| ??? 02/20/07 16:48 Read: times |
#133341 - Current limiting resistors Responding to: ???'s previous message |
The processors have protection diodes to both supplies. The ROM has a PMOS depletion device as an output. Depending on the internal thresholds, the ROM output "0" level could be closer to the ROM Vdd, and below the uP Vss.
If that happens, current will flow through the protection diode and into the ROM. This is not good, but if it is limited by a resistor, then it is not bad either. A 10K resistor will limit the current to well below 1 mA, and so you should be quite safe. For reference, the original PMOS devices (1972 - 1982) were designed to interface into 5 volt TTL systems. Many times there were 3 supplies - +5, Ground, and -12. |
| Topic | Author | Date |
| 9 volt to 5 volt bidirectional logic interface? | 01/01/70 00:00 | |
| rather than "wierd old ROM" why not part number? | 01/01/70 00:00 | |
| if you need to use a misspelled pseudonym | 01/01/70 00:00 | |
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| something similar just occured to me | 01/01/70 00:00 | |
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| great, I think I've got it | 01/01/70 00:00 | |
| read the datasheet twice ... | 01/01/70 00:00 | |
| voltage question and datasheet errata | 01/01/70 00:00 | |
| Why Vss is positive and Vdd is negative | 01/01/70 00:00 | |
what I meant was ... | 01/01/70 00:00 | |
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| naw ... just use the old RS232 level shifters | 01/01/70 00:00 |