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IDT DMA Controller
External DMA Operations
79RC32438 User Reference Manual
9 - 16
November 4, 2002
Notes
External DMA Operations
An external DMA operation is one in which the DMA controller is used to transfer data between an
external peripheral and memory. The DMA controller supports two external DMA channels: external DMA
channel 0 (uses DMA channel 0) and external DMA channel one (uses DMA channel 1).
The DMA descriptor DS field is used to select the direction of the DMA transfer. When DS is zero, data
is transferred from the external peripheral to memory. When the DS field is one, data is transferred from
memory to the external peripheral.
The DMA descriptor DEVCS field, shown in Figure 9.11, holds the address of the external DMA periph-
eral. The DMA descriptor DEVCMD field, shown in Figure 9.12, contains a Transfer Size (TS) field that
specifies the DMA transfer burst size for external peripherals. The width of the TS field must be greater than
or equal to the width of the external DMA peripheral. During DMA burst transactions on the memory and
peripheral bus, the address field remains constant throughout the entire transaction and is equal to the
value in the Peripheral Address (ADDR) field.
Device Control and Status Field for External DMA
Figure 9.11 Device Control and Status Value for External DMA Descriptors
Device Command Field for External DMA
Figure 9.12 Device Command Field for External DMA Descriptors
NDPTR
Description:
Next Descriptor Pointer.
This 32-bit field contains the address of the first descriptor in the
descriptor list to be used for chaining. If this field is a zero, DMA chaining is disabled.
Writing to this register when the DMA is not running causes the DMA to start and a chaining
operation to take place.
Writing a zero to this field modifies its contents but does not cause DMA descriptor processing to
start.
Initial Value:
Undefined
Read Value:
Address of next descriptor in descriptor chain
Write Effect:
Modify value
ADDR
Peripheral Address.
This 32-bit field specifies the address of the external DMA peripheral. The
address must map to a device on the memory and peripheral bus. The address should be
aligned to the size of the external DMA peripheral (e.g., address bit zero must be zero for a 16-bit
external DMA peripheral).
DEVCS
0
31
32
ADDR
DEVCMD
2
3
TS
0