Specifications

7.4. SOFTWARE DESIGN

interrupt, thereby providing exactly one second de lay between decrements.

Once the timer value has decremented down to zero, it is time to poll that

peripheral. This timer array is a 16 bit number and the maximum polling

period is thus 65535 seconds (approximately 18 hours). The status array is

checked to ensure that a poll is not currently already under way. If not, the

status byte is changed to “request peripheral poll” and the peripheral’s timer

is reset to the value stored in the p eripheral poll period array. This occurs

during the following timer de crement procedure.

An FSM in the main system loop constantly checks the status bytes of

all installed peripherals. If one of them is found to be “request peripheral

poll” and the I

C bus is available, then the I

C bus is initialised with a data

request using that peripheral’s address (as extracted from the peripheral

address array). That peripheral’s poll request status is then changed to

“request issued, awaiting response”. The peripheral poll FSM now waits for

the I

C FSM to complete the exchange. If the I

C returns a “successful”

result, then the exchange is complete; the peripheral’s poll status is changed

back to idle and the received data is stored for reporting to the device

operator. I f not, an error message is displayed on the LCD display and

the poll request is cancelled.

If a peripheral requires polling, but the I

C bus is occupied, then the

request is queued until the bus is free. This is simply achieved by ignoring the

poll request until the I

C bus is available. Its request to be polled is stored in

its status byte and remains pending until the I

C bus is available. Priorities

are inherently catered for: if two peripherals require polling simultaneously,

then peripheral with a lower value is polled ﬁrst as it’s status byte is checked

ﬁrst. The second request will be ignored until the ﬁrst one has completed or

has returned an error.

7.4.5 I

C FSM

The choice to use I

C simpliﬁes software development as the generation of

bus events is catered for by on-board hardware. The Microchip C18 C-

compiler package includes I

C functions, however, we were unable to use

them reliably. Thus, custom libraries were written which invoke the hardware