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ST72321 Datasheet(PDF) 69 Page - STMicroelectronics |
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ST72321 Datasheet(HTML) 69 Page - STMicroelectronics |
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69 / 189 page  ST72321 69/189 10.4 16-BIT TIMER 10.4.1 Introduction The timer consists of a 16-bit free-running counter driven by a programmable prescaler. It may be used for a variety of purposes, including pulse length measurement of up to two input sig- nals (input capture) or generation of up to two out- put waveforms (output compare and PWM). Pulse lengths and waveform periods can be mod- ulated from a few microseconds to several milli- seconds using the timer prescaler and the CPU clock prescaler. Some ST7 devices have two on-chip 16-bit timers. They are completely independent, and do not share any resources. They are synchronized after a MCU reset as long as the timer clock frequen- cies are not modified. This description covers one or two 16-bit timers. In ST7 devices with two timers, register names are prefixed with TA (Timer A) or TB (Timer B). 10.4.2 Main Features ■ Programmable prescaler: fCPU divided by 2, 4 or 8. ■ Overflow status flag and maskable interrupt ■ External clock input (must be at least 4 times slower than the CPU clock speed) with the choice of active edge ■ 1 or 2 Output Compare functions each with: – 2 dedicated 16-bit registers – 2 dedicated programmable signals – 2 dedicated status flags – 1 dedicated maskable interrupt ■ 1 or 2 Input Capture functions each with: – 2 dedicated 16-bit registers – 2 dedicated active edge selection signals – 2 dedicated status flags – 1 dedicated maskable interrupt ■ Pulse width modulation mode (PWM) ■ One pulse mode ■ Reduced Power Mode ■ 5 alternate functions on I/O ports (ICAP1, ICAP2, OCMP1, OCMP2, EXTCLK)* The Block Diagram is shown in Figure 43. *Note: Some timer pins may not be available (not bonded) in some ST7 devices. Refer to the device pin out description. When reading an input signal on a non-bonded pin, the value will always be ‘1’. 10.4.3 Functional Description 10.4.3.1 Counter The main block of the Programmable Timer is a 16-bit free running upcounter and its associated 16-bit registers. The 16-bit registers are made up of two 8-bit registers called high & low. Counter Register (CR): – Counter High Register (CHR) is the most sig- nificant byte (MS Byte). – Counter Low Register (CLR) is the least sig- nificant byte (LS Byte). Alternate Counter Register (ACR) – Alternate Counter High Register (ACHR) is the m ost significant byte (MS Byte). – Alternate Counter Low Register (ACLR) is the least significant byte (LS Byte). These two read-only 16-bit registers contain the same value but with the difference that reading the ACLR register does not clear the TOF bit (Timer overflow flag), located in the Status register, (SR), (see note at the end of paragraph titled 16-bit read sequence). Writing in the CLR register or ACLR register resets the free running counter to the FFFCh value. Both counters have a reset value of FFFCh (this is the only value which is reloaded in the 16-bit tim- er). The reset value of both counters is also FFFCh in One Pulse mode and PWM mode. The timer clock depends on the clock control bits of the CR2 register, as illustrated in Table 16 Clock Control Bits. The value in the counter register re- peats every 131072, 262144 or 524288 CPU clock cycles depending on the CC[1:0] bits. The timer frequency can be fCPU/2, fCPU/4, fCPU/8 or an external frequency. |
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