Multi-Function Timer - Motorola MC68HC05T16 Technical Data Manual

5.2

MULTI-FUNCTION TIMER

Multi-Function Timer Register
The MFT provides miscellaneous function to the MC68HC05T16 MCU. It includes a timer overflow
function, real-time interrupt, and COP watchdog. The external interrupt (IRQ) triggering option is
also set by this Multi-Function Timer register.
The clock base for this module is derived from the bus clock divided by four. For a 4.2MHz E (CPU)
clock, the clock base is 1.05 MHz. This clock base is then divided by an 8-stage ripple counter to
5
generate the timer overflow. Timer overflow rate is thus E/1024. The output of this 8-stage ripple
counter then drives a 4-stage divider to generate real time interrupt. Hence, the clock base for real
time interrupt is E/16384. Real time interrupt rate is selected by RT0 and RT1 bits of Multi-Function
Timer register. The interrupt rates are E/16384, (E/16384)/2, (E/16384)/4, and (E/16384)/8. The
selected real time interrupt rate is then divided by 8 to generate COP reset.
Register bit definitions:
TOF - Timer Overflow
1 (set)
0 (clear) –
This bit is set when the 8-bit ripple counter overflows from $FF to $00; a timer overflow interrupt
will occur, if TOFIE (bit 5) is set. TOF is cleared by writing a '0' to the bit.
RTIF - Real Time Interrupt Flag
1 (set)
0 (clear) –
When RTIF is set, a CPU interrupt request is generated if RITE is set. The clock frequency that
drives the RTI circuit is E/16384 giving a maximum interrupt period of 3.9ms at a bus rate of
4.2MHz. RTIF is cleared by writing a "0" to the bit.
TOFIE - Timer Overflow Interrupt Enable
1 (set)
0 (clear) –
RTIE - Real Time Interrupt Enable
1 (set)
0 (clear) –
MOTOROLA
5-10
Address bit 7
$1C TOF
– 8-bit ripple timer overflow has occurred.
No 8-bit ripple timer overflow has occurred.
– A real time interrupt has occurred.
A real time interrupt has not occurred.
– TOF interrupt is enabled.
TOF interrupt is disabled.
– Real time interrupt is enabled.
Real time interrupt is disabled.
bit 6 bit 5 bit 4 bit 3
RTIF TOFIE RTIE IRQN WDOG
TIMERS
State
bit 2 bit 1 bit 0
on reset
RT1 RT0 0000 0011
MC68HC05T16
TPG