Direct Memory Access (DMA) on PIC MCUs [Part 2] - UART to PWM

This is part 2 of 2 of an example showing how to set up Direct Memory Access (DMA) using the Microchip Code Configurator (MCC) on a PIC18F57Q43 microcontroller to transfer data from the devices UART recieve buffer to the PWM duty cycle register to alter the brightness of an LED based on the incoming serial messages from the device we set up in part 1. The data transfer is triggered directly by the hardware UART module when the Rx buffer register is full, meaning we achieve this with zero CPU utilization.

Peripherals Used
Analog-to-Digital Converter (ADC)
Direct Memory Access (DMA)
Pulse-Width Modulation (PWM)
Universal Synchronous Asynchronous Serial Transmitter (USART)

Related Documentation

• TB3242: Configuring the DMA Peripheral
• TB3164: Direct Memory Access on 8-bit PIC® Microcontrollers
• AN3312: Arbitrary Waveform Generator Using DAC and DMA
• AN3382: ADCC Context Switching Using DMA
• AN3398: Building Hardware State Machines with CIPs

Software Used

All software used in this example is listed here:

• MPLAB® X IDE 5.30 or newer
• MPLAB® XC8 2.10 compiler or newer
• MPLAB® Code Configurator (MCC) 3.95.0 or newer
  • MCC Device Libraries PIC10 / PIC12 / PIC16 / PIC18 MCUs
• Microchip PIC18F-Q Series Device Support (1.4.109) or newer

Hardware Used

• PIC18F57Q43 Curiosity Nano (DM164150)
  • NOTE: There is second device used in part 1
• Potentiometer (x1) - used in part 1
• [Optional] Curiosity Nano Base for Click boards™ (AC164162) - this was used in lieu of a breadboard.

Setup

The hardware is setup as shown in more detail in this video. Below is a diagram of the specific pins used. You'll notice the Curiosity Nano Baseboard for clicks is missing from this diagram as they were only used as alternative to a breadboard in this cases

Potentiometer	Q43 (part 1)		Q43 (part 2)
+	3.3V
gnd	gnd
output	RA0	(Yellow Wire)
	RA3 (UART3 TX)	----------->	RA4 (UART3 RX)

Operation & Summary

As noted before - this is part 2 of 2 of an example showing how to set up Direct Memory Access (DMA) using the Microchip Code Configurator (MCC) on a PIC18F57Q43 microcontroller to transfer data from the devices UART recieve buffer to the PWM duty cycle register to alter the brightness of an LED based on the incoming serial messages from the device we set up in part 1.

Walkthrough

Create New MPLAB X Project and Open MCC

1. Same as before in part 1, but restated here for clarity.
2. Open the MPLAB X IDE
3. Create new project by clicking the icon:
   1. Alternatively: file >> New Project
4. In Choose Project window
   1. Select Microchip Embedded category
   2. Select Standalone Project
   3. Hit Next>
5. In Select Device window
   1. Select PIC18F57Q43 as your device
   2. Pro-tip: quickly filter for devices by inputting the last 3-4 characters of the device name and then selecting from the drop-down list (e.g. Q43).
   3. Hit Next>
6. In Select Tool (Optional) window
   1. If device is connected – select Microchip Kits -> PIC18F57Q43 Curiosity Nano (PKOB nano)
   2. If not, you can do this later when programming the device.
   3. Hit Next>
7. In Select Compiler window
   1. Select XC8 (v2.10)
   2. Hit Finish

Configure MCC

8. In System Module window
9. Set Oscillator Select to HFINTOSC
   1. While this example uses a system clock of 1MHz, it should be noted that any larger value for the system clock should provide similar results.
10. Under Device Resources left-hand pane
    1. Open the Peripherals drop-down
    2. Double-click the (PIC10/PIC12/…) instance of the PWM and USART3 peripherals to add them your project.

11. In UART window/tab
    1. Set baud rate to 9600

12. In the PWM window / Easy Setup tab
    1. Check the box next to Enable PWM
    2. In the Registers tab, Set PWM1LDS to DMA1
    3. Warning: Make this change only after doing the previous step where you configure the model in the Easy Setup window.

13. In the Pin Manager: Grid View window, connect pins:
    1. RX3 -> A4
    2. TX3 -> A3 (not needed necessarily since we are just receiving)
    3. PWM -> RF3 (pin tied to CNano board LED0 as per schematic)

14. In the DMA window manager
    1. Enable DMA CHANNEL 1 and make the changes shown in the table and screenshot below with explanations

	Module	Region	SFR	VarName	VarSize	Address	Mode	Message Size	Start Trigger	Abort Trigger
Src	UART3	SFR	U3RXB	-	-	-	unchanged	1	U3RX	None
Dst	PWM_16bit	SFR	PWMS1P1	-	-	-	increment	2	(see above)	(see above)

15. What we just did:
    1. UART3 is the data source module of DMA channel 1
    2. U3RXB is the SFR (Special function register) region that we want data to be sourced from.
    3. The U3RXB SFR register is 1-byte, therefore the mode is unchanged since we don’t need to increment over multiple bytes or registers.
    4. Similarly, the UART RX Buffer is 1-byte, so we need to indicate that the message size is 1-byte
    5. PWM1_16bit is the data destination module, with the PWM1S1P1L being the specific destination SFR.
    6. The PWM period register is 16-bits, meaning we will need to increment in order to load both the low register that we started with and the PWM1S1P1H (high), which also means our message size is 2-bytes since we are loading two 8-bit registers.
    7. Set the DMA start trigger to fire every time the RX Buffer is full (U3RX).
    8. NOTE** - we did not need to set VarSize, VarName, and Address parameters as these are only required when working with user-defined values in memory as opposed to fixed SFRs in this case.
16. Hit Generate Project and
17. Hit Program the device

Conclusion

There you have it. You just implemented some systems coms, automatic memory operations, and waveform without writing any lines of code. Feel free to build on the project as you see fit, find more detailed applications in the documents linked in the resource section, or check out other code examples in the repo.