The Microcontroller Idea Book - Jan Axelson's Lakeview Research

problems with EPROMs that have a 10 percent power-supply tolerance; in other words, ones
that are guaranteed to operate from supplies of 4.5 to 5.5V. You do want to be sure that your
main supply is a solid +5V, or even a little higher.
The data sheets for some EPROMs specify 5-percent tolerance: the supply must be between
4.75 and 5.25V to guarantee operation within the specifications. In this case, you will be
operating near or just below the recommended supply voltage, especially if your main supply
is slightly under +5V. When you are not programming the EPROM, you can move J6 to
connect pin 28 directly to +5V. But overall, 10-percent-tolerance EPROMs are a better
choice for this circuit.
If you are using a NVRAM or EEPROM, set J6 to +5V, since VCC must remain at 5V for
these devices.
Power Supplies for Programming
You have several options for creating the programming power supplies of +12.5V and,
optionally, +6V.
Benchtop Supply
For occasional use, if you have a benchtop supply that can supply the needed outputs, you
can add terminals to the appropriate connections in your BASIC-52 system, and connect the
supply leads to them when needed.
Adjustable Regulator
Figure 4-6 shows a circuit that regulates a DC supply of 15 to 18V to 12.5V or 6V. For the
15V supply, you can use a benchtop supply, a wall-transformer AC-to-DC adapter, or even
two 9-volt transistor batteries connected in series. The supply must have a DC output, but
it doesn’t have to be regulated. You’ll need one LM317 and an R1 and R2 for each output
voltage, but you can power both LM317’s from the same supply.
Typical current requirements for programming an NMOS 2764A are 50 milliamperes at
12.5V and 75 milliamperes at 6V, or 125 milliamperes total. For a CMOS 27C64, it’s 30
milliamperes for each, or 60 milliamperes total.
Each regulating circuit uses an LM317 adjustable regulator. You set the output voltage of
the LM317 with R1 and R2, using the formula shown. The LM317 creates a constant 1.25V
reference across R1. The current through R1 also flows through R2, and the voltage across
the pair of resistors is the regulator’s output.
Intel’s EPROM data sheets specify this range for the programming voltages:
Saving Programs
The Microcontroller Idea Book 61