DMA Controller
Am186™CC/CH/CU Microcontrollers User’s Manual
8-41
8.5.7.7
SmartDMA Channel Descriptor Polling
When any of the SmartDMA channels on the Am186CC/CH/CU microcontrollers require a
new buffer and the owner semaphore (OWN) bit for the next descriptor in the descriptor
ring is not set, the channel does a periodic poll (read) of the descriptor to determine if
software has set the OWN bit in the intervening time. To assure that this polling has a
minimal effect on interrupt latency and system performance, the DMA uses a single counter
to trigger the poll. Each channel is given a unique timer value that is used to initiate the
poll; these values are evenly dispersed throughout the timer period. This behavior
guarantees that only a single SmartDMA channel attempts to poll its descriptor ring at any
given time. The automatic poll timer completes one cycle every 64K processor clocks. This
results in a potential poll cycle every 8K clocks.
The SmartDMA Channel Control (SDxCON) register provides bits that allow software to
request an immediate poll of one or both of the current descriptors (transmit channel and/
or receive channel). This poll does not affect the polling status of any other channel.
A poll is never performed if the SmartDMA channel does not currently need the next buffer.
This is true even if software sets the bit requesting an immediate poll.
7–0
HADR
The HADR (High Order) field contains the eight high-order address bits of the data
buffer pointed to by this descriptor. The highest four bits of the address must be set
to 0000b. These address bits do not exist in the microcontroller’s 20-bit address space
but are provided for LANCE compatibility. The HADR field is written by the software
and not changed by the SmartDMA channel.
Receive Buffer Byte Count (Word 2)
15
RTCE
0 = Disable RTC interrupt.
1 = Enable RTC interrupt.
This bit is used to enable the Receive Terminal Count interrupt.
14–0
BCNT
The BCNT (Buffer Byte Count) field contains the length in bytes of the buffer pointed
to by this descriptor. This number is expressed in 2’s complement format and indicates
the number of bytes allocated for this buffer. This field is written by the software and
not changed by the SmartDMA channel. For example, if you want to transfer 64 bytes,
take the number 64 (40h), complement it (7FBFh), and increment it by 1 (7FC0h).
Place this number (7FC0h) in the BCNT field.
Receive Buffer Message Count (Word 3)
15
Reserved
Read/Write as zero.
14–0
MCNT
The MCNT (Message Byte Count) field contains the length in bytes of the frame. The
MCNT field is valid only when the ERR bit is 0 and the ENP bit is 1. This field is written
by the SmartDMA channel and cleared by software.
Notes:
1. The address programmed in the LADR and HADR fields is a linear address, not a segment:offset address. For
example, if a receive data buffer starts at segment address C000h and offset 1000h, it would be programmed as
follows:
Linear address= (segment address x 16) + offset address
= (C000h x 16) + 1000h = C1000h
LADR = 1000h
HADR = 0Ch
Table 8-14
SmartDMA Receive Channel Descriptor Format (Continued)
Bit Number
Bit Name
Description
Содержание Am186 CC
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Страница 24: ...Introduction xxiv Am186 CC CH CU Microcontrollers User s Manual...
Страница 39: ...Architectural Overview Am186 CC CH CU Microcontrollers User s Manual 1 15 Figure 1 6 32 Channel Linecard CH CC...
Страница 40: ...Architectural Overview 1 16 Am186 CC CH CU Microcontrollers User s Manual...
Страница 86: ...System Overview 3 36 Am186 CC CH CU Microcontrollers User s Manual...
Страница 92: ...Emulator Support 4 6 Am186 CC CH CU Microcontrollers User s Manual...
Страница 112: ...DRAM Controller 6 8 Am186 CC CH CU Microcontrollers User s Manual...
Страница 134: ...Interrupts 7 22 Am186 CC CH CU Microcontrollers User s Manual...
Страница 186: ...Programmable I O Signals 9 8 Am186 CC CH CU Microcontrollers User s Manual...
Страница 200: ...Watchdog Timer 11 6 Am186 CC CH CU Microcontrollers User s Manual...
Страница 232: ...Asynchronous Serial Ports UARTs 13 24 Am186 CC CH CU Microcontrollers User s Manual...
Страница 242: ...Synchronous Serial Port SSI 14 10 Am186 CC CH CU Microcontrollers User s Manual...
Страница 264: ...High Level Data Link Control HDLC 15 22 Am186 CC CH CU Microcontrollers User s Manual...
Страница 332: ...Universal Serial Bus USB 18 34 Am186 CC CH CU Microcontrollers User s Manual...
Страница 348: ...Register Summary A 16 Am186 CC CH CU Microcontrollers User s Manual...
Страница 376: ...Index Index 18 Am186 CC CH CU Microcontrollers User s Manual...