| ??? 06/29/05 19:17 Modified: 06/29/05 19:19 Read: times |
#96226 - which should'nt be a problem Responding to: ???'s previous message |
but one more subtlety: while at the 16-bit external memory addressing (MOVC, MOVX @DPTR, program fetch) the whole PORT2 is output strong - both 0s and 1s; when doing the MOVX @Ri (8-bit addressing), PORT2 remains as usual, i.e. esentially open collector with the weak pullups
Which should'nt be a problem because the pullups have had one full instruction cycle to "pull up". Whereas in the Movx @dptr case they only have a few clocks. Thus the "strong output" is required at the "high address in same instruction" case but not in the "whatever is in p2" case. If you use some standard TTL gates for some address decoding you may get in trouble if the set p2 and movx @Ri are sequential, but with CMOS I can not see that happen. Anyhow, using P0 and P2 as port I/O in an external memory environment is, at best, tricky san should be avoided. MMIO, however, is, of course, possible. Erik |
| Topic | Author | Date |
| function of 8051 pins | 01/01/70 00:00 | |
| double functions | 01/01/70 00:00 | |
| a requirement | 01/01/70 00:00 | |
| Yeah... and it's really big too! :) | 01/01/70 00:00 | |
| port0 | 01/01/70 00:00 | |
| thanks | 01/01/70 00:00 | |
| do not SHOUT | 01/01/70 00:00 | |
| A couple more little things | 01/01/70 00:00 | |
| PORT2 during MOVX @Ri | 01/01/70 00:00 | |
| which should'nt be a problem | 01/01/70 00:00 | |
| Same thought, almost same time | 01/01/70 00:00 | |
| Port 2 in memory access | 01/01/70 00:00 | |
| double function of SBUF | 01/01/70 00:00 | |
thank you jan waclawek | 01/01/70 00:00 |