Dangerous Prototypes

Dangerous Prototypes => Bus Blaster JTAG debugger => Topic started by: wwortel on January 18, 2021, 01:51:47 pm

Title: BB v4.1a (JTAG Key) simple EJTAG Flatcable
Post by: wwortel on January 18, 2021, 01:51:47 pm
Just wanted to describe a simple connecting cable from the  20 pin Bus Blaster's connector to a standard 14 pin EJTAG connector as used with many Broadcom MIPS systems.
Initially had made a cable on basis of a flat cable with many cuts and leads crossing to get to the right pins, as the order of the BB's 20 pin connector differs from the EJTAG standard.
But of course the programmable CPLD logic allows to redefine the BB's pinout such that a flat cable can be connected in a straightforward way.

On the EJTAG side the flatcable is 14 leads into a standard 14 pin flat cable connector, lead 1 (red on my cable) to connector pin 1.
On the BB side lead 1 goes to pin 15 of a standard 20 pin connector. This means that lead 14 (target Vcc) connects to BB's pin 2 (Vtarget). Pin 1 and pins 16-20 of the BB connector remain free. These get RTCK and DBGACK but those are signals not present on EJTAG.
The ground leads of the BB, even pin numbers, connect to the ground pins of EJTAG, also with even pin numbers.
No need to manipulate any of the 14 flatcable leads; Using ISE compiled a jtagkey compatible svf file. It has redefined the BB signals to match the EJTAG signals.
Do set jumper JP4 to 'self power' as the target will supply Vt to the bank of the CPLD that provides the EJTAG signals. Make sure the voltage on EJTAG pin 14 does not exceed 3.3 V.

The picture shows the BB v4.1a connected to a router with Broadcom mips SoC.

To see that this CPLD logic is loaded pressing PB will light the blue LED.
This .svf redefines the pins of the BB v4.1a as follows:
3 DBGREQ, 5 SRSTn, 7 TCK, 9 TMS, 11 TDO, 13 TDI, 15 TRSTn, 17 RTCK, 19 DBGACK
Title: Re: BB v4.1a (JTAG Key) simple EJTAG Flatcable
Post by: wwortel on January 18, 2021, 04:15:40 pm
and here the .svf for the ktlink compatible buffer logic for this cable arrangement