The Microcontroller Idea Book - Jan Axelson's Lakeview Research

U11 is a 74HCT138 3-to-8-line decoder that generates individual chip-enable signals for
eight 1K blocks in the memory area from E000h to FFFFh. It works the same way that U6
generates chip selects for 8K blocks in the main circuit.
U11 is enabled whenever pin 7 of U6 is low, which occurs when the 8052-BASIC reads or
writes to addresses from E000h to FFFFh.
Address lines A10, A11, and A12 determine which of U11’s outputs goes low when the chip
is enabled. As with U6, each output is low for a different memory area. For example, pin 15
is low only when addresses from E000h to E3FFh are being read or written to.
One possible point of confusion is that both pin 7 of U6 ( in Figure 3-1) and pin 15 of U11
are chip-selects with a starting address of E000h. One controls memory accesses in the entire
8K block from E000h to FFFFh, while the other controls only the 1K block from E000h to
E3FFh. To distinguish the two, I’ve labeled U11’s pin 15 as E000H-1K.
U12 and U14 are examples of input and output ports that U11 can enable.
An Input Port
Inputs and Outputs
U12 is a 74LS244 octal buffer that adds eight inputs to the circuit. The buffer’s Y outputs
connect to the system’s data bus (D0-D7). The chip has two groups of four buffers, with each
having its own enable input (1G and 2G). For this application, the enables are tied together,
and all eight buffers are accessed as a group.
I used an LSTTL device for the buffer rather than a CMOS 74HC244 for a couple of reasons.
The LSTTL chip has Schmitt-trigger inputs, which are less sensitive to noise. Plus, unlike
CMOS, the LSTTL buffer requires no pull-up resistors at unused inputs.
OR gate U9B ensures that U12 is enabled only when U2 (the 8052-BASIC chip) reads an
address from E000h to E3FFh. When this occurs, output-enable pins 1 and 19 go low, the
buffer’s outputs follow its inputs, and U2 reads the data that U12 has placed on D0-D7.
After a read operation, pins 1 and 19 go high and U12’s outputs are high-impedance. In other
words, the outputs are electrically similar to an open circuit, which ensures that they won’t
interfere with other operations on the data bus.
In the circuit, U12 is read-only. Write operations to E000h have no effect. To access U12 at
a different address, wire pin 4 of U9B to a different output of U11.
Memory decoding options. Note that although U12 holds just one byte of information to
read, you can access it at any address from E000h to E3FFh. This may seem like a waste of
The Microcontroller Idea Book 91