FMS7401L_05 Datasheet, PDF (34/81 Pages) Fairchild Semiconductor – Digital Power Controller

FMS7401L
PRODUCT SPECIFICATION
Table 14. Timer 1 Prescale Selection (PS) Bits
PS[2]
0
0
0
0
1
1
1
1
PS[1]
0
0
1
1
0
0
1
1
PS[0]
0
1
0
1
0
1
0
1
Timer 1 Clock
FT1CLK / 1
FT1CLK / 2
FT1CLK / 3
FT1CLK / 4
FT1CLK / 5
FT1CLK / 6
FT1CLK / 7
FT1CLK / 8
6.1.2 PWM Cycle Configuration Registers
The PWM Timer 1 circuit has three 12-bit (T1CMPA, T1CMPB, T1RA) and one 5-bit (DTIME) configuration registers used to
specify the duty cycle and period of the Timer 1 output signals. Upon a system reset, all four registers are initialized with ones
(0xFFF, 0x1F). All configuration registers must be programmed with their appropriate values prior to enabling the Timer 1
circuit. Except for the DTIME register, the T1CMPA, T1CMPB and T1RA registers may be changed by software at any time;
however, if the Timer 1 circuit is in run mode, the register values will not change the Timer 1 output signal’s attributes until
after the TMR1 counter overflows ending the current PWM cycle. The last register values at the TMR1 counter overflow will
dictate the output signal’s attributes for the next PWM cycle. When reading the registers, the value reported will be the last
value written by software and may not necessarily reflect the output signal’s attributes for the current PWM cycle.
The 12-bit T1RA is Timer 1’s reload/capture register (depending on the selected operating mode). All 12 bits are accessible by
software through the 4-bit T1RAHI and 8-bit T1RALO memory mapped registers where T1RA= {T1RAHI, T1RALO}.¹ In
PWM Mode, T1RA configures the period of the T1HS1, T1HS2 and ADSTROBE outputs and determines after what TMR1
count it will overflow (reinitialize to 0x000) to begin the next PWM cycle. In Input Capture Mode, T1RA contains the captured
value of the TMR1 count at the time of the trigger defined by the rising or falling edge of the T1HS2 input.
The 12-bit T1CMPA is Timer 1’s Compare A register that dictates the length of the resting (off) state of the T1HS1 and T1HS2
output signals. All 12 bits are accessible by software through the 4-bit T1CMPAHI and 8-bit T1CMPALO memory mapped
registers where T1CMPA={T1CMPAHI, T1CMPALO}.¹ In PWM Mode, the TMR1 counter is compared against T1CMPA
(TMR1=T1CMPA) to determine when the first transition of the T1HS1 and T1HS2 output signals should be triggered. In Input
Capture Mode, the T1CMPA value is still used to configure T1HS1 but has no affect on the T1HS2 signal since it is used as an
input of the Timer 1 circuit in this mode. Software must ensure that the total T1CMPA plus TDT time is not greater than or equal
to the total T1RA plus TDT times.
The 12-bit T1CMPB is Timer 1’s Compare B register that dictates the length of the resting (off) state of the ADSTROBE out-
put signal. All 12 bits are accessible by software through the 4-bit T1CMPBHI and 8-bit T1CMPBLO memory mapped regis-
ters where T1CMPB={T1CMPBHI, T1CMPBLO}.¹ The TMR1 counter is compared against T1CMPB to determine when the
first transition of the ADSTROBE output signal should be triggered. If enabled, at the rising edge of ADSTROBE
(TMR1=T1CMPB) an Analog-to-Digital Converter (ADC) conversion may be initiated.⁴ Software must ensure that the total
T1CMPB plus TDT time is not greater than or equal to the total T1RA plus TDT times.
Bits 4-0 (DT[4:0]) of the DTIME register determines the amount of dead time (TDT) delay, if any, between the T1HS1 and
T1HS2 output signal transitions (see Table 13). In PWM Mode, once the TMR1 counter equals the T1CMPA value, the T1HS1
signal transitions ending its resting (off) state. The dead time delay counter is then initiated incrementing with each edge of the
FT1CLK up to the programmed DT[4:0] value.⁵ Once TDT has passed, the T1HS2 signal will then transition ending its resting
(off) state. As the TMR1 counter continues, once the TMR1 counter equals the T1RA value, the T1HS2 signal transitions end-
ing its active (on) state. The dead time delay counter is then reinitiated incrementing to the DT[4:0] value. Once TDT has
passed, the T1HS1 signal will then transition ending its active (on) state.
34
REV. 1.0.3 1/24/05