SPI interface 3-wire serial Si471X

Question

I tried to find an answer to this question by searching SDIO which is related to my question based on Silicon Labs naming of data channels. However, I was flooded with topics related to the SDIO protocol for serial comms with an SD memory card - a totally different concern.

I want to interface a PIC MCU (SPI bus) with Silicon Labs FM TX/RX 47XX chips which describe a serial control interface and the multiple ways to use it - what they call 2-wire (basically I2C) or what they call 3-wire and refer to as "SPI". The 3 lines are SLCK, SEN and SDIO (clock, enable, and data input/output half-duplex over the same wire).

My problem is that SPI is full duplex capable - TX and RX simultaneously. Even though every implementation I've seen of this is really half-duplex - send then receive - the hardware is wired with SDI/SDO or MISO/MOSI or called whatever lines in the SPI module; a separate data IN and a data OUT wire. Along with SCLK and SEN (clock and enable).

I'm not sure how to wire this up - and whether it will work. Do I wire both the SDI and SDO pins of the PIC (SPI module) to the same SDIO on the Si47XX? Since the Si47XX is half-duplex that would make sense; but I've never seen the two lines tied on an SPI interface. Not sure if the pins are always Hi-Z for example and I don't want to fry something out. Would I need to add pull ups/downs?

Oddly, I can't seem to find an actual wiring diagram of such a case. When I search 3-wire SPI I always get examples where SEN is tied since only one slave device is being used - not my situation. Any advice would be - as always - greatly appreciated. -Rick

Answer 1

One can interconnect SPI and 3-wire devices using the following schematic. It does not matter which device is master and which is slave.

SPI device             3-wire device
----+
SDO |--/\/\/\-+  3k
    |         |        +---
SDI |---------*--------| SDIO
    |                  |
SCK |------------------| SCK
SEN |------------------| SEN
----+                  +---

If a 3-wire device is in reception state, then SDO signal comes via resistor to (Hi-Z) SDIO pin and back to SDI pin. If the 3-wire device is in transmission state, then SDIO output overrides SDO value.

The example of such connection one can see in FTDI datasheet for FT2232H , page 37 (Figure 4.1RS232 Configuration) — 93С46 to FT2232H connection.

At lower SPI frequencies one can use higher resistor value for lower overriding current. It's depend also from traces length but in usual cases 3k is in very good marging for frequences up to 2 MHz.

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ps It seems that the question is not about programming. Could it be moved to electronics.stackexchange.com?