Using 64 × 8 ROM chips with an enable input, construct a 512 × 8 ROM with eight chips and a decoder.
The Correct Answer and Explanation is:
To construct a 512 × 8 ROM using 64 × 8 ROM chips with eight chips and a decoder, we need to arrange the chips and decoder correctly.
✅ Correct Answer:
Use 8 of the 64 × 8 ROM chips and connect their enable inputs to the outputs of a 3-to-8 decoder. Feed the 3 most significant address lines (A6, A5, A4) into the decoder, and feed the remaining 6 address lines (A3 to A0 and A2 to A0, depending on naming) into each ROM chip.
🧠 Detailed Explanation (300+ words):
A 64 × 8 ROM means it has 64 memory locations (or addresses), each holding 8 bits of data. To access 64 locations, you need 6 address lines, because 26=642^6 = 64.
You want to build a 512 × 8 ROM, which means:
- 512 memory locations
- Each location stores 8 bits
- To address 512 locations, you need 9 address lines because 29=5122^9 = 512
💡 Step-by-Step Construction:
- Number of Chips Needed:
- Each chip can store 64 bytes.
- 51264=8\frac{512}{64} = 8, so you need 8 ROM chips.
- Decoder Requirement:
- You need to select one ROM chip at a time, based on the upper address bits.
- A 3-to-8 decoder (like 74LS138) takes 3 input lines and activates one of 8 outputs.
- Connect the 3 most significant bits of the address lines (A8, A7, A6) to the inputs of the decoder.
- ROM Enable Connections:
- Connect each of the 8 decoder outputs to the enable (chip select) input of each ROM chip.
- This ensures only one chip is active at a time.
- Address Lines to Chips:
- Each chip still needs to know which of its 64 locations to access.
- Connect the lower 6 address lines (A5 to A0) to the address inputs of all the ROM chips in parallel.
- Data Lines:
- Connect the 8 data output lines from each chip to a common 8-bit data bus.
- Only the enabled chip will drive the data bus (since only one chip is active).
✅ Summary:
- Use a 3-to-8 decoder for selecting among 8 ROM chips
- Use upper 3 address lines to drive the decoder
- Use lower 6 address lines for ROM address inputs
- Connect data outputs to a common bus
- Result: A working 512 × 8 ROM system using eight 64 × 8 ROM chips