SPRUIZ1 User guide

10.3 Address Pointer and Transfer Control

The DMA state machine is, at the most basic level, two nested loops.

Burst (Inner) Loop:

The burst (inner) loop transfers a programmable number of words set by (BURST_SIZE + 1) register when a DMA channel trigger (Peripheral or Software trigger) is received. The BURST_SIZE register allows a maximum of 32 sixteen-bit words to be transferred in one burst. Each DMA channel supports both 16-bit or 32-bit word burst that can be controlled by MODE.DATASIZE bit field. Each DMA channel contains a shadowed address pointer for the source (SRC_ADDR_SHADOW) and the destination (DST_ADDR_SHADOW) address. At the beginning of each transfer, the shadowed version of each pointer is copied into the respective active (SRC_ADDR_ACTIVE or DST_ADDR_ACTIVE) register. During the burst loop, after each word is transferred, the signed value contained in the appropriate source or destination BURST_STEP register is added to the active register:

SRC_ADDR_ACTIVE = SRC_ADDR_ACTIVE + SRC_BURST_STEP

DST_ADDR_ACTIVE = DST_ADDR_ACTIVE + DST_BURST_STEP

The burst (inner) loop transfers a burst of data when a DMA Channel Trigger (Peripheral or Software trigger) is received.

Transfer (Outer) Loop:

The Transfer (outer) loop transfers a programmable number of bursts set by (TRANSFER_SIZE + 1) register for each channel. Since TRANSFER_SIZE is a 16-bit register, the total size of a transfer allowed is well beyond any practical requirement. During the transfer loop, after each burst is complete, there are two methods that can be used to modify the active address pointer:

Method 1 (Default): When address wrapping is disabled (SRC_WRAP_SIZE or DST_WRAP_SIZE is greater than TRANSFER_SIZE), active address pointer is updated as shown below

SRC_ADDR_ACTIVE = SRC_ADDR_ACTIVE + SRC_TRANSFER_STEP

DST_ADDR_ACTIVE = DST_ADDR_ACTIVE + DST_TRANSFER_STEP

Method 2: Address wrapping gets enabled when SRC_WRAP_SIZE or DST_WRAP_SIZE is less than TRANSFER_SIZE. This allows the channel to wrap multiple times within a single transfer. When the number of bursts is equal to (SRC/DST_WRAP_SIZE + 1) register, the state machine modifies the active address pointers as:

SRC_BEG_ADDR_ACTIVE = SRC_BEG_ADDR_ACTIVE + SRC_WRAP_STEP

DST_BEG_ADDR_ACTIVE = DST_BEG_ADDR_ACTIVE + DST_WRAP_STEP

SRC_ADDR_ACTIVE = SRC_BEG_ADDR_ACTIVE

DST_ADDR_ACTIVE = DST_BEG_ADDR_ACTIVE

At the end of DMA transfer, DMA can have transferred (BURST_SIZE + 1) x (TRANSFER_SIZE + 1) words.

OneShot Mode:

OneShot mode is disabled by default.

When OneShot mode is disabled (MODE.CHx[ONESHOT] = 0), DMA transfers one burst [(BURST_SIZE + 1) words] of data each time a DMA Channel Trigger is received. After the burst is completed, the state machine moves on to the next pending channel in the priority scheme, even if another trigger for the channel just completed is pending. This feature keeps any single channel from monopolizing the DMA bus.

When OneShot mode is enabled (MODE.CHx[ONESHOT] = 1), DMA transfers all the bursts [(BURST_SIZE + 1) x (TRANSFER_SIZE + 1) words] on a single DMA channel trigger. Be careful when using this mode, since this can create a condition where one trigger uses up the majority of the DMA bandwidth.

Continuous Mode:

Continuous mode is disabled by default.

When Continuous mode is disabled (MODE.CHx[CONTINUOUS] = 0), DMA state machine disables channel after all bursts in a transfer loop (TRANSFER_COUNT = 0) are complete. The channel must be re-enabled by setting the RUN bit in the CONTROL register before another transfer can be started on that channel.

When Continuous mode is enabled (MODE.CHx[CONTINUOUS] = 1), DMA state machine keep channel active even after all bursts in a transfer loop (TRANSFER_COUNT = 0) are complete.

Each DMA channel can trigger an EPIE interrupt for each DMA transfer either at start of DMA transfer or end of DMA transfer using MODE.CHx[CHINTMODE] bit.

Source/Destination Address Pointers (SRC/DST_ADDR)

The value written into the shadow register is the start address of the first location where data is read or written to.

At the beginning of a transfer the shadow register (SRC/DST_ADDR_SHADOW) is copied into the active register (SRC/DST_ADDR_ACTIVE). The active register performs as the current address pointer.

Source/Destination Begin Address Pointers (SRC/DST_BEG_ADDR)

This is the wrap pointer.

The value written into the shadow register (SRC/DST_BEG_ADDR_SHADOW) is loaded into the active register (SRC/DST_BEG_ADDR_ACTIVE) at the start of a transfer. On a wrap condition, the active register (SRC/DST_BEG_ADDR_ACTIVE) is incremented by the signed value in the appropriate SRC/DST_WRAP_STEP register prior to being loaded into the active register (SRC/DST_ADDR_ACTIVE).

For each channel, the transfer process can be controlled with the following size values:

Source and Destination Burst Size (BURST_SIZE)

This specifies the number of words to be transferred in a burst.

