1. Memory Section 7.2

2. Types of Memories Definitions – Write: store new information into memory – Read: transfer stored information out of memory Random-Access Memory (RAM) – Can read and write Read-Only-Memory (ROM) – Read only

3. Interesting Facts Interesting fact of RAM: the time it takes to transfer information to or from any desired random location is always the same A word =a group of bits – A group of 8 bits is called a byte. –

4. Two Types of RAM Static RAM (SRAM): Stored information remains valid as long as power is applied to the unit. Dynamic RAM (DRAM): Stores binary information in the form of electric charges on capacitor provided inside the chip by MOS transistors.

5. 74LS189 RAM 64 bits=16 4-bits words.

6. Block Diagram of a Memory Unit

7. 74LS189 RAM [A3,A2,A1,A0]=address inputs [D3,D2,D1,D0]=data inputs [S3,S2,S1,S0]=outputs ME,WE control the direction of transfer VCC=power GND=ground

8. Logic Diagram memory cell Each word is enabled by the 4-input AND

9. Write →Read

10. Logic Diagram memory cell Each word is enabled by the 4-input AND

11. Switch Characteristics

12. Switching Time Waveforms 17 nS23 nS -7 nS for address -14 nS for data A negative hold time means that the address/data can change before the rising edge of WE because the there is internal delay through the chip.

13. Write (ME=0, WE=0) 0 0 1 0 1 1 1 1 D1 D2 D3 D4 [hi Z ? ]

14. READ (ME=0, WE=1) 1 0 0 1 Complement of data stored

15. HOLD (ME=1, WE=X) X 1 0 Hi-Z output

16. Memory Description in Verilog memory depth: 64 words word length 4-bits Need 6 bits address for 2 6 =64 words. EnableReadWrite 11Read 10Write 0XHi-Z