DMA Controller
Am186™CC/CH/CU Microcontrollers User’s Manual
8-29
The owner semaphore (OWN) bit is a single-bit field in each buffer descriptor. This bit is
set by software when the buffer is valid—either it contains valid data for transmission or it
is available to be overwritten by the receiver. The SmartDMA controller never sets the OWN
bit.
Software must never clear the OWN bit while the SmartDMA controller is active—the
software should first stop the DMA operation by resetting the TXST or RXST bit in the
SDxCON register. (If the SmartDMA controller is already working on that buffer, clearing
the OWN bit has no effect; if the SmartDMA controller was going to get the buffer, it would
be in poll mode and wait until the buffer is available.) The SmartDMA controller clears the
OWN bit when it releases control of the buffer.
Some systems may need to have DMA transfer continue even if software has not kept up
with the DMA. This can be accomplished by setting the TXS0 bit in SDxCON for the transmit
DMA channel or the RXS0 bit in SDxCON for the receive DMA channel. Setting these bits
inhibits the associated SmartDMA channel from clearing the OWN bit after it is through
processing a buffer.
Note: Take care when setting these bits, because you may lose received data or transmit
stale data.
Figure 8-6
SmartDMA Channel Descriptor Ring Example
1
2
3
4
4-Entry Descriptor Ring
Memory
Own=0
Own=1
Own=1
Own=0
Software is processing here
DMA is processing here
Buffer 1 (packet x)
Buffer 2 (packet x)
Buffer 3 (packet x)
Buffer 4 (packet y)
Summary of Contents for Am186 CC
Page 1: ...Am186 CC CH CU Microcontrollers User s Manual Order 21914B...
Page 4: ...iv Am186 CC CH CU Microcontrollers User s Manual...
Page 18: ...Table of Contents xviii Am186 CC CH CU Microcontrollers User s Manual...
Page 24: ...Introduction xxiv Am186 CC CH CU Microcontrollers User s Manual...
Page 40: ...Architectural Overview 1 16 Am186 CC CH CU Microcontrollers User s Manual...
Page 86: ...System Overview 3 36 Am186 CC CH CU Microcontrollers User s Manual...
Page 92: ...Emulator Support 4 6 Am186 CC CH CU Microcontrollers User s Manual...
Page 112: ...DRAM Controller 6 8 Am186 CC CH CU Microcontrollers User s Manual...
Page 134: ...Interrupts 7 22 Am186 CC CH CU Microcontrollers User s Manual...
Page 186: ...Programmable I O Signals 9 8 Am186 CC CH CU Microcontrollers User s Manual...
Page 200: ...Watchdog Timer 11 6 Am186 CC CH CU Microcontrollers User s Manual...
Page 232: ...Asynchronous Serial Ports UARTs 13 24 Am186 CC CH CU Microcontrollers User s Manual...
Page 242: ...Synchronous Serial Port SSI 14 10 Am186 CC CH CU Microcontrollers User s Manual...
Page 264: ...High Level Data Link Control HDLC 15 22 Am186 CC CH CU Microcontrollers User s Manual...
Page 332: ...Universal Serial Bus USB 18 34 Am186 CC CH CU Microcontrollers User s Manual...
Page 348: ...Register Summary A 16 Am186 CC CH CU Microcontrollers User s Manual...
Page 376: ...Index Index 18 Am186 CC CH CU Microcontrollers User s Manual...