professionalpyroman

Oh you should see the stuff I'm procrastinating on...

professionalpyroman

Alright, progress update. Made a decent amount of headway.

I'm almost 100% certain that the two controller chips are NEC uPD75108B maskrom microcontrollers. To replace the microcontroller, I need to address two functions.

1. Interfacing audio with existing DAC and filters. This part will be the more straightforward half. It will probably be easier to desolder and remove these items and place on a new pcb than have an adapter ribbon to install and then have to disable all the other components on the existing board. It would also simplify things a bunch. The audio connections I would need from the new microcontroller are the 3 standard I2S lines; data, l/r, and clock, along with two aperture lines to help with any excessive oscillations (from what I have found, these may not be needed with modern digital audio sources). I'd have the aperture lines hooked up as an option, and if they are not needed, they can be left out of the programming. There are two other lines that will need hooked up, the deemphasis and mute circuits. I'm still trying to understand these, but it looks like the deemphasis is to quiet down "noisy" signals, and the mute is engaged whenever the optical pickup is moving around, or the CD unit isn't playing music. I'll have to read more about how they work to implement these, but deemphasis and mute only need one circuit each to run. This should allow music to be fed from an I2S audio source, which a lot of available hobbyist microcontrollers seem to support. Good deal there. Also by removing all the extra circuitry, it frees up the 12v lines to be used for whatever. Currently just looking at bluetooth, but that might be able to supply enough power to enable other devices like a microphone or something for answering a call (feature for a later revision).

2. Integrating with controls. This is the difficult part, but should be a lot more possible since I believe I have ID'd the two control chips.
I have been looking to see where the Vdd for these chips comes from. It looks like all the main board and tape deck chips are all powered by a voltage regulator on the main PCB. Also the Vdd for the CD control chip on the CD pcb is also supplied with this Vdd. The power for all the other CD chips comes from a +5v regulator that only the CD controller chip can turn on and only powers stuff on the CD pcb. It keeps the CD chips powered down unless they are in use, probably to keep the thing from draining your battery from them running all the time. I think that both control chips are powered on when the unit receives +13v Batt power. It looks from the datasheets that the 75108 chips have a low power mode, and probably don't turn all the way on until it sees the signal from +13v ACC. This would make sense because if you unplug the unit, you lose your radio stations, but with battery power, it would keep your stations. Kinda like how your old nintendo cartridges have the coin cell batteries to keep the game saves in the static ram. If we got rid of the extra stuff in the CD portion, it frees up a bunch of power headroom that we could use at our discretion. The main power consumer at that point would be the amp units. I've just got to confirm the power-on status with a scope or voltmeter and that should be pretty straightforward. There is a reset line that runs to both controller chips that ensures both chips are reset to their starting point after a loss of power. I'm pretty sure it doesn't do anything after the initial power from +Batt. This would likely just be a status indicator input for the new microcontroller.
There is an LED output from the CD microcontroller that turns on the DISC SET in the upper left corner. Easy straight output from the microcontroller. Thinking this would work well as a bluetooth connected indicator.
That leaves 5 lines that I still need to figure out their function. From what I have seen, only 2 or 3 of these could be for the purposes of serial interface. I am trying to find out more information on this. It is referenced in the datasheets as NEC Standard Serial Bus Interface (SBI). If you guys could do some digging, that would be helpful. I haven't quite found the right thing. I have found SPI, and I2C interfaces, I wonder if SPI is what I am looking for. The lines I have are BRXEN, BRST, BDATA, BSCK, and BSRQ. BDATA and BSCK are fairly obvious as data and clock for the serial communications. BRXEN is an auxiliary enable output from the CD controller, purpose unknown. BRST Bus reset and BSRQ Bus request I also have to figure out. If we can tack these down, there's a pretty good shot at decoding it, or at least decoding what we need.

I am trying to get ahold of another professor at the university who would have grad students that could help me. It's more about who you know, than what you can do by yourself. I am still out of my depth, but I think I've made some big progress here. Let me know what you think, or if you guys find some relevant info.

beew

Hope your project is going well... Only posting because I am also looking for the this bus protocol documentation or explanation.
Got here because you're the only mention of BSRQ on the internet.
I'm trying to adapt a bluetooth device to my own old radio (Pioneer KE-83) in a 1990 E32 BMW 750iL and have matched all the lines except the BSRQ line, and want to know if it's redundant for my needs.. https://maakbaas.com/bluetooth-cd-ch...-to-prototype/.

From what I can find the NEC SBI is also called the 3-line SIO or 3-wire SPI... hope it gets you in the right direction.

professionalpyroman

Welcome to the forum. I'm glad there are other people looking into this as well. Kinda felt like I was one of the only ones.

I've read that forum post. A lot of good information there. That guy went for a somewhat different use case, adapting a newer head unit to the cd changer. I think for our stuff it might be similar, but until I get a signal decoder like that, we won't know for sure. Can you tell me a little bit about your radio? Is it similar to ours, running a pair of 75108's for control?

Thank you for the SBI clue. I'm still looking through it. It's getting to be the busy season for me again, so updates are probably going to slow down more than they already have.

beew

​​​​​​Yes I think it runs on a variant of the 75108. The schematic only shows 1 chip.
​​Good thing too, I've got enough duplicate computers with V12 as is.

Anyway, I briefly spoke to my Electrical engineer friend about this project and she said to just wire it up and test. (I am a little worried that pricey lil Bluetooth amp will get fried... but that's half the fun right?)
The SQR line
​​​might not be essential to function, but she also said it would be essentially impossible to manually spoof.
The mbus protocol notes I found say that the SQR line is only essential to tell the head unit that the peripheral is present and its ok to send data, since the head unit doesn't automatically search for peripherals on startup. The later KE-91 radio for which the Bluetooth adapter was designed does this automatically and has no the SQR pin.
​

professionalpyroman

Thank you very much for this info. It looks like we are at least in the same ballpark, if not pretty close in design. I'm sure this will be a big leap forward on this project. I haven't had a lot of time to look through, but from what I did review of the BMW project, BRXEN is likely the same thing as his ENBL pin. As far as BSRQ, I think it might be used as a sort of chip select. I haven't done anything to confirm it yet. On our Rx7 Pioneer units, we can boot the CD unit, which is built like a peripheral, into test mode. This pin might be used when the CD unit is in test mode to take over as the master chip (entirely speculation). Do your radios have a test mode so you can adjust and configure your CD changers?

beew

BSRQ in our unit is used to search for a peripheral. Process goes like this (i think)... Power on - Headunit sends SRQ out - gets response from peripheral (CD changer) - sends BRXEN/ENBL signal, this is the "handshake", the data lines light up and the 2 units start talking.
The unit in MaakBaas project is more advanced and skips the SQR part, automatically initiating the BXRN/ENBL when switched into CD mode.

​​​​​​Afaik our units do not have a test mode. Couldn't find mention of one in the document.

When I set this up, I'm going to test with and without the SQR pin plugged in. Hopefully it's not needed. Just waiting on AliExpress parts to make the patch cables. ​