Reading peripheral register of Cortex M0 MCU using JLink and GDB

Question

I'm trying to read the MCUs ADC register using GDB but I can't seem to find how it's done.

Using x\10x 0x40012708 in gdb just returns zeroes, as do any memory mapped peripheral register I try to read.

It this possible to do? If so, how is it done?

Thanks!

Answer 1

I would suggest to validate that your setup is functional first. For example, you could try to read the CPUID register which is present on all Cortex-M0 processors - see section 4.3 of the Cortex™-M0 DevicesGeneric User Guide.

It is located at address 0xE000ED00, and it shall contain value 0x410CC200. If you can read it, this would be a good sign.

You probably should try using J-Link commander first, then with GDB-Server and GDB. I ran the procedure hereafter on an Olimex board using an LPC1114/301 Cortex-M0. I was using J-Link software V5.10n, and arm-none-eabi-gdb version 7.10.1.20151217-cvs, which is part of Linaro gcc 5.2 for ARM Cortex-M processors package.

Start JLink.exe using the following options:

jlink.exe -if SWD -speed 4000  -AutoConnect 1 -device Cortex-M0

You should see something similar to:

SEGGER J-Link Commander V5.10n (Compiled Feb 19 2016 18:39:46)
DLL version V5.10n, compiled Feb 19 2016 18:39:11<br/>
Connecting to J-Link via USB...O.K.
Firmware: J-Link ARM V8 compiled Nov 28 2014 13:44:46
Hardware version: V8.00
S/N: [My J-Link EDU Serial Number]
License(s): FlashBP, GDB
OEM: SEGGER-EDU
Emulator has Trace capability
VTref = 3.313V
Device "CORTEX-M0" selected.

Found SWD-DP with ID 0x0BB11477
Found Cortex-M0 r0p0, Little endian.
FPUnit: 4 code (BP) slots and 0 literal slots
CoreSight components:
ROMTbl 0 @ E00FF000
ROMTbl 0 [0]: FFF0F000, CID: B105E00D, PID: 000BB008 SCS
ROMTbl 0 [1]: FFF02000, CID: B105E00D, PID: 000BB00A DWT
ROMTbl 0 [2]: FFF03000, CID: B105E00D, PID: 000BB00B FPB
Cortex-M0 identified.
J-Link>

You can now read the CPUID register using the J-Link commander mem32 command, and verify the CPUID register does contain the expected value:

J-Link>mem32  0xE000ED00,1

You should get:

E000ED00 = 410CC200

If this is the case, I would say that your J-Link/Cortex-M0 setup is likely functional. We can now verify GDB Server and GDB are working fine too.

Launch JLinkGDBServerCL.exe with the following options:
JLinkGDBServerCL.exe -if SWD -speed 4000 -device Cortex-M0
SEGGER J-Link GDB Server V5.10n Command Line Version JLinkARM.dll V5.10n (DLL compiled Feb 19 2016 18:39:11)

-----GDB Server start settings-----
GDBInit file:                  none
GDB Server Listening port:     2331
SWO raw output listening port: 2332
Terminal I/O port:             2333
Accept remote connection:      localhost only
Generate logfile:              off
Verify download:               off
Init regs on start:            off
Silent mode:                   off
Single run mode:               off
Target connection timeout:     0 ms
------J-Link related settings------
J-Link Host interface:         USB
J-Link script:                 none
J-Link settings file:          none
------Target related settings------
Target device:                 Cortex-M0
Target interface:              SWD
Target interface speed:        4000kHz
Target endian:                 little

Connecting to J-Link...
J-Link is connected.
Firmware: J-Link ARM V8 compiled Nov 28 2014 13:44:46
Hardware: V8.00
S/N: [My J-Link EDU Serial Number]
OEM: SEGGER-EDU
Feature(s): FlashBP, GDB
Checking target voltage...
Target voltage: 3.31 V
Listening on TCP/IP port 2331
Connecting to target...Connected to target
Waiting for GDB connection...

Keep the GDB server running, and start GDB in an other Windows console session:

arm-none-eabi-gdb
GNU gdb (GNU Tools for ARM Embedded Processors) 7.10.1.20151217-cvs
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "--host=i686-w64-mingw32 --target=arm-none-eabi".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".

We can now connect to GDB server, and attempt to display the content of the CPUID register again:

(gdb) target remote localhost:2331
Remote debugging using localhost:2331
0xfffffffe in ?? ()
(gdb) monitor halt
(gdb) monitor reset
Resetting target
(gdb) x/1xw  0xE000ED00
0xe000ed00:     0x410cc200
(gdb)

If you are getting 0x410cc200 again, your J-Link/Cortex-m0/GDB Server/GDB setup should be functional, and you should now try again to read your ADC register using this command:

x/1xw 0x40012708

If you were able to read the CPUID register, but not the ADC register, and if the address for the ADC register is correct, I would not have an explanation for this behavior right now - knowing the exact brand/model for the MCU you are using could of course help further.

Please note that from now on, you should specify the exact model for your MCU as the argument to the -device J-Link Commander/GDB Server options instead of Cortex-M0. You can get the list by entering command ? on the J-Link Commander command-prompt.