4.2.3 Principle of Operation
The MEP logic is capable of placing an edge in one of 255 (8 bits) discrete time steps (see device-specific
data sheet for typical MEP step size). The MEP works with the TBM and CCM registers to be certain that
time steps are optimally applied and that edge placement accuracy is maintained over a wide range of PWM
frequencies, system clock frequencies, and other operating conditions.
operating frequencies supported by the HRPWM.
Table 4-3. Relationship Between MEP Steps, PWM Frequency, and Resolution
System
(MHz)
MEP Steps Per
SYSCLKOUT
PWM Minimum
PWM Maximum
(MHz)
Resolution @
60.0
93
916
3.00
10.9
70.0
79
1068
3.50
10.6
80.0
69
1221
4.00
10.4
90.0
62
1373
4.50
10.3
(1)
System frequency = SYSCLKOUT, that is, CPU clock. TBCLK = SYSCLKOUT.
(2)
Table data based on a MEP time resolution of 180 ps (this is an example value. See the device-specific data sheet for MEP limits)
(3)
MEP steps applied = T
SYSCLKOUT
/180 ps in this example.
(4)
PWM minimum frequency is based on a maximum period value, that is, TBPRD = 65535. PWM mode is asymmetrical up-count.
(5)
Resolution in bits is given for the maximum PWM frequency stated.
4.2.3.1 Edge Positioning
In a typical power control loop (for example, switch modes, digital motor control [DMC], uninterruptible power
supply [UPS]), a digital controller (PID, 2pole/2zero, lag/lead, and so on) issues a duty command, usually
expressed in a per unit or percentage terms. Assume that for a particular operating point, the demanded duty
cycle is 0.300 or 30.0% on time and the required converter PWM frequency is 1.25 MHz. In conventional PWM
generation with a system clock of 90 MHz, the duty cycle choices are in the vicinity of 30.0%. In
compare value of 22 counts (that is, duty = 30.6%) is the closest to 30.0% that you can attain. This is equivalent
to an edge position of 244.4 ns instead of the desired 240.0 ns. This data is shown in
By utilizing the MEP, you can achieve an edge position much closer to the desired point of 240 ns.
shows that in addition to the CMPA value of 21 (that is, duty = 29.2% and edge positioning at 233.3 ns), 37 steps
of the MEP (CMPAHR register) will position the edge at 239.96 ns, resulting in almost zero error. In this example,
it is assumed that the MEP has a step resolution of 180 ps.
Tpwm = 800 ns
240 ns
19 20 21 22 23
13.8 ns steps
26.3%
27.8%
29.2%
30.6%
31.9%
Demanded
duty (30.0%)
EPWM1A
0
72
Figure 4-6. Required PWM Waveform for a Requested Duty = 30.0%
High-Resolution Pulse Width Modulator (HRPWM)
384
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Summary of Contents for TMS320 2806 Series
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