Figure 7-6. Injection Current Design Solution Option
REG B
MCU
REG A
VOUT
VIN
ENB
GND
VOUT
VIN
GND
LOGIC
PWRGD
SHDN
100k
1k
MCU
RST
10k
0.1µf
REAL_ICE / ICD4
PWR_SRC_1
PWR_SRC_2
• For relatively low-speed signals (i.e., typically ≤ 2.5 MHz), a series resistor at the MCU input pins is in line
with the remote signals to limit the injection current to ~1.0 mA/pin will usually suf
fice. The sum of all injection
currents must not exceed ~15 mA total.
As a rule of thumb:
R
SERIES(max)
= ((VDD*0.75) /1 mA). Where, then the input signal Frequency(max) ≤ 1 / (1.95E-10 * R
SERIES(max)
).
Limiting the injection current to a sufficiently low value will insure it will not be able to power the MCU, and
impact the internal power on reset logic.
• Another option is to electrically isolate the interconnected signals using components similar to those listed in the
following table:
Table 7-1. Signal Isolation Components
Signal Isolation Components Inductive Coupling Capacitive Coupling Opto Coupling Analog Signal Coupling
ADuM7241 / 40 ARZ (1 Mbps) X
ADuM7241 / 40 CRZ (25 Mbps) X
ISO721 X
LTV-829S (2 Channel) X
LTV-849S (4 Channel) X
FSA266 / NC7WB66 X
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7.5 Crystal Oscillators
Note:
In all MCUs, the crystal Automatic Gain Control (AGC), logic performs the AGC function only once at start-up.
They do not continuously monitor and adjust crystal gain over temperature or voltage changes.
7.5.1 PIC32MZxxEFxx
Problem 5: Using a primary oscillator with a crystal, but some PIC32MZxxEFxx devices are intermittently not starting
up.
Problem 6: Running a PIC32MZxxEFxx at 200 MHz and some devices are experiencing lockups or exception errors.
This MCU family has documented crystal errata that defines both required crystal circuit configurations, hardware
and software, as well as supported crystal frequencies in addition to silicon revision B2 errata concerning restricted
operating frequency versus Flash WS (i.e., wait states as defined in PRECON.PFMWS).
MCU Start-up Problems
© 2022 Microchip T
echnology Inc.
and its subsidiaries
Manual
DS70005439B-page 18
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