Intel VC820 Design Guide - Page 96
RTC External RTCRST Circuit, 2.20.6 RTC Routing Guidelines
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Layout/Routing Guidelines 2.20.5 RTC External RTCRST Circuit The ICH RTC requires some additional external circuitry. The RTCRST# signal is used to reset the RTC well. The external capacitor and the external resistor between RTCRST# and the RTC battery (Vbat) were selected to create an RC time delay, such that RTCRST# will go high some time after the battery voltage is valid. The RC time delay should be in the range of 10-20 ms. When RTCRST# is asserted, bit 2 (RTC_PWR_STS) in the GEN_PMCON_3 (General PM Configuration 3) register is set to 1, and remains set until software clears it. As a result of this, when the system boots, the BIOS knows that the RTC battery has been removed. Figure 2-58. RTCRST External Circuit for the ICH RTC VCC3_3SBY Diode/ Battery Circuit 1K VccRTC 1.0 uF 8.2K RTCRST Circuit RTCRST# 2.2 uF 2.20.6 This RTCRST# circuit is combined with the diode circuit (Figure 2-57) which allows the RTC well to be powered by the battery when the system power is not available. Figure 2-56 is an example of this circuitry that is used in conjuction with the external diode circuit. RTC Routing Guidelines • All RTC OSC signals (RTCX1, RTCX2, VBIAS) should all be routed with trace lengths of less than 1", the shorter the better • Minimize the capacitance between RTCX1 and RTCX2 in the routing (optimal would be a ground line between them) • Put a ground plane under all of the external RTC circuitry • Do not route any switching signals under the external components (unless on the other side of the ground plane) 2-70 Intel®820 Chipset Design Guide