fix: typo

Fixes #2356


Signed-off-by: Frederic Pillon <[email protected]>

Author: fpistm

API.md

@@ -83,9 +83,9 @@ As each pin has not the same capabilities, it uses the best way:
 ```C
   // Assuming Ax pins have PWM capabilities and use a different Timer.
   analogWrite(A1, 127); // Start PWM on A1, at 1000 Hz with 50% duty cycle
-  analogWriteFrequency(2000); // Set PMW period to 2000 Hz instead of 1000
+  analogWriteFrequency(2000); // Set PWM period to 2000 Hz instead of 1000
   analogWrite(A2, 64); // Start PWM on A2, at 2000 Hz with 25% duty cycle
-  analogWriteFrequency(500); // Set PMW period to 500 Hz
+  analogWriteFrequency(500); // Set PWM period to 500 Hz
   analogWrite(A3, 192); // Start PWM on A3, at 500 Hz with 75% duty cycle
 ```
 
@@ -327,7 +327,64 @@ This part describes the STM32 libraries provided with the core.
 
 ## SPI
 
-See [`SPI README.md`](../blob/main/libraries/SPI/README.md)
+STM32 SPI library has been modified with the possibility to manage hardware CS pin linked to the SPI peripheral.
+_We do not describe here the [SPI Arduino API](https://www.arduino.cc/en/Reference/SPI) but the functionalities added._  
+
+User have 2 possibilities about the management of the CS pin:  
+* the CS pin is managed directly by the user code before to transfer the data (like the Arduino SPI library)  
+* the user uses a hardware CS pin linked to the SPI peripheral  
+
+### New SPISetting parameter
+
+* `noReceive`: value can be `SPI_TRANSMITRECEIVE` or `SPI_TRANSMITONLY`. It allows to skip receive data after transmitting.
+
+### New API functions
+
+* `SPIClass::SPIClass(uint8_t mosi, uint8_t miso, uint8_t sclk, uint8_t ssel)`: alternative class constructor  
+_Params_ SPI `mosi` pin  
+_Params_ SPI `miso` pin  
+_Params_ SPI `sclk` pin  
+_Params_ (optional) SPI `ssel` pin. This pin must be an hardware CS pin. If you configure this pin, the chip select will be managed by the SPI peripheral.
+
+##### Example
+
+This is an example of the use of the hardware CS pin linked to the SPI peripheral:
+
+```C++
+#include <SPI.h>
+//            MOSI  MISO  SCLK   SSEL
+SPIClass SPI_3(PC12, PC11, PC10, PC9);
+
+void setup() {
+  SPI_3.begin(); // Enable the SPI_3 instance with default SPISsettings
+  SPI_3.beginTransaction(settings); // Configure the SPI_3 instance with other settings
+  SPI_3.transfer(0x52); // Transfers data to the first device
+  SPI_3.end() //SPI_3 instance is disabled
+}
+```
+
+#### Change default `SPI` instance pins 
+It is also possible to change the default pins used by the `SPI` instance using above API:
+
+[[/img/Warning-icon.png|alt="Warning"]] **Have to be called before `begin()`.**
+
+* `void setMISO(uint32_t miso)`
+* `void setMOSI(uint32_t mosi)`
+* `void setSCLK(uint32_t sclk)`
+* `void setSSEL(uint32_t ssel)`
+* `void setMISO(PinName miso)`
+* `void setMOSI(PinName mosi)`
+* `void setSCLK(PinName sclk)`
+* `void setSSEL(PinName ssel)`
+
+**_Note 1_** Using `setSSEL()` allows to enable hardware CS pin management linked to the SPI peripheral.
+
+##### Example:
+```C++
+    SPI.setMISO(PC_4); // using pin name PY_n
+    SPI.setMOSI(PC2); // using pin number PYn
+    SPI.begin(2);
+```
 
 ## I2C