With more advanced CPU’s there is usually requirement on JTAG connection such that there better match in length, since clock frequency runs up to dozens of MHz.
If using a 1MHz to 12MHz 430 device, what is the frequency for the spi-2-wire JTAG? We checked in MSP430 Programming Via the JTAG Interface User's Guide.pdf and see the 350 kHz ± 100 kHz value, but the document seems only describes this value in the context of flash programming.
For ordinary program load, run, breakpoint, what is this JTAG frequency? If the speed is low, may I not use any standard JTAG header and just use short wires to connect the 430 device with emulator, for which the scheme looks like:
430 spy-bi-wire pins→ two test points on board→ two wires/cables with one end soldered on the test points→ MSP-FET430UIF spi-bi-wire pins
The quality of connection would definite be inferior as comparing to putting a dedicated JTAG header on board, and saves extra space. Had this ever been done in practice and does it work?
SBW is a serialized JTAG. I think I read something in the MHz range. Normal JTAG shouldn't be much slower, since SBW requires three times the clock speed to be equally fast, and SBW is slower than normal JTAG. So normal JTAG frequency likely is at least half of SBW frequency or even the same. (I never read any details).Also, JTAG/SBW frequency is independent of MSP operating frequency.
For the RST pin, the recommendation is to have not more than 2.2nF capacitance on the RST pin or SBW may fail.
John Mielkesee the 350 kHz ± 100 kHz value, but the document seems only describes this value in the context of flash programming.
To program flash through JTAG, the software usually injects a small code snippet into RAM and lets it execute by the CPU. In theory, JTAG could directly program flash too, but since it requires checking for flash controller registers etc, this would be much slower than when executed by the CPU. However, in any case, the controller requires the proper working frequency. On 5x family devices, the flash controller has its own internal oscillator for the proper clock.
Out of curiosity, I just did connect a logic analyzer to my 5438 target board. The maximum frequency I observed for 4-wire JTAG on TCK was 4MHz (50% DC). On TDI or TDO, the maximum was 2MHz. In SBW mode, the observed frequencies were 1MHz (actually 666.66kHz but with a DC of 25%) or less for the clock and less for the data line.This was done with an MSP-FETU430IF and the elprotronic software.Note that after reset, the default CPU frequency of the 5x family is ~1MHz (= much slower than JTAG), on 1x family even only ~600kHz.
About the header, well, in our devices there is a 10-pin combo header that contains the JTAG pins, VCC, and RS485. The FET is connected by an adapter. Sicen we already needed RS485, VCC/GND and RST, adding a few more pins was no problem. And if you need a removable connection (don't want the FET hardlinked forever), a dedicated header is usually better than a test point.
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The datasheet states that the maximum SBW frequency is 20mhz, while the max JTAG frequency is 5mhz (or 10mhz for some newer ones), which makes sense since it is a half duplex line with three signals encoded, and 20mhz would still make it slower than JTAG. However as JMG observed I don't think a FET would approach nearly that fast a frequency when doing SBW.
TonyKaoI don't think a FET would approach nearly that fast a frequency when doing SBW.
Jens and Tony,
Thanks both, and especially to Jens for the values which are truly helpful.
We asked someone doing MSP430 for years and he said that he had succeeded many times even using software connection out of the board for JTAG debugging/programming. Most likely SBW just works slower, rather than at a multiplied speed of JTAG. We will test these ourselves.
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