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Mission 03 / 07

Clock Warrior

One missed pulse, one lost ship. Give it a heartbeat.

TimersOSCCONT0CONT1CON
On this page
  1. The situation
  2. Why this matters
  3. The idea in one picture
  4. Your mission
  5. How you know it worked
  6. Debrief

01The situation

The thrusters fire blind: too early and the station drops, too late and it drifts away. It needs perfect timing again.

02Why this matters

Software can't wait precisely on its own. A timer counts clock ticks in hardware, so the chip always knows exactly when.

  • Blinking & PWMLED dimming and motor speed are timer-driven pulses.
  • TimeoutsA microwave stops at exactly 0:00 because a timer counted it.
  • MeasuringSpeed, RPM and frequency all come from counting ticks.

03The idea in one picture

Every 16 clock pulses → count + 1CLOCK÷16COUNT11111011= 251overflow flag ○8-bit timer · counts 0…255
A ÷16 prescaler turns 16 clock pulses into one count. At 255 the counter overflows to 0.

Think of it as: a car odometer behind a gearbox. The prescaler is the gear (1:16 means one turn of the wheel per 16 turns of the axle), and the overflow is the odometer rolling over to zero.

Go deeper
Overflow time = (1 / Fosc) × prescaler × counts (256 for 8-bit, 65,536 for 16-bit)

At 4 MHz this mission's Timer0 overflows every 1.024 ms and Timer1 every 131.072 ms (highlighted below).

8-bit timer (256 counts)

FoscBypass1:21:41:81:16
1 MHz256 µs512 µs1.024 ms2.048 ms4.096 ms
2 MHz128 µs256 µs512 µs1.024 ms2.048 ms
4 MHz64 µs128 µs256 µs512 µs1.024 ms
8 MHz32 µs64 µs128 µs256 µs512 µs

16-bit timer (65,536 counts)

FoscBypass1:21:41:81:16
1 MHz65.536 ms131.072 ms262.144 ms524.288 ms1.049 s
2 MHz32.768 ms65.536 ms131.072 ms262.144 ms524.288 ms
4 MHz16.384 ms32.768 ms65.536 ms131.072 ms262.144 ms
8 MHz8.192 ms16.384 ms32.768 ms65.536 ms131.072 ms

04Your mission

  • OSCCONStart the clock. Any speed works.
  • T0CONStart Timer0 as an 8-bit counter with a 1:16 prescaler.
  • T1CONStart Timer1 as a 16-bit counter with a 1:8 prescaler.
Hint 1
Each timer needs three things: on, a size (8 or 16 bits) and a prescaler. Careful: the prescaler bits work upside down.
Hint 2
TMRxON is bit 0 and the 8-bit switch is bit 1. PSA = 0 (bit 3) means the prescaler is used. In the prescaler bits (5:4), 00 = 1:16 and 01 = 1:8.
Show the solution
  1. 1Step 1 — OSCCON (0x20)Set it to 0x05 (0000 0101). Switch the internal oscillator on at 4 MHz (any speed wins).
  2. 2Step 2 — T0CON (0x27)Set it to 0x03 (0000 0011). Timer0: on, 8-bit mode, prescaler assigned, 1:16.
  3. 3Step 3 — T1CON (0x28)Set it to 0x11 (0001 0001). Timer1: on, 16-bit mode, prescaler assigned, 1:8.
  4. 4Program & ExecutePress P to compile your configuration into machine code in ROM, then press E to run it. When every register matches you get Target Reached.

The same thing as a program (MOVLW loads a value into W, MOVWF copies W into a register):

MOVLW 0x05
MOVWF 0x20   ; OSCCONMOVLW 0x03
MOVWF 0x27   ; T0CONMOVLW 0x11
MOVWF 0x28   ; T1CON
Bit-by-bit breakdown

OSCCON0x20= 0x05

Switch the internal oscillator on at 4 MHz (any speed wins). 0x05 = 0000 0101 = SCS0 = 1, IRCF = 10

Bit76543210
Value00000101
Field—IRCFSCS0
  • IRCF (bits 2:1) = 10 → 4 MHz
  • SCS0 (bit 0) = 1 → Internal oscillator on
  • Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

T0CON0x27= 0x03

Timer0: on, 8-bit mode, prescaler assigned, 1:16. 0x03 = 0000 0011 = TMR0ON = 1, T08BIT = 1, PSA = 0, T0PS = 00

