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S9S08SG16E1VTLR Datasheet(PDF) 26 Page - Freescale Semiconductor, Inc |
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S9S08SG16E1VTLR Datasheet(HTML) 26 Page - Freescale Semiconductor, Inc |
26 / 328 page Chapter 2 Pins and Connections MC9S08SG32 Data Sheet, Rev. 7 26 Freescale Semiconductor Typically, application systems have two separate capacitors across the power pins. In this case, there should be a bulk electrolytic capacitor, such as a 10- μF tantalum capacitor, to provide bulk charge storage for the overall system and a 0.1- μF ceramic bypass capacitor located as near to the MCU power pins as practical to suppress high-frequency noise. Each pin must have a bypass capacitor for best noise suppression. VDDA and VSSA are the analog power supply pins for MCU. This voltage source supplies power to the ADC module. A 0.1 μF ceramic bypass capacitor should be located as near to the MCU power pins as practical to suppress high-frequency noise. The VREFH and VREFL pins are the voltage reference high and voltage reference low inputs, respectively for the ADC module. For this MCU, VDDA shares the VREFH pin and these pins are available only in the 28-pin packages. In the 16-pin and 20-pin packages, they are double bonded to the VDD pin. For this MCU, VSSA shares the VREFL pin and these pins are available only in the 28-pin packages. In the 16-pin and 20-pin packages, they are double bonded to the VSS pin. 2.2.2 Oscillator (XOSC) Immediately after reset, the MCU uses an internally generated clock provided by the clock source generator (ICS) module. For more information on the ICS, see Chapter 11, “Internal Clock Source (S08ICSV2).” The oscillator (XOSC) in this MCU is a Pierce oscillator that can accommodate a crystal or ceramic resonator. Rather than a crystal or ceramic resonator, an external oscillator can be connected to the EXTAL input pin. Refer to Figure 2-4 for the following discussion. RS (when used) and RF should be low-inductance resistors such as carbon composition resistors. Wire-wound resistors, and some metal film resistors, have too much inductance. C1 and C2 normally should be high-quality ceramic capacitors that are specifically designed for high-frequency applications. RF is used to provide a bias path to keep the EXTAL input in its linear range during crystal startup; its value is not generally critical. Typical systems use 1 M Ω to 10 MΩ. Higher values are sensitive to humidity and lower values reduce gain and (in extreme cases) could prevent startup. C1 and C2 are typically in the 5-pF to 25-pF range and are chosen to match the requirements of a specific crystal or resonator. Be sure to take into account printed circuit board (PCB) capacitance and MCU pin capacitance when selecting C1 and C2. The crystal manufacturer typically specifies a load capacitance which is the series combination of C1 and C2 (which are usually the same size). As a first-order approximation, use 10 pF as an estimate of combined pin and PCB capacitance for each oscillator pin (EXTAL and XTAL). 2.2.3 RESET RESET is a dedicated pin with open-drain drive containing an internal pull-up device. Internal power-on reset and low-voltage reset circuitry typically make external reset circuitry unnecessary. This pin is normally connected to the standard 6-pin background debug connector so a development system can directly reset the MCU system. If desired, a manual external reset can be added by supplying a simple switch to ground (pull reset pin low to force a reset). |
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