So I've been working on getting UART access to the head unit in my car; I've got the schematics of the unit and found that TX3/RX3 on the Renesas SH7750 are presented externally - along with 3 "test bit" pins.

Pulling the test bits low in certain combinations enables certain modes (eval or debug). I'm happy that I am in debug mode as the unit does output on UART in this mode whereas there is nothing at all in normal operation.

Some of this is documented in a workshop manual for another headunit from the same manufacturer - but also includes a "test box" which sits between the unit and the PC (herein potentially lies the problem)

It should output "Hitanykey" and then "xanavi>"

I however only got garbage output... I've been through the baud rates and no luck so have resorted to sticking a logic analyser on it.

Any pointers or suggestions would be greatly appreciated!

Sigrok save: https://www.dropbox.com/s/td6kt2vsnfl450j/uart_test.pvs?dl=0




  • \$\begingroup\$ You signal looks like it might be inverted, which the decoder can presumably be configured to accept. To determine baud rate, crank up the sample rate and measure the width of the narrowest pulse, which would be one baud period. \$\endgroup\$ – Chris Stratton Sep 26 '20 at 15:36
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    \$\begingroup\$ In addition to what Chris said, after determining the correct baud rate which might be non-standard, check if you have the correct frame size, as it might contain parity so frame size could be 11 bits (start, 8 data, parity, stop). With correct framing and speed there should be no "break" or other warnings. \$\endgroup\$ – Justme Sep 26 '20 at 16:30
  • \$\begingroup\$ Thanks for the fast replies, So I tried as suggested with inverting the input in the decoder which does remove quite a few breaks. With regard to narrowest pulse: at the highest sample rate of 24mhz it sees the smallest as 41.667us. Am I lacking the resolution with my cheap analyser ? \$\endgroup\$ – Whiterat Sep 26 '20 at 16:54
  • \$\begingroup\$ It looks to me like like decent enough resolution, and the rate comes out to 24kbits/second. Not exactly a standard rate, but believable. \$\endgroup\$ – Eamon Sep 26 '20 at 17:28
  • \$\begingroup\$ Still struggling to get anything sensical here... apologies for not being much of an EE. Is there anything in the first schematic that shines any light? The "dengen" (test) box referenced in the maintenance manual specifies a pretty standard 9600 8n1 no flow control. \$\endgroup\$ – Whiterat Sep 27 '20 at 21:35

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