The Intel 8755 is a chip I come across quite often inside vintage equipment, but, I’ve never stopped to look at what it does. That is, until recently when I was asked by a reader if I had anything to program one with.
The 8755 is an unusual offering, combining a 16kbit (2kbyte) EPROM and two 8-bit GPIO ports into a single package. Just the kind of thing you’d be wanting if designing a system based on an 8-bit microprocessor. The EPROM would be used to store a simple bootloader, and the GPIOs purposed for various system functions.
There are many other use-cases, for example on pluggable peripherals where more GPIOs are required, along with peripheral specific data or firmware.
From a technology standpoint, the 8755 appears to be the GPIO and EPROM portions of the 8749 MCS-48, minus the MCU core. This is evident from the matching headline specification, and that the programming algorithm, timings and voltages are the same as the MCS-48 family. This means that my MCS-48 programming shield would be able to program it with a simple adapter.
This is also how it was done in the day, with Intel documentation referring to MCS-48 programming hardware and adapters as a programming solution for the 8755.
Unfortunately because of differences in the programming interface, some software changes are required. Those who have already built the MCS-48 shield would need to get the latest host software and re-flash with the latest firmware to get 8755 support.
All build resources for this are on the MCS-48 shield page.
Posted in Vintage microcontrollers
I am a little confused about the PROG pin. The data sheet for the 8748 states that PROG needs to be +18V during programming. However, the only non-5V voltage the 8755 data sheet lists is +25V Vdd during programming only. Does your 8755 shield send 5V or 18V to PROG?