Texas Instruments C28 Series, Student Manual

Page: 124 / 342

General-Purpose Timers
GP Timer Modes of Operation
The C28x event managers (EVA and EVB) each have two General Purpose Timers (GP Timers).
Each timer has four different modes of operation. The most simple is the Stop/Hold mode, where
the operation of the timer stops and holds at its current state. The timer counter, its compare
output, and its prescale counter all remain unchanged in this mode. Two of the remaining three
modes are commonly used in PWM generation, the exception being the directional up/down
counting mode. This mode allows the use of external signals for both count clocking and
direction, and is also employed for recording encoder counts from the on-chip QEP units.
Continuous-Up Counting
Continuous
Continuous
-
-
Up Counting Mode
Up Counting Mode
This example: This example:
TxCON.3 TxCON.3 - - 2 = 00 (reload 2 = 00 (reload TxCMPR TxCMPR on underflow) on underflow)
TxPR TxPR = 3 = 3
TxCMPR TxCMPR = 1 (initially) = 1 (initially)
Prescale Prescale = 1 = 1
0
0
1
1
2
2
3
3
0
0
1
1
2
2
3
3
0
0
1
1
2
2
CPUCLK
CPUCLK
TxCNT
TxCNT
Reg.
Reg.
3
3
0
0
CPU writes a 2 to CPU writes a 2 to
compare reg. buffer compare reg. buffer
anytime here
anytime here
TxCMPR
TxCMPR
=2 is loaded here
=2 is loaded here
TxPWM TxPWM / / TxCMP TxCMP
(active high) (active high)
(Used for Asymmetric PWM Waveforms)
(Used for Asymmetric PWM Waveforms)
♦
♦
Seemless Seemless counting continues counting continues
♦ ♦
Up count period is TxPR+1
Up count period is TxPR+1
The procedure for GP Timer Continuous-Up Counting is as follows:
User sets bit 6 of TxCON register high to initiate counting
Counting begins on next rising clock edge
−
1
st
count is a “Zero” (no increment)
Count up until match with period register
If counter > period counts up to FFFFh
On next rising clock edge:
−
Counter resets to zero
−
Counting continues seemlessly
7 - 10 C28x - Event Manager