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I want to make a pcb, there will be the device with spi and i only need to read it's registers and send on bluetooth's rx pin. It's a little bit expensive to use microcontroller to just read registers. Is there some optimal solution for this? I need a programmable chip that read registers with spi and send values by uart.

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    \$\begingroup\$ I'm afraid you need an MCU - but surely one of the $1 parts will do this? \$\endgroup\$ – pjc50 Mar 16 '17 at 13:34
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    \$\begingroup\$ Considering that MCUs cost sub $1 for a Cortex M0, I doubt that you will find a discrete / programmable logic solution which will be cheaper. There might be some ICs floating around for doing this. \$\endgroup\$ – Arsenal Mar 16 '17 at 13:37
  • \$\begingroup\$ @Arsenal okay, i will search for cheapest mcu then \$\endgroup\$ – GerrDott Mar 16 '17 at 14:01
  • \$\begingroup\$ Some bluetooth modules (such as HC05) have built in microcontrollers and you might be able reprogram it to bit-bang out the necessary spi commands to read your device directly from the module. \$\endgroup\$ – ks0ze Mar 16 '17 at 15:04
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There is an optimal solution, but for some reason you have excluded it. The cheapest and smallest solution by far will be a microcontroller. You should be able to find a micro for under a USD, in a handy SO-8 package that will have everything integrated. I found one in a quick parametric search.

Implementing such a hack with discrete logic will likely be twice as expensive, not to mention the board size and extra circuits needed.

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It's a little bit expensive to use microcontroller to just read registers.

There are cheap MCUs :-)

I need a programmable chip that read registers with spi and send values by uart.

A programmable chip to do this, will effectively either be an MCU, or will need an MCU to program it e.g. the NXP SC16IS740 family.

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If you are going for production i strongly advice against an MCU. The total cost for programming the MCU, handling the SW in a controlled way and keep track of what versions is compatible with what HW will likley be higher than a non-MCU solution.

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    \$\begingroup\$ If you used programmable logic you would have exactly the same issues with the device configuration file. If you use some fixed logic circuit then you are paying a lot more for something that is more likely to have compatibility issues in the future but is a lot more expensive to change. \$\endgroup\$ – Andrew Mar 17 '17 at 12:47

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