Host** or
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**Bidirectional**.
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## 4 Others
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Check the References below for links to the Product Selector, and more
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detailed information on the interfaces used in Hosted. If you have
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other issues with Hosted, you can check the Troubleshooting Guide.
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You can also raise an Issue on the ESP-Hosted Github repository. Check
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that the issue has not already been raised before submitting. The
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solution to your problem may have already been provided.
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## 5 References
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**External Links**
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- ESP Product Selector: https://products.espressif.com/
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- ESP-Hosted Github Issues: https://github.com/espressif/esp-hosted-mcu/issues
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**ESP-Hosted Documentation Links**
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- SPI Full Duplex interface documentation: [spi_full_duplex.md](spi_full_duplex.md)
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- SDIO interface documentation: [sdio.md](sdio.md)
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- SPI Half Duplex interface documentation: [spi_half_duplex.md](spi_half_duplex.md)
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- UART documentation: [uart.md](uart.md)
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- Troubleshooting Guide: [troubleshooting.md](troubleshooting.md)
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