Resister sip network?

[snipersquad100]

Can anyone explain whats going on in this circuit concerning the 10k sip? It doesn't look right to me, why would the common pin be connected to VCC and connected to the pins on the 8515, that would just keep all the pins high all the time.

[Paul Moir]

It's a pullup.  It weakly pulls the UDN2981's inputs high in case the ATMega isn't pulling them low.  For example, in RESET and until defined as an output, the PA0-7 pins are defined as inputs.  This means they won't necessarily drive the UDN2981 inputs reliably during this time.

[Yansi]

No, that is not true.

The pullups are there due to the fact that mega8515 is a direct pin compatible replacement for 8051 MCU. The 8051 MCU does NOT have integrated pullups on port P0 (the port A on the atmega). The 8051 P0 is only open collector outputs.

However the schematic is still wrong, as the 10k pullups are not enough to drive the UDN2981, neither are the integrated ~50k pullups for the low side driver.

As the Atmega has configurable symetrically driven totem-pole output stages, the pullups are pointless and shall be pull-DOWN instead to ensure it won't be lit whenever the MCU pins are floating high impedance.   However the drivers already have pulldown resistors built in.

Therefore when done properly, the drivers shall be isolated from an 8051 device using octal buffers like 74245. With the atmega MCU, it is fine as it is, if one replaces he pullup with a pulldown array.

[snipersquad100]

Any chance you could draw a better schematic??? 

[Paul Moir]

No, that is not true.

Thanks, I admit this is a case of TLDRtD (Too Lazy, Didn't Read the Datasheets).