In this mode the system buses arc controlled by microprocessor and hence the microprocessor is connected to the system bus. In master mode becomes the bus master and hence the microprocessor is isolated from the system bus. This isolation is done by AEN signal. In minimum configuration, DMA controller is used to transfer the data. The peripheral chips are interface as normal 10 ports.
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The function of the various components in the circuit are as follows : The 74LS 1 latch is used to demultiplexed lower byte address and data bus. Another 74LS 2 latch is used to demultiplexes higher byte address and data bus when DMA is master. These connections make it possible to isolate system buses when DMA is in master mode.
Note that with these connections, the programming of the Read bit 15 and write bit 14 bits in the terminal count register will have a different meaning as shown in table. Programming Programming of includes the loading of DMA address, terminal count and DMA transfer mode in the respective channel registers and mode set register.
These registers are accessed by the CPU with unique addresses. Table The least significant three address bits, A0 — A3, indicate the specific register to be accessed. As we know, channel registers are bits two program instruction cycles are required to load or read an entire register contents. The Interfacing of with has flip flop which determines whether the upper or lower byte of the register is to be accessed. Updated: August 22, — am Related Posts.
Microprocessor - 8257 DMA Controller
It is designed by Intel to transfer data at the fastest rate. Then the microprocessor tri-states all the data bus, address bus, and control bus. Each channel has bit address and bit counter. Each channel can transfer data up to 64kb.