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Bluegiga Technologies Oy
Page 19 of 54
6 Host Interfaces
6.1 Host Selection
The MCU selects the UART/SDIO interfaces by reading PIO[4] at boot-time. When PIO[4] is
high, the SDIO interface is enabled; when PIO[4] is low, the UART is enabled.
If in UART mode, the MCU selects the UART transfer protocol automatically using the unused
SDIO pins shown in Table 9
SDIO_CLK SDIO_CMD Protocol
0 0 bcsp
01h4
1 0 h4ds
11h5
Table 9: SDIO_CLK and SDIO_CMD transfer Protocols
6.2 UART Interface
This is a standard UART interface for communicating with other serial devices.
WT21 UART interface provides a simple mechanism for communicating with other serial devices
using the RS232 protocol.
Note:
WT21 uses RS232 protocol, but voltage levels are 0V to VDD_PADS (requires external RS232
transceiver chip.
UART_TX
UART_RX
UART_RTS#
UART_CTS#
Figure 7: Universal Asynchronous Receiver
Four signals implement the UART function, as shown in Figure 8. When WT21 is connected to
another digital device, UART_RX and UART_TX transfer data between the two devices. The
remaining two signals, UART_CTS and UART_RTS, can be used to implement RS232 hardware
flow control where both are active low indicators.