Eliminate voltage swings on your F²MC-16X MCU

Also: To MUX or not to MUX, writing to flash memory, and how do I find all of those Fujitsu MCUs, anyway?

Q. I have an MB96610 series MCU with an F2MC-16FX core.  When A/D conversion is continuously executed, the analog input voltage often increases or decreases according to the voltage. Why is this happening and how can I correct for this?

A.  I hope this will answer your question while giving you some suggestions on how to correct for this.  If the external impedance is too high or the sampling time Tsamp is too short, the analog voltage charged to the internal sample and hold capacitor will be insufficient, adversely affecting the A/D conversion precision. To satisfy the A/D conversion precision, you must select a sufficient Tsamp. The required Tsamp depends on the external driving impedance Rext, the board capacitance of the A/D converter input pin Cext, and the analog supply voltage AVCC. Refer to the following circuit example (figure 1) where:

Rext: External driving impedance

Cext: Capacitance of PCB at A/D converter input

CVIN: Analog input capacitance (I/O, analog switch and ADC are contained)

RVIN: Analog input impedance (I/O, analog switch and ADC are contained)

To determine minimum Tsamp, we can use the following approximation formula for the replacement model:

Tsamp = 7.62 × (Rext × Cext + (Rext + RVIN) × CVIN)


Here are some key considerations:

  • Do not select a sampling time below the absolute minimum permitted value as shown below.

(0.5 μs for 4.5 V ≤ AVCC ≤ 5.5 V, 1.2 μs for 2.7 V ≤ AVCC < 4.5 V)


  • If the appropriate sampling time is not sufficient, you can increase it by connecting a capacitor of about 0.1 μF to the analog input pin.
  • A big external driving impedance also adversely affects the A/D conversion precision as a result of the pin input leakage current IIL (static current before the sampling switch) or the analog input leakage current IAIN (total leakage current of pin input and comparator during sampling). The effect of the pin input leakage current IIL cannot be compensated for by an external capacitor.

The accuracy declines as magnitude of the difference between the analog high reference voltage AVRH and the analog ground voltage AVSS (|AVRH – AVSS|) becomes smaller. For more information, please refer to the datasheet on Spansion.com.

Q: Is the random read access time faster on a non-multiplexed (non-MUXed) or a MUXed memory device?

A: The non-MUXed device will deliver faster random read access time. The tAA (address access time) is actually the access time from when data is latched and ready on the output buffers to the output pins. The preparation time to latch address and data is not factored into the the tAA time, however. On a non-MUXed device, the preparation time for address and data are separate and on dedicated lines, as compared to the MUXed device, for which the preparation time for address and data must be shared on the same lines. Although some random address access time for MUXed and non-MUXed devices may be identical, the preparation time for address and data will be longer on the MUXed device. The non-MUXed device would therefore be a better choice.

Q: How can I write to the flash memory when the FM3 microcontroller is mounted on the board?

A: The methods for writing to the flash memory when the microcontroller is mounted on a board involve onboard write through serial.  The options for onboard write include:

  • Use of a commercial writer (consult the writer manufacturer for details)
  • Write using RS-232C from a PC (see the serial write connection chapter of the FM3 Flash Programming Manual).
  • For models with USB in the microcontroller, write using USB from a PC (see the serial write connection chapter of the FM3 Flash Programming Manual.

Q: What is the Suffix OPN nomenclature breakdown regarding the MB3771PF-G-BND-JN-ERE1?

A: The Suffix Ordering Part Number (OPN) breakdown for MB3771PF-G-BND-JN-ERE1 is as follows:

  • PF = 8pin-SOP
  • G = general purpose
  • BND = bended leads1
  • JN = production identification purposes / fab location (China)
  • ER = tape & reel
  • E1 = lead-free material set

1. Lead disposition was removed from suffix OPN on newer Fujitsu products.

Q: Is there a road map detailing all of Fujitsu's former microcontrollers that are now available at Spansion?

A: For updated information on current (ex-Fujitsu) microcontroller products now available at Spansion, please click here. This page provides market segment information (automotive and industrial), technical resources, CPU core road-maps, and documentation.


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