Introduction to Microcontrollers Lab Manual - Microchip

4.3 THE LAB
4.2.4 Configuration Bits
In general, most device configuration is done at run time by executing various instructions.
However, there are some lower level functions that are only configurable at programming
time. The WDT pre-scaler and post-scaler are two of the configuration
settings that are only configurable at programming time.
The top three words of on-chip program memory in the PIC24FJ256GB110 family of
devices are reserved for configuration information. On device Reset, the configuration
information is copied into the appropriate run-time registers.
The Configuration bits are mapped starting at program memory location F80000h. Note
that this address is beyond the user program memory space. In fact, it belongs to the
configuration memory space (800000h-FFFFFFh), which can only be accessed using
table reads and table writes and cannot be modified during normal code execution.
Each lab has a file named config_bits_pic24fj256gb110.inc that includes the
values used for every Configuration bit available in this family of devices. It also
includes comments that describe each possible setting for each field.
4.3.1 Objective
Modify the file config_bits_pic24fj256gb110.inc to change the WDT period
and write code to monitor the Watchdog Timer. Use the controller itself to measure the
Watchdog Timer period and compare to the expected values.
Since the WDT time out will cause the processor to reset, a clever scheme is needed
to measure the WDT time out. One way to do this is to configure a timer for a 1 ms
period. Each time the timer expires, increment a local variable and send out the value
of that variable through the UART to be monitored using a PC terminal emulator (like
HyperTerminal).
When the Watchdog Timer expires, it will reset the controller and thus the value of the
timer will be reset to zero. By waiting for a button press on startup, it is possible to keep
the controller from continuously overwriting the previous values in the terminal emulator
on power up. In this case, the last counter value that was written to the UART should
be the WDT period.
The UART needs to run at 115200 baud. At that baud rate, a character will take about
87 us to be transmitted. This means there will be just enough time for some processing
and to transmit about 10 characters between the 1 ms interrupts. If more characters are
sent out the UART, then the resolution of the timer must increase. This will decrease
the accuracy of the measurement, but may allow for a more informative message. This
is a typical design trade-off that happens in real embedded system design.
4.3.2 Pertinent Information
The basic program flow is depicted in Figure 4-2.
2011 Microchip Technology Inc. DS51963A-page 35