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MOTOROLA
MC68349 USER’S MANUAL
10-3
MC68349
XFC
VCCSYN
0.1 F 1
NOTE 1: Must be a low leakage capacitor.
VCCSYN
0.1 F
0.01 F
Figure 10-4. XFC and VCCSYN Capacitor Connections
10.1.2 Reset Circuitry
A power-on reset (POR) is generated by the SIM49 module when it detects a positive
going VCC transition—the VCC threshold is typically in the range 2.0–2.7V, and varies
depending on processing and environmental variables. Hysterisis is included in the reset
circuit to prevent reassertion for a monotonically increasing VCC voltage; however,
excessively long VCC rise times (>100 ms) may allow the reset logic to release RESET
before VCC has stabilized. The reset thresholds provided in the SIM49 should not be
relied upon to monitor VCC, since internal logic may fail at voltages between spec VCCmin
and the reset trigger threshold. An external low voltage monitor circuit, such as the
MC34064, should be used instead.
When the MC68349 is used in crystal clock mode, the simplest external reset logic
consists of simply a 1K pullup resistor from RESET to VCC. This solution relies on a
monotonically increasing VCC that has a rise time on the order of 100ms or less - the
actual allowable rise time is dependent on the startup time of the 32.768kHz oscillator
circuit. As noted above, this does not provide rigorous VCC monitoring, and may be
susceptible to sags or glitches in the VCC supply voltage.
In external clock mode, either with or without the PLL, the POR time delay of 328 * T clkin
does not provide adequate time for VCC to stabilize before allowing reset to negate.
Applications using these two clocking modes should include an external reset circuit which
generates an appropriate delay for the power source being used, as well as a voltage
monitor if needed.
10.1.3 SRAM Interface
Although the MC68349 contains internal static random access memory (SRAM), an
optional external SRAM interface may be created using the programmable chip selects.
External circuitry to decode address information and circuitry to return data and size
acknowledge (DSACK≈) is not required. However, external logic is required to provide
write enables for the high and low data bytes.