1.4 Cache Maintenance Operations

This example application shows how to use the cache maintenance APIs to avoid issues related to cache coherency when the data cache is enabled.

The application uses the USART and the DMA PLIBs to demonstrate the cache maintenance APIs provided by the Cache peripheral library. It registers a callback with the DMA transmit and receive channels. The application first transmits a message using the DMA transmit channel and then schedules a read of ten characters using the DMA receive channel. Once the DMA read is complete, it reads the received data and echoes the same on the terminal using the DMA transmit channel.

The application calls the DCACHE_CLEAN_BY_ADDR API on the write buffer before transmitting it. Calling this API copies the data from the cache memory to the main memory, thereby ensuring that the DMA peripheral uses the updated values in the write buffer.

The application calls the DCACHE_INVALIDATE_BY_ADDR API on the read buffer after reception of data is complete by the DMA receive channel. Calling this API invalidates the cache region corresponding to the read buffer, thereby ensuring that the CPU reads the updated values in the read buffer from the main memory and into the cache.

The cache maintenance operations are always performed on a cache line (1 cache line = 32 bytes), the read and write buffers must be aligned to a 32 byte boundary and must be a multiple of 32 bytes. For the same reason, the number of received and echoed back bytes is 10, the size of the receive and echo buffers is 32 bytes.

To clone or download this application from Github, go to the main page of this repository and then click Clone button to clone this repository or download as zip file. This content can also be downloaded using content manager by following these instructions.

Path of the application within the repository is apps/cache/cache_maintenance/firmware.

To build the application, refer to the following table and open the project using its IDE.

The following table shows the target hardware for the application projects.

1. Open the Terminal application (Ex.: Tera term) on the computer
2. Connect to the EDBG Virtual COM port and configure the serial settings as follows:
   • Baud : 115200
   • Data : 8 Bits
   • Parity : None
   • Stop : 1 Bit
   • Flow Control : None
3. Build and Program the application using its respective IDE
4. The console displays the following message

   

5. Type 10 characters in the terminal. It will echo back the received bytes and toggles the LED
   The following table provides the LED names:

   Board	LED Name
   PIC32MZ Embedded Connectivity with FPU (EF) Starter Kit	LED1