Dear Peter,
You can combine 3-wire device with 4-wire master, the functionality of this kind of connections depends on the master microcontroller, typically the one with the 10k resistor between MOSI and MISO as in your design is used. But I cannot guarantee it would work for your application, you may need to consult with your mcu manufacturer.
Also, if the mcu manufacturer confirms if the mcu can work with 3-wire SPI, then check that it is properly configured for this kind of communication.
About the PCF8537, at power-on, the PCF8537 resets to starting conditions as follows:
1. All backplane and segment outputs are set to VSS.
2. Selected drive mode is: 1:8 with 1⁄ 4 bias.
3. Input and output bank selectors are reset.
4. The I2C-bus and SPI-bus interface are initialized.
5. The data pointer is cleared (set logic 0).
6. The internal oscillator is running; no clock signal is available on pin CLK; pin CLK is in 3-state.
7. Temperature measurement is enabled.
8. Temperature filter is disabled.
9. The internal VLCD voltage generation is disabled. The charge pump is switched off.
10. The VLCD temperature compensation is enabled.
11. The display is disabled.
Then, the first command sent to the device after the power-on event must be the Initialize command.
This command generates a chip-wide reset which resets all command values to their default values. After this command is sent, it is possible to send additional commands without the need to re-initialize the interface. The reset takes 100 ns to complete.
After POR and before enabling the display, the RAM content should be brought to a defined status
• by clearing it (setting it all to logic 0) or
• by writing meaningful content (for example, a graphic)
otherwise unwanted display artifacts may appear on the display.
The recommended start-up sequences depending on the application modes, can be found in section 8.2.3 of the datasheet: https://www.nxp.com/docs/en/data-sheet/PCF8537.pdf
Have a great day,
Jose
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