DMA in LPC17xx

Direct Memory Access is a feature which allows the peripherals to access the memory directly or to transfer data to other peripherals without the help of CPU. So whenever we are using DMA, we can be doing computations on the CPU while the data transfer is taking place. Effectively the microcontroller is multitasking.

LPC17xx has a General Purpose DMA (GPDMA) which has 8 channels of which each can perform unidirectional data transfer. All types of data transfers like memory-memory, memory-peripheral, peripheral-peripheral are supported. And we can prioritize the DMA transfers as we wish. It can also perform 8-bit, 16-bit and 32-bit wide transactions. If we want to transfer a stream of data that is not stored in contiguous manner, then we can use the scatter and gather feature of the DMA which uses linked list. There are many many more features available with the GPDMA out of which I will be explaining some basic features in this post. I will talk about other advanced features as and when it is required.

The essential things that you need to do while configuring the DMA are

1. Select the channel to be used for data transfer based on the priority requirement (DMA channels have fixed priorities. So we should choose the channel of required priority for our use.)
2. Set the bus transfer width (8-bit, 16-bit or 32-bit)
3. Set the source and destination
4. Set the DMA request signal (The signal that will start the DMA transfer)
5. Set the endian behaviour (Little endian or Big endian)
6. Set the burst size (Number of bytes that need to be transferred continuously in one DMA transaction)
7. Set the type of transfer (Mem-Mem, Mem-Peripheral or Peripheral-Peripheral)

Now let us go step by step and setup the GPDMA to perform a simple task of toggling the LED. Here lets store a series of alternating 1's and 0's in an array and then transfer it to the GPIO port for which the LED is connected through channel 0 of the DMA.

Setup of the timer which generates the DMA request signal

    LPC_SC->PCONP |= 1 << 2; // Power up
    LPC_SC->PCLKSEL0 |= 0x01 << 4; // CCLK
    LPC_TIM1->MR0 = 1 << 25;
    LPC_TIM1->MCR = 1 << 1; // reset on Match Compare 0

Now lets configure the DMA.
1. Power up the GPDMA (LPC17xx manual table 46, pg 64)

    LPC_SC->PCONP |= 1 << 29;

There is no need to setup the clock for DMA as we were doing for other peripherals. The system clock will be directly given to the DMA.

2. Enable the GPDMA (LPC17xx manual table 557, pg 599)

    LPC_GPDMA->DMACConfig |= 1 << 0;

If you are using big endian mode you need to set the bit 1 of DMACConfig register.

3. Set the Match Compare 0 (MAT1.0 signal) as the DMA request signal

    LPC_GPDMA->DMACSync &= ~(1 << 10); // use MAT1.0 for Sync (LPC17xx manual table 558, pg 599)

    // The above line is actually not needed as DMACSync register will be zero on reset.

    LPC_SC->DMAREQSEL |= 1 << 2; // Timer1 Match Compare 0 as DMA request (LPC17xx manual table 559, pg 600)

4. Clear the Interrupt Terminal Count Request and Interrupt Error Status register. (Writing 1 to the clear registers will clear the entry in the main register. i.e writing 0xff to DMACIntErrClr register will clear the DMACIntErrStat register)

    LPC_GPDMA->DMACIntErrClr |= 0xff; // (LPC17xx manual table 549, pg 596)

    LPC_GPDMA->DMACIntTCClear |= 0xff; // (LPC17xx manual table 547, pg 595)

5. Set the source and destination addresses (LPC17xx manual table 560 and 561, pg 601)

    LPC_GPDMACH0->DMACCDestAddr = (uint32_t) &(LPC_GPIO1->FIOPIN3); // LED is connected to P1.29

    LPC_GPDMACH0->DMACCSrcAddr = (uint32_t) &data[0]; // data[] is the array where I have stored alternating 1's and 0's

6. Clear the Linked List as we are not using it. (LPC17xx manual table 562, pg 602)

    LPC_GPDMACH0->DMACCLLI = 0;

7. Set the burst transfer size, source burst and destination burst size, source and destination width, source increment, destination increment (LPC17xx manual table 563, pg 603)

In our case, let the transfer size be 200 bytes, source burst and destination burst sizes are 1, source and destination width are 8-bits and we need to do source increment and there is no need for destination increment.

    LPC_GPDMACH0->DMACCControl = 200 | ( 1 << 26 );

8. Set the type of transfer as Memory to Peripheral and the destination request line as MAT1.0 (LPC17xx manual table 564, pg 605)

    LPC_GPDMACH0->DMACCConfig = ( 10 << 6 ) | ( 1 << 11); // 10 corresponds to MAT1.0 and it is selected as the destination request peripheral (LPC17xx manual table 543, pg 592)

9. Enable the channel (LPC17xx manual table 564, pg 605)

    LPC_GPDMACH0->DMACCConfig |= 1; //enable ch0

You can setup an interrupt for DMA transfer completion so that after the DMA transfer is complete and interrupt request is generated and in the interrupt service routine you need to disable the channel by writing

    LPC_GPDMACH0->DMACCConfig = 0; // stop ch0 dma

and then repeat all the steps of configuration to setup another transaction.

The compiled code which toggles the LED connected to pin P1.29 with the help of DMA is here.

Lot more details about DMA will be discussed later as and when required. I do not know what I would be doing without DMA because I need it for almost all purposes in the Game Console project. It is being used to put out Luminance and Chrominance signals. It will be used to put out audio, SD card interface, etc etc. May be at some time I will run out of channels!!