| ??? 03/18/07 17:11 Read: times |
#135205 - There's a special "once" mode ... for emulators Responding to: ???'s previous message |
The hardware manual will discuss this mode. Since I'm not a big emulator user, I don't know much about it, but it's entirely possible that the emulator uses this feature to alter the behavior of the system. Back in the day when the MCU was developed, it was common for emulators to use a separate MCU that took over the functions of the in-circuit MCU, but attached through the in-circuit MCU's pins. This necessitated that the in-circuit MCU "go away" and it's my understanding, however limited, that that's what this "once" mode does.
The hardware guide says, " ONCE Mode
The ONCE (“on-circuit emulation”) mode facilitates testing and debugging of systems using the device without the device having to be removed from the circuit. The ONCE mode is invoked by: 1. Pull ALE low while the device in in reset and PSEN is high; 2. Hold ALE low as RST is deactivated. While the device is in the ONCE mode, the Port 0 pins go into a float state, and the other port pins and ALE and PSEN are weakly pulled high. The oscillator circuit remains active. While the device is in this mode, an emulator or test CPU can be used to drive the circuit. Normal operation is restored after a normal reset is applied. That might explain things. A physical look inside the emulator hardware might reveal that it's using an external program store, since, years ago, they didn't have flash- or RAM-based MCU's. There were, of course, special bond-outs, that allowed substituting emulator RAM for the ROM program store, and they were intended for emulators. That, in turn, might explain why the P0 and P2 behavior is different from the P1 and P3 behavior. RE |
