Date   : Tue, 31 Dec 1985 15:22:32 GMT
From   : jp@LANL.ARPA
Subject: Re:  8251 Usart

It's been a while, but I recall that the 8251 had a problem with reset. 
The solutions I've seen are 1: send reset several (3?) times, 2: go through 
the reset-program sequence twice.  Don't know exactly what the problem with 
the 8251 is but that seems to fix it. 
AH, I found some code from my HAL MCEM-8080 (ca 1976).


	UARTI	EQU	8EH	;INITIAL UART MODE WORD
	URTMO	EQU	0EAH	;7 BITS, EVEN PARITY, 2 STOP
	URTCT	EQU	0BH	;UART CONTROL PORT
	URTDA	EQU	0AH	;UART DATA PORT
	UARTR	EQU	55H	;RESET COMMAND
	TXRXE	EQU	27H	;UART RX AND TX ENABLE 


INIT:	MVI	A,UARTI		;UART MODE OUT	
	OUT	URTCT 
	MVI	A,UARTR		;RESET 
	OUT	URTCT 
	MVI	A,URTMO		;FINAL UART MODE
	OUT 	URTCT
	MVI	A,TXRXE		;ENABLE UART
	OUT	URTCT
	RET


The above code works.  I have seen a slightly different way, but can't
lay my hands on it this morning.

I remember now that the 8251 comes on with its brains slightly scrambled.
There are two control bytes that are sent to the same address sequentially
and it doesn't know where it is in that sequence when first initialized.

Hope this is some help.

Jim Potter  jp@lanl.arpa