This value is loaded into the BURST_COUNT register at the beginning of each burst. The BURST_COUNT decrements each word that is transferred and when the register reaches a zero value, the burst is complete, indicating that the next channel can be serviced. The behavior of the current channel is defined by the ONE_SHOT bit in the MODE register. The maximum size of the burst is dictated by the type of peripheral. For the ADC, the burst size can be all 16 registers (if all 16 registers are used). For RAM, the burst size can be up to the maximum allowed by the BURST_SIZE register, which is 32. See Table 10-2 to understand how BURST_SIZE register affects the number of 16-bit words transferred with respect to DATASIZE.

Table 10-2 BURSTSIZE versus DATASIZE Behavior

BURSTSIZE	Number of 16-bit words transferred in
BURSTSIZE	DataSize = 16-bit data	DataSize = 32-bit data
0	1	2
1	2	2
2	3	4
3	4	4
4	5	6
5	6	6
6	7	8
7	8	8
8	9	10
9	10	10
10	11	12
11	12	12
*	*	*
*	*	*
*	*	*
30	31	32
31	32	32

Source and Destination Transfer Size (TRANSFER_SIZE)

This specifies the number of bursts to be transferred per CPU interrupt (if enabled).

Whether this interrupt is generated at the beginning or the end of the transfer is defined in the CHINTMODE bit in the MODE register. Whether the channel remains enabled or not after the transfer is completed is defined by the CONTINUOUS bit in the MODE register. The TRANSFER_SIZE register is loaded into the TRANSFER_COUNT register at the beginning of each transfer. The TRANSFER_COUNT register keeps track of how many bursts of data the channel has transferred and when the register reaches zero, the DMA transfer is complete.

Source/Destination Wrap Size (SRC/DST_WRAP_SIZE)

This specifies the number of bursts to be transferred before the current address pointer wraps around to the beginning.

This feature is used to implement a circular addressing type function. This value is loaded into the appropriate SRC/DST_WRAP_COUNT register at the beginning of each transfer. The SRC/DST_WRAP_COUNT registers keep track of how many bursts of data the channel has transferred and when the registers reach zero, the wrap procedure is performed on the appropriate source or destination address pointer. A separate size and count register is allocated for source and destination pointers. To disable the wrap function, assign the value of these registers to be larger than the TRANSFER_SIZE.

Note: The value written to the SIZE registers is one less than the intended size. So, to transfer three 16-bit words, the value 2 can be placed in the SIZE register.

Regardless of the state of the DATASIZE bit, the value specified in the SIZE registers are for 16-bit addresses. So, to transfer three 32-bit words, the value 5 can be placed in the SIZE register.

For each source/destination pointer, the address changes can be controlled with the following step values:

Source/Destination Burst Step (SRC/DST_BURST_STEP)

Within each burst transfer, the address source and destination step sizes are specified by these registers.

This value is a signed 2s compliment number so that the address pointer can be incremented or decremented as required. If no increment is desired, such as when accessing the data receive or transmit registers in a communication peripheral, the value of these registers can be set to zero.

Source/Destination Transfer Step (SRC/DST_TRANSFER_STEP)

This specifies the address offset to start the next burst transfer after completing the current burst transfer.

This is used in cases where registers or data memory locations are spaced at constant intervals. This value is a signed 2s compliment number so that the address pointer can be incremented or decremented as required.

Source/Destination Wrap Step (SRC/DST_WRAP_STEP)

When the wrap counter reaches zero, this value specifies the number of words to add/subtract from the SRC/DST_BEG_ADDR pointer and hence sets the new start address.

This implements a circular type of addressing mode, useful in many applications. This value is a signed 2s compliment number so that the address pointer can be incremented or decremented as required.

Note: Regardless of the state of the DATASIZE bit, the value specified in the STEP registers are for 16-bit addresses. So, to increment one 32-bit address, a value of 2 can be placed in these registers.

Channel Interrupt Mode (CHINTMODE)

This mode bit selects whether the DMA interrupt from the respective channel is generated at the beginning of a new transfer or at the end of the transfer.

If implementing a ping-pong buffer scheme with continuous mode of operation, then the interrupt can be generated at the beginning, just after the working registers are copied to the shadow set. If the DMA does not operate in continuous mode, then the interrupt is typically generated at the end when the transfer is complete.

All of the previous features and modes are shown in Figure 10-4. The following items are in reference to Figure 10-4.

The HALT points represent where the channel halts operation when interrupted by a high priority channel 1 trigger, or when the HALT command is set, or when an emulation halt is issued and the FREE bit is cleared to 0.
The SRC/DST_ADDR_ACTIVE registers are not affected by SRC/DST_BEG_ADDR_ACTIVE at the start of a transfer. SRC/DST_BEG_ADDR_ACTIVE only affects the SRC/DST_ADDR_ACTIVE registers on a wrap. Following is what happens when a transfer first starts:
- SRC/DST_BEG_ADDR_SHADOW remains unchanged.
- SRC/DST_ADDR_SHADOW remains unchanged.
- SRC/DST_BEG_ADDR_ACTIVE = SRC/DST_BEG_ADDR_SHADOW
- SRC/DST_ADDR_ACTIVE = SRC/DST_ADDR_SHADOW
The active registers get updated when a wrap occurs. The shadow registers remain unchanged. Specifically:
- SRC/DST_BEG_ADDR_SHADOW remains unchanged.
- SRC/DST_ADDR_SHADOW remains unchanged.
- SRC/DST_BEG_ADDR_ACTIVE += SRC/DST_WRAP_STEP
- SRC/DST_ADDR_ACTIVE = SRC/DST_BEG_ADDR_ACTIVE
The best way to remember this is:
- The shadow registers never change except by software.
- The active registers never change except by hardware, and a shadow register is only copied into the active register, never an active register by another name.

Figure 10-4 DMA State Diagram