OLIMEX© 2015
ARM-USB-OCD user's manual
2.2.2 JTAG connector
The JTAG connector is a 20-pin male one. It has the standard ARM JTAG 20 at 2.54mm (0.1'')
pitch, specified by IEEE 1149.1. There is a female-female ribbon cable included in the box of
ARM-USB-OCDfor easier connection to the target board.
There is a small mark over the connector that indicates where the first pin is located. It might be
difficult to spot it at first glance (because of the casing) so please also consider that the cable that
comes with the debugger has the first wire colored in red.
The pinout of the JTAG connector is shown in the next table (#3):
Table 3. JTAG connector pinout
PIN # Signal name
PIN # Signal name
1 VREF
2 VREF
3 TTRST_N
4 GND
5 TTDI
6 GND
7 TTMS
8 GND
9 TTCK
10 GND
11 TRTCK
12 GND
13 TTDO
14 GND
15 TSRST_N
16 GND
17 NOT CONNECTED
18 GND
19 TARGET SUPPLY
20 GND
Pins 1 and 2 of the JTAG connector are voltage reference that probes if the target is already
powered by another source. If it is not, the ARM-USB-OCD would attempt to power it by 5V at pin
19 of the connector. Please note that the USB standard allows limited amount of power. Powering
the board from the debugger is not always reliable (especially if the target board has a lot of power-
hungry peripherals). It is recommended to use external power supply for bigger target boards.
If instead of the standard 20-pin 2.54mm (0.1'') JTAG connector your board has the mini version –
the 10-pin 1.27mm (0.05'') – then you might use the ARM-JTAG-20-10 adapter which is sold
separately. This adapter does not provide SWD capabilities (look for ARM-JTAG-SWD for such an
expansion). The ARM-JTAG-20-10 adapter can be used with debuggers manufactured by other
companies as long as they follow the standard 20-pin JTAG layout suggested by ARM Holdings.
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