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ST72E331 Datasheet(PDF) 46 Page - STMicroelectronics |
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ST72E331 Datasheet(HTML) 46 Page - STMicroelectronics |
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46 / 107 page  46/107 ST72E331 ST72T331 16-BIT TIMER (Cont’d) 5.4.3.4 Output Compare In this section, the index, i, may be 1 or 2 because there are 2 output compare functions in the 16-bit timer. This function can be used to control an output waveform or indicate when a period of time has elapsed. When a match is found between the Output Com- pare register and the free running counter, the out- put compare function: – Assigns pins with a programmable value if the OCIE bit is set – Sets a flag in the status register – Generates an interrupt if enabled Two 16-bit registers Output Compare Register 1 (OC1R) and Output Compare Register 2 (OC2R) contain the value to be compared to the counter register each timer clock cycle. These registers are readable and writable and are not affected by the timer hardware. A reset event changes the OCiR value to 8000h. Timing resolution is one count of the free running counter: ( fCPU/CC[1:0]). Procedure: To use the output compare function, select the fol- lowing in the CR2 register: – Set the OC iE bit if an output is needed then the OCMP i pin is dedicated to the output compare i signal. – Select the timer clock (CC[1:0]) (see Table 1). And select the following in the CR1 register: – Select the OLVL i bit to applied to the OCMPi pins after the match occurs. – Set the OCIE bit to generate an interrupt if it is needed. When a match is found between OCRi register and CR register: – OCF i bit is set. – The OCMP i pin takes OLVLi bit value (OCMPi pin latch is forced low during reset). – A timer interrupt is generated if the OCIE bit is set in the CR2 register and the I bit is cleared in the CC register (CC). The OCiR register value required for a specific tim- ing application can be calculated using the follow- ing formula: Where: ∆t = Output compare period (in seconds) fCPU = CPU clock frequency (in hertz) PRESC = Timer prescaler factor (2, 4 or 8 de- pending on CC[1:0] bits, see Table 1) If the timer clock is an external clock, the formula is: Where: ∆t = Output compare period (in seconds) fEXT = External timer clock frequency (in hertz) Clearing the output compare interrupt request (i.e. clearing the OCF i bit) is done by: 1. Reading the SR register while the OCF i bit is set. 2. An access (read or write) to the OC iLR register. The following procedure is recommended to pre- vent the OCF i bit from being set between the time it is read and the write to the OCiR register: – Write to the OC iHR register (further compares are inhibited). – Read the SR register (first step of the clearance of the OCF i bit, which may be already set). – Write to the OC iLR register (enables the output compare function and clears the OCF i bit). MS Byte LS Byte OC iROCiHR OC iLR ∆ OCiR = ∆ t * fCPU PRESC ∆ OCiR = ∆t * fEXT 46 |
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