not everything requires a full blown 8051 system. the project defined for the students is a smart, 4-zone thermostat.

Thermostats are simple mechanical sensors that flip a contact switch on and off. its not cosmic - don't make it cosmic.

so, no rtc (real-time clock) is required. temperature accuracy is mundane, and response and calculate time only needs to match that of the bi-metal thermostatic coil. easy!

timing: recursive counter loops.
speed: 12 Mhz or slower.
powersave: lots of idle mode time.
sleepmode: generally not worth the cost.
sensor failure: software - guard time for contact closure mode. if exceeded, use exception handling.

the project is basically a 4 signal hysterisis problem.

assuming it runs off a battery, the major concern is how long the battery would last. the biggest factor may be the contact switching. the microcontroller could either run in KHz speeds to save power (thermostaic updates can run to every 90 seconds to even 5 minutes). You'd probably want to balance the tradeoffs against additional circuitry to interrupt the microprocessor out of sleep mode every 90 seconds (555 timer) against saving the money and just using idle mode.

a real product design should examine the temperature sensors for one with a wakeup/alarm signal. the micro could said the alarm threshold and sleep until the sensor threshold is exceeded. this keeps the micro sleeping until action is required. if the temperature sensor has some eeprom, better to save settings. I2C, SPI or 1-wire interfaces would be best to span the distances to the 4 zones.

code:

this is a very easy application. just break it down to small routines and write that.

if you write it first in pseudo code, then write it a second time in more detail, then write the assembly program (yeah I know they only TEACH C) you'll probably get it well organized.

duh