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diff --git a/docs/helios64/spi.md b/docs/helios64/spi.md
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+!!! notice
+	***Preliminary Info***
+
+RK3399 has SPI controller with following features:
+
+- 6 on-chip SPI controllers
+
+- Support master and slave mode, software-configurable
+
+- DMA-based or interrupt-based operation
+
+- Embedded two 32x16bits FIFO for TX and RX operation respectively
+
+On Helios64 the bus are divided into three types: unused, system (internal bus) and user application (external)
+
+| Bus | Usage |  Remarks |
+|-----|-------|-----------|
+| 0 | Not Used | |
+| 1 | System | Bootable SPI Flash |
+| 2 | External | User Application |
+| 3 | Not Used | |
+| 4 | Not Used | |
+| 5 | System | Reserved for Production use only |
+
+
+## Internal Bus
+
+| Bus | Chip Select |  Device | Remarks |
+|-----|-------------|---------|---------|
+|  1  | 0 | W25Q128JV | Bootable SPI Flash |
+|  5  | 0 | W25X20CL | SATA Controller ROM |
+
+
+### Bootable SPI Flash
+
+Helios64 is equipped with [Winbond W25Q128JV 3V 128M-Bit Serial Flash Memory](https://www.winbond.com/hq/product/code-storage-flash-memory/serial-nor-flash/?__locale=en&partNo=W25Q128JV)
+as a Bootable SPI NOR Flash.
+
+This flash contains
+
+- preloader & bootloader
+
+| Binaries | Offset | Remarks |
+|----------|--------|---------|
+| Preloader | 0x0040 | U-Boot TPL & SPL |
+| Bootloader | 0x4000 | U-Boot & ATF |
+
+- electronic nameplate data in Security Register
+
+
+## External Bus
+
+| Bus | Chip Select | Remarks |
+|-----|-------------|---------|
+|  2  | 0 | User Application |
+
+SPI Bus for user application is exposed on [UEXT Connector (P2)](/helios64/uext)
+
+![!UEXT](/helios64/img/spi/spi_on_uext.png)
+
+| Pin | Signal Name |
+|-----|-------------|
+| 7 | MISO |
+| 8 | MOSI |
+| 9 | SCK |
+| 10 | SSEL |
+
+The pin voltage level is 3.3V
+
+
+## Usage in Linux
+
+### Enable SPIDEV in device tree
+
+To make SPI device accessible by user space application, we need to create a device tree node.
+After the node has been created, user application can access the device at
+
+`/dev/spidev5.0`
+
+!!! note
+	*To Be Updated*
+
+
diff --git a/mkdocs.yml b/mkdocs.yml
index 2bf9e6f..6cbe23a 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -80,6 +80,7 @@ nav:
     - I2C : 'helios64/i2c.md'
     - LED : 'helios64/led.md'
     - USB : 'helios64/usb.md'
+    - SPI : 'helios64/spi.md'
     - UEXT : 'helios64/uext.md'
     - Front Panel : 'helios64/front-panel.md'
 - Helios4: