3.2 Power-on and Manual Reset Circuit
To ensure that the state of the host interface selection signal is sampled correctly at the rising edge of the WFM200S RESETn signal, a
power-on reset circuit has been added to the WFM200S Wi-Fi Expansion Kit. This circuit achieves this by
• Adding a delay of 1ms to the rising edge of the RESETn signal with respect to the rising edge of the power supply
• Isolating the host from the WFM200S DAT2/HIF_SEL pin during the rising edge of the RESETn signal
The figure below shows the circuit diagram for the power-on and manual reset circuit. Its functionality is as follows:
• NCP_RESETn is the active-low reset signal of the WFM200S. The WFM200S RESETn pin has an internal pull-up of approximately
43 kOhms. The on-board reset button is connected to this signal.
• HIF_SEL_CTRL is the signal from the HIF selection switch
• HIF_OEn is the active-low output enable signal of the HIF multiplexer circuits
• WF_DAT2_HIF_SEL is the combined SDIO DAT2 signal and HIF selection signal of the WFM200S
• U114 is an open-drain active low output reset monitor which with the installed capacitor connected to the CD pin keeps NCP_RE-
SETn tied to ground for about 1 ms after VMCU_NCP has exceeded the threshold voltage of 0.9 V
• U115 is a tri-state output buffer with an active low output enable signal connected to NCP_RESETn which pulls the CD pin of U116
low while NCP_RESETn is low
• U116 is a push-pull active high output reset monitor which drives HIF_OEn high for 1 ms after the output of U115 is disabled
• U109 is a tri-state output buffer with an active high output enable signal which connects the HIF_SEL_CTRL signal to the
WF_DAT2_HIF_SEL signal as long as HIF_OEn is high
The NCP_RESETn signal is available on both the EXP header and the Raspberry Pi connector and can be used for issuing a manual
reset sequence by pulling it low for at least 1 ms.
Note:
Reset button is effective when board is not connected to MCU or Raspberry Pi boards. When connected, change of host inter-
face is effective after reboot.
Figure 3.2. Power-on and Manual Reset Circuit Diagram
UG407: WFM200S Wi-Fi
®
Expansion Kit User's Guide
WFM200S Wi-Fi NCP Expansion Kit
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