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.. SPDX-License-Identifier: GPL-2.0
==================
JTAG userspace API
==================
JTAG master devices can be accessed through a character misc-device.
Each JTAG master interface can be accessed by using /dev/jtagN.
JTAG system calls set:
* SIR (Scan Instruction Register, IEEE 1149.1 Instruction Register scan);
* SDR (Scan Data Register, IEEE 1149.1 Data Register scan);
* RUNTEST (Forces the IEEE 1149.1 bus to a run state for a specified number of clocks.
open(), close()
---------------
Open/Close device:
::
jtag_fd = open("/dev/jtag0", O_RDWR);
close(jtag_fd);
ioctl()
-------
All access operations to JTAG devices are performed through ioctl interface.
The IOCTL interface supports these requests:
::
JTAG_SIOCSTATE - Force JTAG state machine to go into a TAPC state
JTAG_SIOCFREQ - Set JTAG TCK frequency
JTAG_GIOCFREQ - Get JTAG TCK frequency
JTAG_IOCXFER - send/receive JTAG data Xfer
JTAG_GIOCSTATUS - get current JTAG TAP state
JTAG_SIOCMODE - set JTAG mode flags.
JTAG_IOCBITBANG - JTAG bitbang low level control.
JTAG_SIOCFREQ
~~~~~~~~~~~~~
Set JTAG clock speed:
::
unsigned int jtag_fd;
ioctl(jtag_fd, JTAG_SIOCFREQ, &frq);
JTAG_GIOCFREQ
~~~~~~~~~~~~~
Get JTAG clock speed:
::
unsigned int jtag_fd;
ioctl(jtag_fd, JTAG_GIOCFREQ, &frq);
JTAG_SIOCSTATE
~~~~~~~~~~~~~~
Force JTAG state machine to go into a TAPC state
::
struct jtag_tap_state {
__u8 reset;
__u8 from;
__u8 endstate;
__u8 tck;
};
reset: one of below options
::
JTAG_NO_RESET - go through selected endstate from current state
JTAG_FORCE_RESET - go through TEST_LOGIC/RESET state before selected endstate
endstate: any state listed in jtag_tapstate enum
::
enum jtag_tapstate {
JTAG_STATE_TLRESET,
JTAG_STATE_IDLE,
JTAG_STATE_SELECTDR,
JTAG_STATE_CAPTUREDR,
JTAG_STATE_SHIFTDR,
JTAG_STATE_EXIT1DR,
JTAG_STATE_PAUSEDR,
JTAG_STATE_EXIT2DR,
JTAG_STATE_UPDATEDR,
JTAG_STATE_SELECTIR,
JTAG_STATE_CAPTUREIR,
JTAG_STATE_SHIFTIR,
JTAG_STATE_EXIT1IR,
JTAG_STATE_PAUSEIR,
JTAG_STATE_EXIT2IR,
JTAG_STATE_UPDATEIR
};
tck: clock counter
Example:
::
struct jtag_tap_state tap_state;
tap_state.endstate = JTAG_STATE_IDLE;
tap_state.reset = 0;
tap_state.tck = data_p->tck;
usleep(25 * 1000);
ioctl(jtag_fd, JTAG_SIOCSTATE, &tap_state);
JTAG_GIOCSTATUS
~~~~~~~~~~~~~~~
Get JTAG TAPC current machine state
::
unsigned int jtag_fd;
jtag_tapstate tapstate;
ioctl(jtag_fd, JTAG_GIOCSTATUS, &tapstate);
JTAG_IOCXFER
~~~~~~~~~~~~
Send SDR/SIR transaction
::
struct jtag_xfer {
__u8 type;
__u8 direction;
__u8 from;
__u8 endstate;
__u32 padding;
__u32 length;
__u64 tdio;
};
type: transfer type - JTAG_SIR_XFER/JTAG_SDR_XFER
direction: xfer direction - JTAG_READ_XFER/JTAG_WRITE_XFER/JTAG_READ_WRITE_XFER
from: jtag_tapstate enum representing the initial tap state of the chain before xfer.
endstate: end state after transaction finish any of jtag_tapstate enum
padding: padding configuration. See the following table with bitfield descriptions.
=============== ========= ======= =====================================================
Bit Field Bit begin Bit end Description
=============== ========= ======= =====================================================
rsvd 25 31 Reserved, not used
pad data 24 24 Value used for pre and post padding. Either 1 or 0.
post pad count 12 23 Number of padding bits to be executed after transfer.
pre pad count 0 11 Number of padding bit to be executed before transfer.
=============== ========= ======= =====================================================
length: xfer data length in bits
tdio : xfer data array
Example:
::
struct jtag_xfer xfer;
static char buf[64];
static unsigned int buf_len = 0;
[...]
xfer.type = JTAG_SDR_XFER;
xfer.tdio = (__u64)buf;
xfer.length = buf_len;
xfer.from = JTAG_STATE_TLRESET;
xfer.endstate = JTAG_STATE_IDLE;
if (is_read)
xfer.direction = JTAG_READ_XFER;
else if (is_write)
xfer.direction = JTAG_WRITE_XFER;
else
xfer.direction = JTAG_READ_WRITE_XFER;
ioctl(jtag_fd, JTAG_IOCXFER, &xfer);
JTAG_SIOCMODE
~~~~~~~~~~~~~
If hardware driver can support different running modes you can change it.
Example:
::
struct jtag_mode mode;
mode.feature = JTAG_XFER_MODE;
mode.mode = JTAG_XFER_HW_MODE;
ioctl(jtag_fd, JTAG_SIOCMODE, &mode);
JTAG_IOCBITBANG
~~~~~~~~~~~~~~~
JTAG Bitbang low level operation.
Example:
::
struct tck_bitbang bitbang
bitbang.tms = 1;
bitbang.tdi = 0;
ioctl(jtag_fd, JTAG_IOCBITBANG, &bitbang);
tdo = bitbang.tdo;
THANKS TO
---------
Contributors to Linux-JTAG discussions include (in alphabetical order,
by last name):
- Ernesto Corona
- Jiri Pirko
|
