USBN9604SLBX Datasheet, PDF (18/62 Pages) Texas Instruments – Low EMI, low standby current, 24 MHz oscillator

4.0 Direct Memory Access (DMA) Support (Continued)
4.2 AUTOMATIC DMA MODE (ADMA)
The ADMA mode allows the CPU to transfer independently large bulk or isochronous data streams to or from the USB bus.
The application’s DMA controller, together with the ADMA logic, have the capability to split a large amount of data and trans-
fer it in (FIFO size) packets via the USB. In addition, automatic error handling is performed in order to minimize firmware
intervention. The number of transferred data stream bytes must be of a modulo 64 size. The maximum amount of data is
restricted to 256*64 bytes = 16 Kbytes.
To enable an ADMA transfer, the following steps must be performed:
1. The local CPU programs the DMA controller for fly-by demand mode transfers. In this mode, transfers occur only in re-
sponse to DMA request via the DRQ pin. The data is read/written from/to the receive/transmit FIFO and written/read in-
to/from local memory during the same bus transaction.
2. The DMA address counter is programmed to point to the destination memory block in the local shared memory, and the
Byte Count register is programmed with the number of bytes in the block to be transferred. The DMA Count register must
be configured with the number of packets to be received or transmitted. If required, the Automatic Error Handling register
must also be configured at this time.
3. The ADMA enable bit must be set prior to, or at the same time as the DMA enable bit. The DMA enable bit must be
cleared before enabling ADMA mode.
4. The DMA Request Enable bit and DMA Source bits are set in the device.The respective endpoint Enable bit must also
be set.
5. The USB host can now perform USB bulk or isochronous data transfers over the USB bus to the receive FIFO or from
the transmit FIFO. Steps 5 to 7 of the normal DMA mode are perfromed automatically. The ADMA is stopped either when
the last packet is received or when the DMA Count register has reached the value zero.
See Figures 10 and 11 for the transmit and receive sequences using ADMA mode. See Figures 12 and 13 for the basic
DMA write timing and read timing.
Microcontroller DMA
USB
DMA
USB
USB
Set up ADMA Fill FIFO Transaction Fill FIFO ^Transaction
Figure 10. Transmit Operation Using ADMA Mode
Last
time
Transaction
Microcontroller USB
DMA
USB
DMA
DMA
Set up ADMA Transaction Read FIFO Transaction Read FIFO
Figure 11. Receive Operation Using ADMA Mode
Last
time
Read FIFO
17
www.national.com