The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 6
1023 (3FEh) addresses. Indeed, if using every byte of memory is critical, you can use other
methods in place of U11 to more completely decode, or divide, the memory space.
Programmable logic is one possibility, for those who have access to a logic compiler and
device programmer. But for many circuits, complete decoding of memory isn’t necessary,
and less-than-complete decoding using off-the-shelf parts is more practical.
An Output Port
To complement U12’s input byte, U14 provides eight bits of output. U14 is a 74LS374 octal
flip-flop, which is very similar to the 74HCT373 octal latch at U4. On the ’373, 1Q-8Q follow
1D-8D until pin 11 goes high, after which 1D-8D no longer change. In contrast, on the ’374,
1Q-8Q change only on pin 11’s rising edge, when 1D-8D are latched to 1Q-8Q.
Here again, I chose LSTTL over CMOS, this time because the LSTTL device can sink 12ma
at 0.25V, compared to 6ma at 0.2V for the 74HC374. However, the 74HC374 can also source
6ma at 4.2V, so it’s a better choice if you need to draw current from a logic-high output.
U14’s eight data inputs connect to the data bus (D0-D7). Its output control (pin 1) is tied low
so that the outputs are always enabled. If you want to be able to disable the outputs, you can
instead tie pin 1 to an unused bit on Port 1.
NOR gate U13A clocks U14 only when U2 writes to addresses from E400h to E7FFh. When
this occurs, the data written is latched to the outputs of U14. The outputs do not change until
the next time the chip is written to.
In the circuit, U14 is write-only. Reading address E400h will return the value 0FFh. To
access U14 at a different address, wire pin 2 of U13A to a different output of U11.
Since U12 is read-only and U14 is write-only, you could use the same address for both, by
having them share the same output of U11. For example, if you wire both pin 2 of U13A
and pin 4 of U9B to pin 7 of U11, both will be accessed at FC00h. Write operations will
access U12, and reads will access U14. In this way, you can add up to eight input ports and
eight output ports, with eight chip-select addresses in all.
Wiring Tips
Add Figure 6-2’s circuits to Chapter 3’s circuits, including the connections shown to D0-D7,
A10-A12, E000h, READ, and WRITE in the main circuit. The schematic continues the component
numbering sequence from the earlier schematics. Use sockets for the ICs. Wire U13’s unused
inputs (pins 5, 6, 8, 9, 11, and 12) to GND or +5V. Also add a couple of 0.1-microfarad ceramic
decoupling capacitors from +5V to GND, near the added ICs.
92 The Microcontroller Idea Book