Broadcom 7405 and 7401 chips

[jane]

Has anyone tried to use  JTAG with  Broadcom 7405 or 7401 chips?

[merkin]

The full datasheet and bcm97405 development board schematics can be found on web. 
It can be configured for three different modes via stapping resistors. 
In normal mode output from TDO is always 0.
In jtag mode you can read IDCODE, but nothing else...which leads me to believe TDI is disabled.
In ejtag mode output from TDI is always 1.

YMMV, its possible the manufacturer of your device did not disable the debug module, or blow any OTP fuses.

[jane]

In jtag mode you can read IDCODE, but nothing else...which leads me to believe TDI is disabled.
In ejtag mode output from TDI is always 1.[/quote
if the JTAG were disabled, I would not EVEN be able to read  IDCODE, I think.And what if the Broadcom's CPU only response in a NON standard ways?

[merkin]

By spec, if jtag device has IDCODE then its mandatory for the register to be loaded when tap is reset.  If no IDCODE than BYPASS register is loaded.  This means TDI is not needed to read IDCODE, but other tap commands will not work.

Btw in my previous post I meant to say TDO in ejtag mode .

[is0-mick]

I believe the bcm range of chips can have the jtag interface "passworded" so it will only unlock with a certain command / key sequence.

I did have a poke around with a bcm7335, which would only give a response when set to jtag mode, and I got nothing with Ejtag mode. (there are jumpers to select).

Mick