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Chapter 18 Flash EEPROM
XVIII - 18
Onboard Serial Interface Programming Mode
xTo connect resistors in series with communication pins (NRST, P53, P54)
When resistors are connected in series with communication pins, signal spectrum speed drops
affected by the load capacity. To ensure effecting reliable communication, the required time in
which the operating voltage reaches to 63% of the time constant must be shorter than 1/8 of
communication cycle. For example, the maximum resistor value, RsMax, derived from following
formula is 208 Ω (at load capacity is 50 pF, communication cycle is12 MHz).
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8 X Communication frequency X Load capacity
Large resistors can be connected in series with communication pins by lowering the communica-
tion speed. The resistor value should be smaller than 1/10 of the pull-up resistors.
xOpen-drain output through writer's communication pin (P53, P54)
During open-drain output through writer's communication pin, signal spectrum speed drops af-
fected by the pull-up resistors and the load capacity. To ensure effecting reliable communication,
the required time in which the operating voltage reaches to 63% of the time constant must be
shorter than 1/8 of communication cycle. The maximum communication cycle derived from foll-
owing formula is 1.38 MHz (at load capacity is 50 pF, pull-up resistor is 1.8 kΩ).
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8 X Pull-up resistor (RupMin) X Load capacity
The communication frequency needs to be lowered for open-drain output, therefore, select push-
pull output, if possible.
RsMax =
fMax =