sdio_slave: add documentation for the standard init process

docs/en/api-reference/peripherals/esp_slave_protocol.rst

@@ -1,5 +1,79 @@
+Communication with ESP SDIO Slave
+====================================
+
+.. _esp_slave_init:
+
+ESP SDIO slave initialization
+------------------------------
+
+The host should initialize the ESP32 SDIO slave according to the standard
+SDIO initialization process (Sector 3.1.2 of `SDIO Simplified
+Specification <https://www.sdcard.org/downloads/pls/>`_). In this specification
+and below, the SDIO slave is also called an (SD)IO card. All the
+initialization CMD52 and CMD53 are sent to Func 0 (CIA region). Here is an
+brief example on how to do this:
+
+1. SDIO reset
+    CMD52 (Write 0x6=0x8)
+
+2. SD reset
+    CMD0
+
+3. Check whether IO card (optional)
+    CMD8
+
+4. Send SDIO op cond and wait for card ready
+    CMD5 arg = 0x00000000
+
+    CMD5 arg = 0x00ff8000 (according to the response above, poll until ready)
+
+    **Example:**
+        Arg of R4 after first CMD5 (arg=0x00000000) is 0xXXFFFF00.
+
+        Keep sending CMD5 with arg=0x00FFFF00 until the R4 shows card ready (arg bit 31=1).
+
+5. Set address
+    CMD3
+
+6. Select card
+    CMD7 (arg address according to CMD3 response)
+
+    **Example:**
+        Arg of R6 after CMD3 is 0x0001xxxx.
+
+        Arg of CMD7 should be 0x00010000.
+
+7. Select 4-bit mode (optional)
+    CMD52 (Write 0x07=0x02)
+
+8. Enable func1
+    CMD52 (Write 0x02=0x02)
+
+9. Enable SDIO interrupt (required if interrupt line (DAT1) is used)
+    CMD52 (Write 0x04=0x03)
+
+10. Set Func0 blocksize (optional, default value is 512 (0x200))
+     CMD52/53 (Read 0x10~0x11)
+
+     CMD52/53 (Write 0x10=0x00)
+
+     CMD52/53 (Write 0x11=0x02)
+
+     CMD52/53 (Read 0x10~0x11, read to check the final value)
+
+11. Set Func1 blocksize (optional, default value is 512 (0x200))
+     CMD52/53 (Read 0x110~0x111)
+
+     CMD52/53 (Write 0x110=0x00)
+
+     CMD52/53 (Write 0x111=0x02)
+
+     CMD52/53 (Read 0x110~0x111, read to check the final value)
+
+.. _esp_slave_protocol_layer:
+
 ESP SDIO slave protocol
-=======================
+-----------------------
 
 The protocol is based on Function 1 access by CMD52 and CMD53, offering 3 services: (1) sending and receiving FIFO, (2) 52 8-bit R/W
 register shared by host and slave, (3) 8 general purpose interrupt sources from host to slave and 8 in the oppsite direction.
@@ -7,10 +81,11 @@ register shared by host and slave, (3) 8 general purpose interrupt sources from
 The host should access the registers below as described to communicate with slave.
 
 Slave register table
---------------------
+^^^^^^^^^^^^^^^^^^^^
 
 32-bit
-^^^^^^^
+#######
+
 - 0x044 (TOKEN_RDATA): in which bit 27-16 holds the receiving buffer number.
 - 0x058 (INT_ST): holds the interrupt source bits from slave to host.
 - 0x060 (PKT_LEN): holds the accumulated length (by byte) to be sent from slave to host.
@@ -18,7 +93,8 @@ Slave register table
 - 0x0DC (INT_ENA): mask bits for interrupts from slave to host.
 
 8-bit
-^^^^^
+#####
+
 Shared general purpose registers:
 
 - 0x06C-0x077: R/W registers 0-11 shared by slave and host.
@@ -31,7 +107,7 @@ Interrupt Registers:
 - 0x08D (SLAVE_INT): bits for host to interrupt slave. auto clear.
 
 FIFO (sending and receiving)
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+############################
 
 0x090 - 0x1F7FF are reserved for FIFOs.
 
@@ -59,7 +135,7 @@ or discard (receiving).
        supports that) to address 0x1F7F9=0x1F800-**7**.
 
 Interrupts
-----------
+^^^^^^^^^^
 
 For the host interrupts, the slave raise the interrupt by pulling DAT1 line down at a proper time (level sensitive).
 The host detect this and read the INT_ST register to see the source. Then the host can clear it by writing the INT_CLR
@@ -72,7 +148,7 @@ For the slave interrupts, the host send transfers to write the SLAVE_INT registe
 the slave hardware and driver will detect it and inform the app.
 
 Receiving FIFO
---------------
+^^^^^^^^^^^^^^
 
 To write the receiving FIFO in the slave, host should work in the following steps:
 
@@ -86,7 +162,7 @@ To write the receiving FIFO in the slave, host should work in the following step
 4. Calculate used buffers, note that non-full buffer at the tail should be seen as one that is used.
 
 Sending FIFO
-------------
+^^^^^^^^^^^^
 
 To read the sending FIFO in the slave, host should work in the following steps:
 

docs/en/api-reference/peripherals/sdio_slave.rst

@@ -89,11 +89,17 @@ The SDIO slave driver uses the following terms:
   and from slave to host (called host interrupts below). See more in :ref:`interrupts`.
 - Registers: specific address in Function 1 access by CMD52 or CMD53.
 
-ESP SDIO Slave Protocol
-^^^^^^^^^^^^^^^^^^^^^^^
+Communication with ESP SDIO Slave
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-The communication protocol slave used to communicate with the host is ESP32 specific, please refer to
-:doc:`esp_slave_protocol`, or example :example:`peripherals/sdio` for designing a host.
+The host should initialize the ESP32 SDIO slave according to the standard
+SDIO initialization process (Sector 3.1.2 of `SDIO Simplified
+Specification <https://www.sdcard.org/downloads/pls/>`_), which is described
+briefly in :ref:`esp_slave_init`.
+
+However, there's an ESP32-specific upper-level communication protocol upon
+the CMD52/CMD53 to Func 1. Please refer to :ref:`esp_slave_protocol_layer`,
+or example :example:`peripherals/sdio` when programming your host.
 
 .. toctree::
     :hidden: