Confused about fed margin call

Can someone tell me what fed margin call actually means in practice. I know you need front up 50% for the stock you're gonna buy using margin. But does this 50% need to be in cash? I haven't really used margin but just learning using some models and calucators so not sure if this is very obvious or stupid question. Lets say my brokerage account value is $40,000. All of it is in stocks and options, so $0 in cash. My margin is $20,000. If I wanna buy $1,000 of XYZ using margin, do I need to have $500 in cash in my acount? OR at least $500 in account value? If it's the latter then I'm already satisfying it right? Thanks for your help. I've been looking up definitions but this is not quite clicking for me.