162CHAPTER 7 CLOCK GENERATOR7.4.2 Subsystem clock oscillatorThe subsystem clock oscillator oscillates with a crystal resonator (standard: 32.768 kHz) connected to the XT1and XT2 pins.External clocks can be input to the main system clock oscillator. In this case, input a clock signal to the XT1 pinand an antiphase clock signal to the XT2 pin.Figure 7-7 shows an external circuit of the subsystem clock oscillator.Figure 7-7. External Circuit of Subsystem Clock Oscillator(a) Crystal oscillation (b) External clockExternalClockXT2XT1PD74HCU04μXT2XT132.768kHzICCautions 1. When using a main system clock oscillator and a subsystem clock oscillator, carry out wiringin the broken line area in Figures 7-6 and 7-7 to prevent any effects from wiring capacities.l Minimize the wiring length.l Do not allow wiring to intersect with other signal lines. Do not allow wiring to come nearchanging high current.l Set the potential of the grounding position of the oscillator capacitor to that of V SS . Donot ground to any ground pattern where high current is present.l Do not fetch signals from the oscillator.Take special note of the fact that the subsystem clock oscillator is a circuit with low-levelamplification so that current consumption is maintained at low levels.Figure 7-8 shows examples of resonator having incorrect connection.Figure 7-8. Examples of Resonator with Incorrect Connection (1/2)(a) Wiring of connection (b) Signal lines intersect each othercircuits is too longRemark When using a subsystem clock, replace X1 and X2 with XT1 and XT2, respectively. Further, insertresistors in series on the side of XT2.X2 ICX1 X2 ICX1PORTn(n = 0 to 7, 12, 13)
