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EM6682 Datasheet(PDF) 11 Page - EM Microelectronic - MARIN SA |
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EM6682 Datasheet(HTML) 11 Page - EM Microelectronic - MARIN SA |
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11 / 59 page  EM6682 Copyright 2008, EM Microelectronic-Marin SA 04/08 – rev.D 11 www.emmicroelectronic.com 5. Oscillator and Prescaler 5.1 RC Oscillator or external Clock EM6682 can use the internal RC oscillator or external clock source for its operation. The built-in RC oscillator without external components generates the system operating clock for the CPU and peripheral blocks. The RC oscillator is supplied by the regulated voltage in medium voltage version. The RC oscillator can generate 2 basic frequencies selectable by metal option or by registers 512kHz or 800kHz selected by RegMFP1[3]. In the output of the RC oscillator, a selectable divider allows generating 512kHz, 256kHz, 128kHz, 64kHz or 32kHz. If the basic frequency is 800kHz the divider can generate 800kHz, 400kHz, 200kHz, 100kHz or 50kHz. The division factor is given by the register RegMFP1[2:0] (see table 5.2.1). These frequencies can be used for the CPU clock (if the external clock is not selected). Another divider generates automatically SysClk as close as possible to 32kHz or 50kHz depending on the basic frequency selected. This clock is used by the 15 stages prescaler that generated clock for the peripherals. For a full software compatibility with the EM6680 and EM6681, the division factor of the RC clock can be fix by metal option. It means that it is not necessary to set the RC frequency by software. To get a good frequency stability over the supply in low voltage it is possible, by metal option, to select a 128kHz (or 200kHz) RC oscillator. In this case the division factor is fixed to 1 meaning that the CPU runs at 128kHz (or 200kHz) and SysClk at 32kHz (or 50kHz). It is not permitted to use the EM6682 at LV over 128kHz (200kHz). Please note that Vddmin must be higher when working with higher frequencies – see figure 30. After POR the circuit always starts with the internal RC oscillator, but it can be switched to the external clock by setting the ExtCPUclkON bit in the register RegPresc. The external clock is input at PA[1] and must be in range from min. 10Khz to max. 1MHz. With this external frequency input all timing for peripherals change and the special 4 stage freq. divider must be adapted to best suit the applied external frequency to keep 32/50kHz System clock as close as possible. The system clock must be less than 64kHz. The external clock source must be a square wave with full amplitude from Vss to Vdd. See Table 5.2.2 for advised special divisions depending on the external clock frequency. Switching from internal RC oscillator to External clock or back from External clock to RC oscillator is made without generating a glitch on the internal clock. Once the circuit is running on the external Clock one can disable the RC oscillator by settin g the RCoscOff bit in RegSCntl2 to „1‟. In sleep mode the oscillator is stopped. It can be stopped also by setting the RCoscOff bit. This bit can be set only if ExtClkOn was set before, indicating that the CPUck was switched from the internal RC oscillator to the external clock which MUST be present. If the External Clock stops without going into Sleep mode first the EM6682 can block and only POR can reset it. Figure 8 below shows the connection of the RC oscillator and external clock and generation of CPUclk and System clock = SysClk which is divided by the special 4 stage Freq. Divider if needed as described in 97H 5.2 and prescaler described in 5.3. Figure 8. Clock source for CPU or system peripherals MUX RegOscTrim1 [3:0] RegOscTrim1 [3:0] RC oscilator 512kHz – 800kHz OscTrim[7:0] RegMFP1 [3] RegMFP1 [2:0] Frequency divider MUX Metal option or register selector Metal option or register selector RC SYSTEM External clock 10kHz – 1MHz ExtCPUclkON Frequency divider MUX Automatic clock selection SysClk = 32kHz or 50kHz CPUclk PRESCALER LOW 15 stages divider MUX foutSel[1:0] MUX Sout PAout[2] ResSys PA[2] Peripherals SysClk Ck[14] MUX PAout[2] PA[5] PA[1] |
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