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PWM GENERATION WITH THE ST62 16-BIT AUTO-RELOAD TIMER

Figure 2. PWM Timer Operation with PWMPOL=0, MASK=FFFFh and RELOAD mode

(Reload bit=1)

Example:

The purpose of this example is to use the PWM as an 8-bit precision DAC for a Telecom

DTMF application.

DTMF is a succession of breaks and tones. The tone is made of two sinus waves for each digit

to send.

To make the sinus waves, we use a window look-up table of 64 samples. For each frequency

the phase is calculated with 16-bit precision and the look-up table is indexed with the 8 most

significant bits. The sum of the two values obtained from the look-up table is loaded in the

CMP register.

The PWM is selected with the set/reset mode.

With a 8MHz clock frequency, a PSC value of 4 and a RLCP register value at 177h

(RLCPH=100h and RLCPL=77h) the PWM gives a sample frequency of:

8MHz/(4*(177h+1)) = 5319Hz

This frequency is the highest one in order to have 115 CPU cycles to calculate the tone.

To calculate the sinus wave with only 8 bits, the sinus table gives data between 1 and 7Fh.

The addition of the two sinus waves gives an 8-bit data stored in the CMP register when an

overflow occurs.

To avoid the TC register going below the maximum value of the CMP register (which is the

maximum value of the DTMF result FFh) when the program wants to reload the CMP register

after an ARTimer overflow, the CPU cycle time between the overflow and TC=FFh equals:

(177h-FFh)*4/8MHz*8MHz/13 = 36 cycles

Reload

Register

Compare

Value

0

t

up to 65536

Overflow

PWM

ouput

t

MASK&TC=MASK&CMP