1. EXTERNAL COMMUNICATIONS DESIGNING AN EXTERNAL 3 BYTE INTERFACE Mark Neil - Microprocessor Course 1 External Memory & I/O

2. You already have an external 1 Byte memory Mark Neil - Microprocessor Course 2 CP (Write Data) Byte coming in (PORTA) Byte coming in (PORTA) Byte Stored (PORTE) Byte Stored (PORTE) Output Enable* (Read Data) * = INVERTED Control lines (PORTD) Control lines (PORTD)

3. Computer Bus Mark Neil - Microprocessor Course 3 The processor within a computer communicates with the other computer modules via a device called: Bus. Many different Bus architectures exist in the market such as PCI; cPCI; VME…. All bus architectures include a control bus, a data bus and an address bus.

4. The BUS of a computer: Mark Neil - Microprocessor Course 4 Address Lines Data Lines Control Lines Bus Master (CPU/Controller) Bus Slave (e.g. Memory)

5. Typical Write Sequence Mark Neil - Microprocessor Course 5 The bus master places the address of the memory where the datum is to be written on the bus and validates it using the control lines. The master signals using the control lines that this is a write sequence. The master places the datum to be written onto the bus and validates it using the control lines. Each slave compares the address on the bus with its own address. If the write refers to it, it takes the data and signals (acknowledges) using the control lines that this has been done.

6. Typical Read Sequence Mark Neil - Microprocessor Course 6 The master places the address of the memory from where the datum is to be read on the bus and validates it using the control lines. The master signals using the control lines that this is a read sequence. The master signals that it is ready to accept the datum using the control lines. The slave compares the address on the bus with its own address. If the read refers to it, it places the datum on the bus and validates it using the control lines (acknowledges the data). At the end the Master latches the datum.

7. Parallel Transfer Mark Neil - Microprocessor Course 7 In a bus all bits of a byte or word are transferred simultaneously. If we have a byte wide bus and a transfer frequency of 1MHz then we have a bus speed of 1Mbytes/sec Hence, this is called Parallel Data Transfer (opposed to Serial Data Transfer that we will learn next)

8. Interface to External SRAM Mark Neil - Microprocessor Course 8 Bus is implemented by using two of the ports!

9. Electronics Exercise Mark Neil - Microprocessor Course 9 We will now make a 3 Byte interface that you can use for external read and write from/to the ATmega128 board. This is a challenging exercise and will take some time to construct and test

10. High Level Design :3 Byte Memory Mark Neil - Microprocessor Course 10 We want to construct : ATmega103 Board ATmega103 Board 8-Bits of data_in 8-Bits of data_out PORTA (Output) PORTE (Input) PORTD (Output Only) AS, READ WRITE, A0,A1 AS, READ WRITE, A0,A1 Control and Address bus Control and Address bus 1-Byte Memory 1-Byte Memory 1-Byte Memory 1-Byte Memory Data Bus Out Data Bus In 1-Byte Memory 1-Byte Memory

11. The data sheet for the 74F138 Mark Neil - Microprocessor Course 11 You will need to select which memory chip to write to using a binary address You will convert the 2 bits of “Address” into 3 lines which can be used to control the Enable lines for the three memory chips To do this use an address decoder The 74F138 converts a 3 bit address into 8 control output lines  When a given 3 bit number is written on the Address lines one of the 8 output lines is set high

12. The 74F138 truth table Mark Neil - Microprocessor Course 12

13. Memory Design Data to write to the memory is placed on PORTA and this is wired to the memory Data Bus The data is read from the memories into PORTE from the output Data Bus The control lines for all this are sent on PORTD 13 Mark Neil - Microprocessor Course PORTE PORTA

14. Control Bus We need to set four signals to control the memories A0:A1 – The Address Bits. These are used to define which memory chip will be addressed (00,01,10) AS* – The address strobe to select the memory R*/W to Read/Write the selected memory\ We will use bits on PORTD to control these signals – you chose which bits are attached to which control line 14 Mark Neil - Microprocessor Course

15. 15 Control Signals AS,A1,A0 R/W On PORTD Control Signals AS,A1,A0 R/W On PORTD Input Data (D0-D7) Bus – from PORTA Output Data Bus (O0-O7) on PORTE Output Data Bus (O0-O7) on PORTE Basic Circuit

