| ??? 02/17/08 21:46 Read: times |
#150966 - Is it the priviledge of noobs... Responding to: ???'s previous message |
to ask noob questions? Are the same questions asked by the pros a shame? :-)))
Brian Reid said:
First, I will be using two PWM outputs on my board for DC motor speed control but have to configure the board first. I need to configure bit 6 of the CFG831 SFR to set the PWM pin out to P2.6 and P2.7 by setting that bit to zero. However, the datasheet for my processor says I should not modify the EPM0, EPM1, or EPM2 bits of the CFG831 SFR as they are set automatically so I don't think I can use the MOV instruction here. I think I need to use the ORL instruction here but I'm not sure how to do that properly. The CFG831 SFR is not bit addressable.... anl CFG831,#0BFh you would use orl if you would want to set the bit to 1 Brian Reid said:
Also, I will be connecting a 5V input(from DVdd) via a normally open solid state relay with 12V control to P0.7 but have a question on the proper interfacing. The datasheet on my board says: "In general-purpose I/O port mode, Port 0 pins that have 1s written to them via the Port 0 SFR will be configured as “open drain” and will therefore float. In this state, Port 0 pins can be used as high impedance inputs." Does this mean I need to use a pulldown resistor to keep the port from floating when no 5V signal is applied to the pin?
Yes; although the common practice is to use a pullup and use active low signalling. Then, you would not need a series resistor (which value, 1MOhm, is unusually high, btw.) You should also read the datasheet of the mcu, don't rely on datasheet of your board. Brian Reid said:
I'm also a little confused when they say to write a 1 to P0.7 to configure it as an input. Does this mean I need to write a 1 to it before checking the state of the pin in my code? If so, can I just do something like the code below to accurately check the state of P0.7?
SETB P0.7 JNB P0.7,label You don't need to set it before each read; it's enough to do it once at the beginning (or do nothing at all - 1 is the default state after reset), and never overwrite it into 0. JW |
