| ??? 11/22/08 11:22 Read: times |
#160198 - unexistent external memory Responding to: ???'s previous message |
Ravinder Singh said:
what will happen if there is no external memory attached/burn out memory is attached and the program is written to access external memory??
i mean what would be the results of MOVC and MOVX instructions with no ext. memory attached?? As Kai said above, it depends on what do you have attached to port 0 pins. If it is truly floating (note that even 74LS inputs present some sort of a pullup, but 74HC won't), usually the pins and attached ICs and buses capacitance "retains" the state of the pins during the reading phase, so it will effectively read back the low address (i.e. reading from addresses xx00, xx01, xx02, xx03.. will return 00, 01, 02, 03...). Of course, there is no guarantee for this behaviour. But, beware, this is true only for the "classical" 8051 and very similar chips (e.g. AT89C51/52/55). For other derivatives, you must refer to the particular datasheet of that derivative. For example, for AT89C2051/4051, which has NO external memory bus, datasheet explicitly states, that executing a MOVX might have unpredictible results. In such case, a "hang" may quite well happen. JW |
| Topic | Author | Date |
| Micro-controller Hang conditions?? | 01/01/70 00:00 | |
| What do you mean by "hang"? | 01/01/70 00:00 | |
| Hang-up vs. lock-up | 01/01/70 00:00 | |
| colloquialisms | 01/01/70 00:00 | |
| hard-and-fast defitionion | 01/01/70 00:00 | |
| Micro-controller Hang conditions?? | 01/01/70 00:00 | |
| What do *you* think? | 01/01/70 00:00 | |
| ?? | 01/01/70 00:00 | |
| Learning to think for oneself | 01/01/70 00:00 | |
| Hand out tools | 01/01/70 00:00 | |
| Unless one is really willing to hand over | 01/01/70 00:00 | |
| Yes! That's the point! | 01/01/70 00:00 | |
| Depends on whether pull-ups are used or not | 01/01/70 00:00 | |
| unexistent external memory | 01/01/70 00:00 | |
| No table | 01/01/70 00:00 | |
I like that analogy! | 01/01/70 00:00 |