Dangerous Prototypes

Dangerous Prototypes => Bus Blaster JTAG debugger => Topic started by: Rachotilko on January 02, 2021, 05:45:44 pm

Title: BB3 KT-link buffer logic design question
Post by: Rachotilko on January 02, 2021, 05:45:44 pm
Hello,

looking at schematic (http://dangerousprototypes.com/docs/images/f/fa/Bbv2-ktlink-v1.png) of KT-link buffer logic (implemented by CPLD) I came across question of ADBUS2 (FT_TDO) pin configuration. It seems this pin is always driven to '1' or '0' by the multiplexer output - it is never 'high Z'. In case the BBv3 is not connected to target, ADBUS2 ((FT_TDO) is driven to high or low randomly, due to the fact that input 'TDO' is floating; not pulled up. However, before the OpenOCD is started, both channels of the FT2232H are configured as UART, with ADBUS2 being configured as RTS# output -> driven high. That means that in case BBv3 (in KT-link configuration) is powered (connected to USB), but disconnected from the target, there is a real danger of damaging FT2232H or CPLD, because their pins (both of which are in the role of outputs) are connected, possibly driven to opposite rails. Is my reasoning correct ?

Edit: I fixed a typo,