Low-cost circuit programs EEPROMs

Jarrod Eliason, Ramtron, Colorado Springs, CO -- EDN, 1/4/2001

When you migrate to 3.3V system supplies, you must usually replace your old, reliable EEPROM programmer with a new, overly flexible and expensive universal programmer. We could not find a 3.3V programmer for less than $1000. For less than $100, the circuit in Figure 1 extends the functional life of any 5V EEPROM programmer. You can apply the circuit to any bidirectional 5 to 3.3V level-translating application. The key to the circuit lies in choosing the correct logic families. The 74VHC and 74LVC families handle the 5-to-3.3V conversion better than previous logic families, such as the 74HC series. The 74HC family accommodates 3.3V operation, but the input-protection diodes clamp the input voltage within a diode drop of V_DD (Figure 2a). So, applying 5V to the input of a 74HC part powered from 3.3V results in much undesired current. An external resistor could limit this current, but this fix would impact bus speed. The 74VHCT and 74LVC families do not use a reverse-biased diode to V_DD (Figure 2b), so the input voltage can safely rise to 5.5V, regardless of the supply level.

The 74HCT family handles the 3.3-to-5V conversion. This 5V CMOS logic family uses input switching levels skewed to accommodate TTL-level inputs. The low and high levels are 0.8 and 2.4V, respectively, in comparison with the typical CMOS levels of 1.5 and 3.5V. Because the inputs receive high levels of 3.3V at most, CMOS-optimized 74HC logic would not guarantee recognition of logic 1 inputs. On the other hand, to a 74HCT powered from 5V, a 3.3V input level represents a solid logic 1. We selected the tristatable buffer function for the EEPROM-programmer level translation. The circuit in Figure 1 programs a 3.3V, 64-kbit EEPROM, using a 5V programmer. For the address and control pins, the output-enable pin of the 74VHC chips is constantly active. For the bidirectional data bus, the OEB5 and OE3 signals control the in/out selection. When OEB5 is low and OE3 is high, a read operation takes place, and the EEPROM has control of the data bus. When OEB5 is high and OE3 is low, a write operation takes place, and the programmer drives the data bus.

A 28-pin DIP socket, IC₂, connects to the 5V EEPROM programmer. The circuit uses an additional adapter to interface to the 32-pin PLCC target device, IC₁. The 74VHC and 74LVC logic parts are not readily available in DIP form, so you can use SOIC-to-DIP adapters for breadboarding. If the 74HCT541 is not available, you can use the alternate-pinout 241 or 244.