| ??? 01/18/03 01:52 Read: times |
#36702 - RE: Absolute Beginner |
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PSEN* (Program Store Enable) is active low (in the 8051 manual it has a bar across the top of the name PSEN, that means active low; i am indicating active low by appending an asterisk). when the 8051 is trying to read a code byte, it will pull PSEN* low. when it the 8051 is accessing (reading or writing) external data memory it will leave PSEN* high. to use PSEN* in a circuit application, it should be wired to the external code PROM chips and to the external data RAM chips in such a way that they know when to allow access to their data. for example, i am looking at "MCS 51 Microcontroller Family User's Manual" dated February 1994. in chapter 1, "MCS-51 Architectural Overview", page 1-7, figure 4, "Executing from External Program Memory", it shows PSEN* wired to OE* of the EPROM chip containing the code. OE* stands for "Output Enable". the EPROM should only read its address port and write the code byte to its output port while its OE* line is pulled low; the 8051 does this with PSEN* while reading a code byte. the reverse should be true for RAM chips for external data. since the external RAM is shown figure 5 as also using OE* (i.e., Output Enable is active low), this chip would have to have an inverter connected between its OE* and the 8051's PSEN* if the RAM chip is to supply data. if you wire it wrong so that the PROM and RAM chips both think they are supposed to be supplying data, i suppose they could damage some of the chips attached to the bus by overdriving the bus lines (since they would both be trying to talk at the same time). (i am not a circuit engineer, but this is what i remember and what i see from reading the '51 manual). <table><tr><td>james</td><td> <td> www.jameshinnant.com</td></tr></table> </html> |
| Topic | Author | Date |
| Absolute Beginner | 01/01/70 00:00 | |
| RE: Absolute Beginner | 01/01/70 00:00 | |
| RE: Absolute Beginner | 01/01/70 00:00 | |
| RE: Absolute Beginner | 01/01/70 00:00 | |
| RE: Absolute Beginner, FYI | 01/01/70 00:00 | |
Meanwhile, back in the 21st Century... | 01/01/70 00:00 |