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Could somebody explain how an FTDI chip converts USB signals into JTAG signals please (is that a correct statement)?

On the development board for my micro-controller the USB connector goes to the FTDI and JTAG signals come out of a port and go to the chip.

I thought FTDI only converted USB to the UART.

Thank you

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  • \$\begingroup\$ Might want to take a look into this, although you didn't specify which FTDI IC you are talking about. \$\endgroup\$ – Wesley Lee Feb 7 '17 at 23:26
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"An FTDI chip" could be any number of things. FTDI makes a lot of different chips, some of which aren't even related to USB.

The "FTDI chip" you're probably most familiar with is the FT232R, which is a pretty basic USB-to-UART part. However, most FTDI-based JTAG devices are based on the FT232H (or the related FT2232H/FT4232H), which supports an interface mode known as MPSSE. The MPSSE mode supports a number of complex synchronous-serial operations, some of which are specifically designed for use with JTAG. For instance, there are a couple of MPSSE operations specifically designed to operate on the JTAG TMS pin.

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  • \$\begingroup\$ thank you, I didn't know about MPSSE - the chip I have is the FT4232H. \$\endgroup\$ – lucozade Feb 8 '17 at 9:24
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Although the most popular FTDI products are USB-to-UART converters, they also make USB-to-JTAG converters, and even some chips which can serve both purposes from a single part. The functions implemented in silicon are similar, but not identical, and different API calls may be used from the PC.

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