16. Logic Gates Mark Neil - Microprocessor Course 16 You will also use some basic logic gates in your circuit to build the control bus  74HC04 Hex Not  74HC08 Quad And  74HC32 Quad OR

17. Logic Table for Control Bus To read a memory, o Address it on A0/A1 o set AS* to 0 o set R*W to 0 To Write a memory, o Address it on A0/A1 o Set AS* to 0 o Change R*W from 0 to 1 Make sure you understand the Logic table and how it is implemented using the logic gates AS* R*W A1A0 O0*O1* R0* W0 R1* W1 Function 0000010010Read Memory 0 (OE*) 0100011110Write Memory 0 (CP) 1000111010 1100111010 0001101000Read Memory 1 (OE*) 0101101011Write Memory 1 (CP) 1001111010 1101111010 17 Mark Neil - Microprocessor Course Exercise: Fill in the Table for Read2*/Write2

18. Sequence Diagram: Mark Neil - Microprocessor Course 18 Unneccessary - need another D-Flip-Flop to remove

19. The Memory Test Program Mark Neil - Microprocessor Course 19 Start Set data pattern on PORTA Set data pattern on PORTA Clock in memory 1 Set data pattern on PORTA Set data pattern on PORTA Set data pattern on PORTA Set data pattern on PORTA Clock in memory 2 Clock in memory 3 Enable memory 1 output Enable memory 3 output Enable memory 2 output Copy PINE to PORTB LEDs

20. Task Plan: Mark Neil - Microprocessor Course 20 Construct the 3 byte memory and connect it to your ATmage128 Board  Always connect to the ports through the special cable-resistor- pack units Proceed in steps:  First the Control Bus, test it and verify that it works!  Next add the first memory byte and test it  Add the second memory and test it  Add the third memory and test it Write programs as you go along with the aid of a top-down- modular diagram  Use a main section  Use driver subroutines that read, write, and loop data through your memory bytes.

21. Some Comments: Mark Neil - Microprocessor Course 21 Make a detailed schematic of your device with all ICs (logic diagrams) and the IC pin assignments. Make sure that you understand your design BEFORE you start building it. Build the device in pieces which you can check using your software. Don’t build the entire design before testing.  Follow the steps – don’t leap ahead and build too much before checking that everything works at each step. Use the oscilloscope and the DVM to check what is going on and debug your device.

22. How to Draw Schematics: Mark Neil - Microprocessor Course 22

23. Step 1: The control bus Address lines Mark Neil - Microprocessor Course 23 Start by setting the Address Strobe* high Set the correct address on your bus. Now lower the AS* and check that the appropriate output bits are selected for the address of each memory

24. The control bus Read/Write Mark Neil - Microprocessor Course 24 Add the Read/Write control logic  First build Read0*/Write0 Set AS* high and R*/W high. Set the address on your bus Lower R*W and AS*  This is a read memory setting  Check that Read0*/Write0 are what you expect Bring *R/W High for 250 nsec and then back Low  This is a write memory  Use a scope to verify the timing Complete the Read1*/Write1 and Read2*/Write2 circuits  Test that they are working as expected when you select each of the memory addresses. you are ready to add a memory  Don’t this before your control bus works!

25. Step 3: Adding the first memory Mark Neil - Microprocessor Course 25 Connect the Data Bus ( D0-D7 ) to the 8 bits on PORTA Connect the OE* to the Read* and CP to the Write lines of your control bus Using your program  Set AS* high and R*/W high.  Set the correct address and put some data on your bus.  Lower R*/W and AS*  Bring R*/W high for 250 nsec and then back to Low Your data should now be stored in the selected register, and visible on the outputs Before you connect the output together use scope/LED+Resistors to check if the data are on the outputs and correct Connect the Output Bus to the pins on PORTE Try reading the data with the AVR

26. Step 4 Add the second Memory Chip Verify that it is working as expected Finally Add the third memory chip You should now be able to write data to the memory of your choice, and verify that it is properly stored 26 Mark Neil - Microprocessor Course

27. Here is an example… When you build your board Think about  Where inputs arrive  Where the outputs go  Where the memory buses will be  Where the control bus will be  What color wire you will use for what function 27 Mark Neil - Microprocessor Course