Figure
1 uses only two transistors and a few resistors to provide the three-level
voltage sequence.
Design Ideas: August 1, 1996
Flash µCs are gaining in popularity because of
their low cost and ease of use. For example, Atmel's 8051-compatible mCs range
from 40-pin drop-in replacements to 20-pin devices. The problem with new flash µCs
devices is the need for support tools, such as suitable flash-memory
programmers. The AT89C2051 µC, for instance, requires a three-level
programming voltage on its RST pin: TTL logic 0, logic 1, and a 12V pulse during
the actual programming operation. The circuit in Figure
1 uses only two transistors and a few resistors to provide the three-level
voltage sequence.
| Table 1—Output voltage vs programming bits | ||
|---|---|---|
| B1 | B0 | VOUT |
| 0 | 0 | 12V |
| 0 | 1 | Logic 0 |
| 1 | 0 | Logic 1 |
| 1 | 1 | Logic 0 |
The circuit uses a regulated 12V supply. Transistors Q1 and Q2 are fast, small-signal npn devices. R1 and R2 require 1% tolerance; all other resistors are 5% carbon-film units. Any general-purpose, TTL-compatible device can drive the switch circuit. Table 1 gives the condition of the two input bits, B0 and B1, and the resulting output voltage. You can modify resistors R1 and R2 and the supply voltage to generate any programming-voltage sequence. (DI #1904)