Hi Boys and Girls,
got a quick question with the BTM-182.
I’ve been running it happily at 3.3V. It seems quite happy to recieve 3.3V TTL UART from my micro.
however it seems to only want the TX pin to operate between 0 and 1.5V, which is slightly frustrating because my micro will not pick it up.
The MAX232 does step it upto 12V necessary for me to recieve it on my laptop. but I need this communicating with my micro.
Have I got some configuration wrong? or do i need to logic level convert downward?
Cheers,
YF
bcr
2
Yeah, I had the same problem with my BTM-182. Apparently the BC04 UART_TX is not really push-pull;
it’s some kind of open collector/tri-state driver with a weak pullup: http://www.oemblue.com/download/BC04Class1Module.pdf
1.5V is really marginal for Vih on a 3.3V micro, let alone a 5V micro. But since its not a push/pull driver in the BTM, you just
need to add an external ~1K pullup to 3.3V, or if you are cheap/lazy, enable on the internal pullup inside your micro.
The internal pullup in my 5V ATmega48 brought the pin to about 3.8V, which is a solid logic ‘1’, and now it’s working for me.
3.8V is technically 0.2V above spec for the BTM-182, but I doubt it will harm it as there’s virtually no current flowing.
In the MCU->BTM direction I am using a resistor divider to drop the level (390 / 680 ohm for 5.0 → 3.3V).
-bcr