8535 ISP issues w/ SPI UART & Power Supervisor Attached

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I have an 8535 design that will require ISP via the SPI port. The difficulty lies in the fact that the RESET line on the 8535 is driven by a MAX707 MCU supervisor, and I have a MAXMAX3110 (SPI driven UART) attached to the SPI pins to add an additional uart to my system.

In looking at the STK500 ISP connections & Reading the 8535 Datasheet, RESET, MISO, MOSI, SCLK are all used for ISP. Does anyone have any configuration suggestions to prevent any contention between the MAX707 & ISP on the RESET line. Also any thoughts if I need to isolate my SPI UART while programming the 8535?

Regards,

Erick

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I have a similar design. A supervisor with a push-pull output will cause trouble. You need an open drain or open collector MCU supervisor and a pull-up resistor. Check the MAX707 datasheet. I expect that the UART has an enable pin. If it isn't enabled, you don't need isolation.

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Robert,

Any suggestion fo an open collector / drain MCU supervisor (the max707 is push-pull)? or could I just put a series resistance between the ISP/RESET Node and the RESET output of the PUSH/PULL supervisor?

ISP-Reset Pin
|
_________________ | __________
| | |
| Rs | | 8535 MCU
MAX707 RESETBAR|---/\/\/\----@-------| RESET
| |

Regards,

Erick

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Clarification on the diagram thanks to fabulous wysisyg formatting of the post box (it removed all my manual spacing for a pretty picture).

ISP-Reset Node is tied to MCU Reset pin directly.
MCU Supervisor Reset output tied to MCU Reset through a series resistance Rs

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Your reset circuit is OK. The Maxim 3110 has a chip select pin that should be one of the outputs on the 8535. This pin should have a pullup resistor so that while the processor is reset during programming (output pins high impedance) the UART is disabled.
Dave Raymond

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Thanks,
Robert and Dave.

Regards,

Erick