Top pinning,bottom pinning and piggy backing on an MAB8440 MCU - Page 1
Forum section: electronics / general

author: djsb
author_rank: Super Contributor
timestamp: December 15, 2024, 04:39:14 pm
content: Does anyone remember or can anyone explain the concept of top pinning, bottom pinning or piggy backing mentioned in this MAB8440 MCU data sheet. I guess it's some way of connecting a 2732 or 2734 EEPROM. It's not very clear how this is meant to work. The MAB8440 MCU in the FM tuner I'm studying is mask programmed. However, I'm wondering if I can somehow replace it with an OTP 8048 compatible equivalent (not sure if UV erasable 28 pin type is available). Just wan to understand the pinning and piggyback idea first.Can anyone help. Thanks. Data sheet attached.MAB8440_MCU.pdf(90.51 kB - downloaded 33 times.)PS Considering a PI PICO, small FPGA with an 8048 soft core or a PIC as a replacement. Looking at some MCS-48 manuals on bitsavers.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 16, 2024, 02:01:31 am
content: Quote from: djsb on December 15, 2024, 04:39:14 pmYes, that is what the data sheet states.They had a socket at the top for an EPROM and were $$$ bond-out prototype parts.Quote from: djsb on December 15, 2024, 04:39:14 pm8748 were only in DIP 40 FWIRQuote from: djsb on December 15, 2024, 04:39:14 pmFPGA sounds expensive, but you may be able to emulate an 8048 core in a fast ARM CPU. The IO pins might be trickier.Quote from: djsb on December 15, 2024, 04:39:14 pmDoes that mean you have the HEX code ?The FM tuner must be somehow special ? What else is connected to the MAB8440 ? ALE and io expanders can complicate things.

author: djsb
author_rank: Super Contributor
timestamp: December 16, 2024, 06:56:00 am
content: No hex code, unfortunately. All the connections are shown on the attachments in this posthttps://www.eevblog.com/forum/microcontrollers/why-a-4-433619mhz-xtal-in-this-fm-tuner/The tuner is kind of special to me, as it's something that sounds really nice (live broadcasts on the BBC will probably sound even better). It's just that the digital control is a bit erratic.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 16, 2024, 07:42:34 am
content: Quote from: djsb on December 16, 2024, 06:56:00 amIf you do not have the ROM code, it is a long, uphill slog.There look to be 5 serial interface peripherals, one of which is a 33V EAROM.You could sniff on the serial links, with a logic analyzer, and work backwards.Or use the Pi PICO you mentioned, running as a smarter logic sniffer.Quote from: djsb on December 16, 2024, 06:56:00 amAre you sure the MCU is the issue ?What exactly does 'a bit erratic' mean ?It may be easier to locate another unit with the hope the same part has not failed ?

author: ocelot
author_rank: Contributor
timestamp: December 22, 2024, 09:05:09 am
content: If it is a mask programmed 8048, its fairly unlikely to be the MCU. If its an 8748 then it has an EPROM in it, that could be becoming erratic and then you are stuck.(even if it doesnt have a window for erasure, that would make it OTP, the chip dies would have been test programmed on the production line, then UV erased as a die , then programmed with the final pattern, I worked at Philips on TV set controllers that had EPROMS for customer code )Later on the 8051 had a multiplexing arrangement that allowed for taking any MCU from the family , plugging it into external hardware that multiplexed in a program RAM for instruction fetch and then latching the IO pin states on different clock phases which meant you no longer needed that expensive chip with a socket on top.In your tuner - It could be the EAROM is worn out so the channel memory is flaky. Emulating that may be easier than reverse engineering the complete tuner, although writing code from scratch could be possible.Check power supplies , capacitors and so on before suspecting semiconductors.

author: djsb
author_rank: Super Contributor
timestamp: December 22, 2024, 09:58:03 am
content: It has been recapped, and I've put a new EEPROM in it. Might stick a logic analyser on it when I've got some time and enthusiasm. But for now I'm just leaving it as it is.Thanks everyone for your comments and interest.