Specifications

7.5. RELIABILITY

7.5.1 Software

Watchdog timer A watchdog timer resets the device should a software

error occur, or the program get stuck in a loop. This operates as follows:

the watchdog peripheral runs an independent timer which (under normal

operating conditions) is periodically reset by software. If the timer were to

overﬂow (for whatever reason) it automatically performs a hardware reset of

the microprocessor. Thus, should the software fail at any stage, or the device

become stuck in an ISR or other loop, the timer will not be reset and it will

overﬂow, thereby resetting the device.

The watchdog reset statement has been placed at the beginning of the

main loop and the timer is set for approximately 1 second, which is enough

time to ensure that there is truly a fault condition and that we are not simply

waiting for a peripheral to respond.

Brown out reset The brown out reset (or BOR)’s function is to reset

the microprocess or should the supply voltage drop below a pre-set threshold.

This is performed autonomously by a dedicated hardware peripheral in the

microprocessor. Microchip’s PIC 18F series oﬀer selectable threshold levels

of 2.0V, 2.7V, 4.2V or 4.5V. The BOR has been set to 2.7V since all digital

logic is still able to operate reliably at this voltage, with the exception of

the I

C EEPROM. However, communication failures with the I

C bus are

managed by that FSM.

Power On Self Test Upon power-up, the system checks all settings and

voltage levels for sensible values. Should these fail, appropriate action is

taken.

For example, if the peripheral settings are found to be corrupt or illogical

(for instance, 12 de vices installed when there is a possible maximum of only

8), then the system will default to zero peripherals installed and signal the

user of the error so that it may be corrected. Peripherals are also polled to

check for their presence and correct operation. Should they fail to respond

or return incorrect values, they are disabled and the user notiﬁed. Normal

device operation continues thereafter.