Intel PXA255 - Manual (Page 144)

4-40 Intel® PXA255 Processor Developer’s Manual
System Integration Unit
comparator contains PWM_PERVALn[PV] and clears the PWM_OUT signal low when
PWM_PERVALn[PV] + 1 and the 10-bit up counter are equal. Both PWM_PERVALn[PV] and
PWM_DUTYn[DCYCLE] are 10 bit fields.
Note: Take care to ensure that the value of the PWM_PERVALn register remains larger than
PWM_DUTYn register. In the case where PWM_PERVALn is less than PWM_DUTYn the output
maintains a high state.
4.5.1.2 Reset Sequence
A system reset results in no pulse width modulated signal. During system reset the PWM_CTRLn
and PWM_DUTYn registers are reset to 0x0 and the PWM_PERVALn register is set to 0x004.
This sets the PWM_OUTn pin low with a zero duty cycle. The six bit down-counter is reset to 0x0
and thus the 3.68 MHz input clock directly drives the 10 bit up-counter. The PWM_OUTn pin
remains reset low until the PWM_DUTYn register is programmed with a non zero value.
A basic pulse width waveform is shown in Figure 4-4 .
4.5.1.3 Power Management Requirements
Each PWM may be disabled through a pair of clock enable bits (see Section 3.6.2, “Clock Enable
Register (CKEN)” on page 3-36 ). If the clock is disabled, the unit shuts down in one of two ways:
• Abrupt – the PWM stops immediately.
• Graceful – the PWM completes the current duty cycle before stopping.
Shutdown is selected by PWM_CTRL[PWM_SD] and described in Section 4.5.2.1 .
4.5.2 Register Descriptions
The following paragraphs provide register descriptions for the Pulse Width Modulator.
4.5.2.1 PWM Control Registers (PWM_CTRLn)
The PWM_CTRLn, shown in Table 4-46 , contains two fields:
• PRESCALE – The PRESCALE field contains the 6-bit prescale counter load value. This field
allows the 3.6864 MHz input clock PSCLK_PWMn, to be divided by values between 1
(PWM_CTL[PRESCALE] = 0) and 64 (PWM_CTL[PRESCALE] = 63).
Note: The value of the divisor is one greater than the value programmed into the PRESCALE field.
• PWM_SD – PWMn can shut down in one of two ways, gracefully or abruptly, depending on
the setting of PWM_CTRLn[PWM_SD]. If gracefully is chosen, then the duty cycle counter
completes its count before PWMn is shut down. If abruptly is chosen, then the prescale
counter and the duty cycle counter are reset to the reload values in their associated registers
and PWMn is immediately shut down.
Note: During abrupt shut down the PWM_OUTn signal may be delayed by up to one PSCLK_PWMn
clock period.