Bit76543210
Value00000011
Field—T0PSPSAT0SET08BITTMR0ON
  • T0PS (bits 5:4) = 00 → Prescaler 1:16
  • PSA (bit 3) = 0 → Prescaler ASSIGNED (inverted: 0 = assigned)
  • T08BIT (bit 1) = 1 → 8-bit mode
  • TMR0ON (bit 0) = 1 → Timer on
  • Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

T1CON0x28= 0x11

Timer1: on, 16-bit mode, prescaler assigned, 1:8. 0x11 = 0001 0001 = TMR1ON = 1, T18BIT = 0, PSA = 0, T1PS = 01

Bit76543210
Value00010001
Field—T1PSPSAT1SET18BITTMR1ON
  • T1PS (bits 5:4) = 01 → Prescaler 1:8
  • PSA (bit 3) = 0 → Prescaler ASSIGNED (inverted: 0 = assigned)
  • T18BIT (bit 1) = 0 → 16-bit mode
  • TMR1ON (bit 0) = 1 → Timer on
  • Every remaining bit is 0: those bits are not needed for this level, but they must stay clear for the value to match.

Try it: build the values

Flip the bits until each register matches the mission. It's practice only; nothing is saved.

0 of 3 registers match.

OSCCON 0x20

✗ Not yet – 2 bits differ
7–
6–
5–
4–
3–
2IRCF1
1IRCF0
0SCS0

Hex 0x00 Binary 0000 0000 Target 0x05 (0000 0101)

  • IRCF (bits 2:1) = 00 → 1 MHz
  • SCS0 (bit 0) = 0 → Internal oscillator not enabled

T0CON 0x27

✗ Not yet – 2 bits differ
7–
6–
5T0PS1
4T0PS0
3PSA
2T0SE
1T08BIT
0TMR0ON

Hex 0x00 Binary 0000 0000 Target 0x03 (0000 0011)

  • T0PS (bits 5:4) = 00 → Prescaler 1:16
  • PSA (bit 3) = 0 → Prescaler ASSIGNED (inverted: 0 = assigned)
  • T0SE (bit 2) = 0 → Selects the increment edge: not part of the win value (leave 0)
  • T08BIT (bit 1) = 0 → 16-bit mode
  • TMR0ON (bit 0) = 0 → Timer off

T1CON 0x28

✗ Not yet – 2 bits differ
7–
6–
5T1PS1
4T1PS0
3PSA
2T1SE
1T18BIT
0TMR1ON

Hex 0x00 Binary 0000 0000 Target 0x11 (0001 0001)

  • T1PS (bits 5:4) = 00 → Prescaler 1:16
  • PSA (bit 3) = 0 → Prescaler ASSIGNED (inverted: 0 = assigned)
  • T1SE (bit 2) = 0 → Selects the increment edge: not part of the win value (leave 0)
  • T18BIT (bit 1) = 0 → 16-bit mode
  • TMR1ON (bit 0) = 0 → Timer off

05How you know it worked

  1. 1Clock pulses arriveThe timer clock line shows the raw Fosc pulses.
    Screenshot coming soon
  2. 2The prescaler divides themThe prescaler line you chose lights up and carries the slower clock.
    Screenshot coming soon
  3. 3Count, then overflowThe counter climbs, wraps to 0, and the overflow pipe latches green.TARGET REACHED
    Screenshot coming soon

Simulation note

Counting is slowed down so you can watch it; the "Overflow every…" readout shows the real period.

Stuck?

  • The counter never movesThe timer is off. Set TMRxON (bit 0).
  • It overflows far too fastPSA = 1 skips the prescaler. Clear bit 3.
  • The period is off by a lotWrong prescaler: here 00 = 1:16 and 11 = 1:2.
  • Timer0 counts to 65,535It's in 16-bit mode. Set the 8-bit switch (bit 1).

06Debrief

You just gave the ship a heartbeat.

Almost every product runs on timers: a steady tick for the main loop, PWM for motors and LEDs, timeouts for safety.

// the same idea in CT0CON = 0x03;  // on, 8-bit, prescaler 1:16T1CON = 0x11;  // on, 16-bit, prescaler 1:8

On a real chip the same switches can sit at different bit positions; its datasheet has the map. The idea is identical.

Next mission · 04FlashBack Fighter (EEPROM)