Interfacing of 8257 with 8085 processor

•    A simple schematic for interfacing the 8257 with 8085 processor is shown.

•    The 8257 can be either memory mapped or I/O mapped in the system.

•    In the schematic shown in figure is I/O mapped in the system.

•    Using a 3-to-8 decoder generates the chip select signals for I/O mapped devices.

•    The address lines A4, A5 and A6 are decoded to generate eight chip select signals (IOCS-0
to IOCS-7) and in this the chip select signal IOCS-6 is used to select 8257.

•    The address line A7 and the control signal IO/M (low) are used as enable for decoder.



•    The  D0-D7  lines  of  8257  are  connected  to  data  bus  lines  D0-D7  for  data  transfer  with processor during programming mode.

•    These lines (D0-D7) are also used by 8257 to supply the memory address A8-A15  during
the DMA mode.

•    The 8257 also supply two control signals ADSTB and AEN to latch the address supplied
by it during DMA mode on external latches.

•    Two  8-bit  latches  are  provided  to  hold  the  16-bit  memory  address  during  DMA  mode. During DMA mode, the AEN signal is also used to disable the buffers and latches used
for address, data and control signals of the processor.

•    The  8257  provide  separate  read  and  write  control  signals  for  memory  and  I/O  devices during DMA.

•    Therefore the RD (low), WR (low) and IO/M (low) of the 8085 processor are decoded by
a suitable logic circuit to generate separate read and write control signals f memory and
I/O devices.

•    The output clock of 8085 processor should be inverted and supplied to 8257 clock input for proper operation.

•    The HRQ output of 8257 is connected to HOLD input of 8085 in order to make a HOLD
request to the processor.

•    The HLDA output of 8085 is connected to HLDA input of 8257, in order to receive the acknowledge signal from the processor once the HOLD request is accepted.

•    The RESET OUT of 8085 processor is connected to RESET of 8257.

•    The I/O addresses of the internal registers of 8257 are listed in table.



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