Arun Pandian

The 8085 Microprocessor Architecture : \r\nThe 8085 Microprocessor Architecture www.yesnaraynan.blogspot.com \r\n\r\nThe 8085 and Its Busses : \r\nThe 8085 and Its Busses The 8085 is an 8-bit general purpose microprocessor that can address 64K Byte of memory. It has 40 pins and uses +5V for power. It can run at a maximum frequency of 3 MHz. The pins on the chip can be grouped into 6 groups: Address Bus. Data Bus. Control and Status Signals. Power supply and frequency. Externally Initiated Signals. Serial I/O ports. www.yesnarayanan.blogspot.com \r\n\r\nThe Address and Data Busses : \r\nThe Address and Data Busses The address bus has 8 signal lines A8 ? A15 which are unidirectional. The other 8 address bits are multiplexed (time shared) with the 8 data bits. So the bits AD0 ? AD7 are bi-directional and serve as A0 ? A7 and D0 ? D7 at the same time. During the execution of the instruction these lines carry the address bits during the early part then during the late parts of the execution they carry the 8 data bits. In order to separate the address from the data we can use a latch to save the value before the function of the bits changes. www.yesnarayanan.blogspot.com \r\n\r\nThe Control and Status Signals : \r\nThe Control and Status Signals There are 4 main control and status signals. These are: ALE: Address Latch Enable. This signal is a pulse that become 1 when the AD0 ? AD7 lines have an address on them. It becomes 0 after that. This signal can be used to enable a latch to save the address bits from the AD lines. RD: Read. Active low. WR: Write. Active low. IO/M: This signal specifies whether the operation is a memory operation (IO/M=0) or an I/O operation (IO/M=1). S1 and S0 : Status signals to specify the kind of operation being performed .Usually un-used in small systems. www.yesnarayanan.blogspot.com \r\n\r\nFrequency Control Signals : \r\nFrequency Control Signals There are 3 important pins in the frequency control group. X0 and X1 are the inputs from the crystal or clock generating circuit. The