Microprocessor 68000

          หมายเลขที่กำหนดขึ้นมาสำหรับโปรเซสเซอร์ของโมโตโรลาซึ่งใช้กับเครื่องแมคอินทอชดั้งเดิม มีขนาด 32 บิต, 68000 ใช้หน่วยความจำด้วยวิธีการอ้างตำแหน่งแบบเชิงเส้น ต่างจากไมโครโปรเซสเซอร์ยอดนิยมของอินเทล โมโตโรลา 68000 เป็นไมโครโปรเซสเซอร์ต้นตระกูล ดึงข้อมูลได้ทีละ 32 บิต มีช่องส่งข้อมูลขนาด 16 บิต จึงกำหนดตำแหน่งของหน่วยความจำได้ 16 เมกะไบต์

          68000 ที่ความเร็ว 8 เมกะเฮิรตซ์ ใช้ในเครื่องแมคอินทอชหลาย ๆ รุ่น รวมทั้งแมคอินทอชพลัส และแมคอินทอชเอสอี นอกจากนี้ 68000 ยังไปอยู่ในเครื่องพิมพ์แอปเปิลเลเซอร์ไรเตอร์ทู และเครื่องพิมพ์เอชพีเลเซอร์เจ็ตทุกรุ่นอีกด้วย

The main features of the 68000 board are:

• 8MHz 68000 processor.
• 16K 100ns static RAM.
• 16K 100ns static RAM with PC parallel port interface, effectively behaving as EPROM erasable and programmable in situ.
• 6821 Peripheral Interface Adapter driving 6818 Real Time Clock and a small keypad.
• 6850 Asynchronous Communications Interface Adapter based serial port with external clock generation, 300 to 307200 bps.
• 6522 Versatile Interface Adapter, driving PIC controlled data link and other external devices.
• 16 bit wide output port, connected to LED display.
• 8 bit wide input port, connected to DIP switches.
  

Data Bus:
     The 68000 has a 16 bit Data Bus (D0-D15).

Control Bus:
     The 68000 has many control lines, as shown in the diagram

Asynchronous Control Lines

     The Asynchronous Control Lines are shown below. They are:

     The 68000 uses these control lines when transferring data between itself and 68xxx peripherals. The sequence of events for a memory read is as follows :

1. The 68000 places an address on the Address Bus.
2. The R/W should already be high (default state)
3. The 68000 pulls AS low to indicate that there is a valid address on the Address Bus
4. The 68000 will pull UDS and/or LDS low depending on the data size and the
   address being read.
5. The decoding circuits and memory ICs then go into action. When memory has been
   enabled and the 68000 can complete its cycle, the decoding circuit must pull DTACK low. This input to the 68000 informs the microprocessor that it may now complete the read cycle

Synchronous Control Lines

     The Synchronous Control Lines (also known as the 6800 peripheral control lines) are shown to the right. They are:

     Officially, these three control lines are the Synchronous Control Lines; but, a data transfer to/from a 6800 peripheral cannot take place without AS, UDS, LDS and the R/W lines. The sequence of events that takes place when the 68000 is transferring data to/from a 6800 peripheral is as follows:

1. The 68000 places the address of the 6800 peripheral (i.e.; the 6821 PIA or 6850
   ACIA) on the Address Bus.
2. The 68000 pulls AS low to indicate a valid address is on the Address Bus.
3. The R/W will go into its active state (1 for read, 0 for write).
4. UDS and/or LDS will become active depending on the peripheral device's address
   and the size of the data being transferred.
5. The decoding circuitry will become active to select and enable the peripheral
   device. The decoding circuitry will also pull VPA low to let the microprocessor know that it is trying to access a 6800 peripheral.
6. The 68000 responds by synchronizing the E clock with the system clock (the E
   clock has a 40% duty cycle, and only one cycle for every ten cycles of the system clock).
7. When E is synchronized, the 68000 pulls VMA low to indicate it is ready to
   transfer data. VMA should be used as an enable input to the peripheral device.
8. When E goes high, the data transfer will take place.
   

Interrupt Control Lines

     The Interrupt Control Lines are inputs to the microprocessor and indicate an interrupt level. They can best be summarized by Table 1.

Table 2 - Processor Status

    FC2     FC1    FC0      Cycle Type
      0        0        0        reserved
      0        0        1        user data
      0        1        0        user program
      0        1        1        reserved
      1        0        0        reserved
      1        0        1        supervisor data
      1        1        0        supervisor program
      1        1        1        Interrupt Acknowledge

Function Code Outputs

     The Function Code Outputs are outputs from the microprocessor and indicate the microprocessor's status. They can best be summarized by Table 2.

Bus Arbitration Control Lines

     The Bus Arbitration Control Lines are shown below. These control lines will allow another microprocessor or a DMA controller to take over the 68000's Data, Address, and Control busses. The requesting device, called a Bus Master, requests the use of the 68000's busses by pulling BR low (Bus Request).

System Control Lines

     The System Control Lines include BERR (Bus Error), RESET, and HALT. The BERR signal is used to inform the 68000 that an error occurred during execution of the

current bus cycle. If the HALT line is NOT low when this happens, the 68000 will execute an exception routine and terminate the current cycle. If HALT is asserted, the 68000 will attempt to rerun the failed cycle.

     Both RESET and HALT are bidirectional signals. Therefore, special care must be taken in developing the circuits that are connected to these pins.

     As inputs and after a power is applied to the circuit, both RESET and HALT must stay low for 100ms. This is to ensure that power has stabilized and results in a total processor reset. A total processor reset will cause the 68000 to load the supervisor stack pointer from vector 0 (Addresses $000000-$000003) and the program counter from vector 1 (Addresses $000004-$000007).

     The Clock has three requirements:

     TTL-compatible waveform
     50% Duty cycle
     10ns rise and fall time (12Mhz versions and up require 5ns)
     The 68000 requires approximately 300mA of current. This is a little high for one input pin, so there are two Vcc and two ground pins. Both pins in each set must be connected for proper operation.