AR1780 / XHDATA D-808 tech info & improvements

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Apr 12, 2012
Recently acquired the Digitech AR1780, and I'm quite impressed with how well the thing works. Just a few tweaks here and there and it would be perfect for my requirements. But to do those tweaks means getting to know the radio. REALLY know the radio, from the inside out.
Here's what I've found out about the AR1780 (and by extension, the D-808 too):
  • Silicon Labs Si4735 direct conversion receiver on a chip is used for LW, MW, SW and FM reception. This device is controlled by sending software commands to it, in order to configure it and tune it. The microprocessor running the radio does that for us, so we don't have to think too hard about it all.
  • The FM & Air band uses a CD7358GS 'front end' (copy of Toshiba's TC7358)
  • Air band is down converted to an IF of 10.7 MHz, with a Princeton Tech FS8308 PLL. This 10.7 MHz IF is then sent to the Si4735 as if it were a shortwave signal to be received. The arrangement seems to work pretty well. (It also means that, in theory, you could get the radio to receive SSB on the air band, though I'm not sure why you'd want to!)
  • The audio PA is a CS4863, a copy of LM4863.
  • There is a 24C08 EEPROM. Not sure what it's doing, I read it's contents in an external reader and it was completely blank. I have some memories stored in my AR1780, and I expected to find them in the EEPROM, but didn't. Bit of a mystery...
  • And the MCU / microprocessor that controls the entire production is hidden under a black blob of epoxy, so I can't identify what it is exactly. It does have a serial SWD interface, which (if not memory readout protected) could permit a peek 'n tweak into the inner workings of the software running the radio.

Two minor improvements I'd like to see are:
  • Improved tuning knob. As supplied, the tuning knob isn't all that easy to use to casually tune around the bands. Perhaps some sort of eccentric indentation can be fashioned or some other arrangement made to make tuning with this radio a bit better?
  • AGC action is woeful. It's either way too fast & aggressive, or far too slow to react, I can't decide which. Lightning crashes render the receiver mute for half a second because of this. I reckon CW would be rather unpleasant to listen to on this radio for the same reason. It might be possible to tap into the MCU and rearrange some of the code & commands that are sent to the Si4735 to better configure it and improve the AGC action? A bit more on this below.
Another mod that has been suggested is to see if the muting while tuning could be removed. When the radio mutes every time the frequency is retuned, it results in the unpleasant chuff-chuff-chuffing sound that can cause you to miss a weak signal as you're tuning about. This muting is happening inside the Si4735, and while I've not yet looked into the programming reference for the Si4735, it might be possible to disable this muting. But then again, would it be wise to? It sounds like removing the muting could unleash a barrage of unwanted chirps, pops and noise as the electronics within does it's thing. Probably why the muting is there in the first place... but you never know.

Now we start to get really technical. Trying to identify what breed of MCU is hiding underneath the black epoxy is going to be the first challenge. I traced out some pins of this device to gauge their function. Perhaps someone will recognise the pins and be able to match it up to a possible ID of the MCU?
It's a 100 pin LQFP device, being clocked by a 32.768 kHz crystal.
Vdd on pins 4 & 13
Gnd on pins 3, 14 & 63
Reset on pin 20
SDA on pin 11
SCL on pin 12
Vpp on pin 17 (now that's unusual, surely it must ring a bell with someone? Vpp on a modern MCU must be a rarity)
SDA to EEPROM on pin 9
SCL to EEPROM on pin 10
NRst to Si4735 on pin 96
SDIO to Si4735 on pin 97
SCLK to Si4735 on pin 98

Any takers on trying to identify what this MCU might be?
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