Address Register
Address Register
The address register is a register that has the capability of incrementing and decrementing without the use of the ALU. Unlike the GPRs the address registers do not have access to the X and Y bus however they do have access to the address bus. The address registers can read and rewrite from the data bus and write to the address bus.
Register Control
Seven control signals control the operation of the address register.
d_bus_read |
Active high |
Read the contents of the data bus into the register latch |
|---|---|---|
d_bus_write |
Active high |
Write the contents of the register latch onto the data bus |
a_bus_write |
Active high |
Write the contents of the register latch onto the address bus |
reg_inc |
Active high |
Increment the register by one |
reg_dec |
Active high |
Decrement the register by one |
reg_carry |
Active high |
This line is high if the counter performed a carry in the previous increment/decrement operation |
reg_borrow |
Active high |
This line is high if the counter performed a borrow in the previous increment/decrement operation |
Appendix
Schematic
BOM
| Reference | Quantity | Value | Datasheet | Digikey | Price | Total |
|---|---|---|---|---|---|---|
C1 - C12 |
12 |
1uf |
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C13 - C14 |
2 |
10uf |
||||
D1 - D16 |
16 |
Red LED |
||||
D17 - D23 |
7 |
Amber LED |
||||
RN1 - RN3 |
3 |
220R |
Y1221CT-ND |
|||
U1 - U4 |
4 |
74HCT193 |
||||
U5 - U11 |
7 |
74HCT245 |
||||
U12 |
1 |
74HCT08 |
Notes
-
Breadboard controls (R-L)
-
d_bus_read
-
d_bus_write
-
a_bus_write
-
inc
-
dec
-
carry
-
borrow
-
-
While counting down or up the other should be held high
-
If not the counter will not work
-
Held high via pull up resistor in case of floating input
-
On reset set to known state (important for PC)
-
On the breadboard the address register 74LS193 is very sensitive to address and data bus levels and pull down resistors must be used on the buses.