From: ian <ian>
Date: Thu, 31 Jan 2008 00:58:21 +0000 (+0000)
Subject: document use of PORTB interrupt-on-change
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ijackson/git?a=commitdiff_plain;h=72dc3759b0ed5a7484aac3c55710fdb71696fd10;p=trains.git

document use of PORTB interrupt-on-change
---

diff --git a/detpic/common.inc b/detpic/common.inc
index 055f32b..8fc7b73 100644
--- a/detpic/common.inc
+++ b/detpic/common.inc
@@ -36,6 +36,7 @@ tickdiv_us equ tick_us * tickdiv
 ;   t_dolocal		Low ISR				High ISR
 ;   FSR1		Low ISR				High ISR (detect[1])
 ;   FSR2		High ISR (nmra[1])		High ISR (detect[1])
+;   PORTB		Special read handling[2]	Used normally
 ;
 ;   Main loop		detection scan			detection scan
 ;   High ISR		NMRA output			I2C service
@@ -78,6 +79,10 @@ tickdiv_us equ tick_us * tickdiv
 ;	detect_slave_init.  Likewise FSR2 is reserved exclusively
 ;	for the NMRA output ISR after nmra_init.
 ;
+;  [2]  On the master PIC we the interrupt-on-change feature of PORTB.
+;	This means that routines mustn't casually read PORTB.  Instead,
+;	they should call portb_read from serout.asm.
+;
 ; General-purpose hardware allocation:
 ;
 ;			Master			Slave
@@ -110,11 +115,11 @@ tickdiv_us equ tick_us * tickdiv
 ;
 ; <something>_intrl	Low ISR service routine.
 ;			Checks for any relevant interrupt.
-;			If not, just returns.
+;			If not, just returns
 ;			If found, services it and then does either
 ;			 intrl_handled or intrl_handled_nostack
 ;			 neither of which return; the latter is
-;			 faster but implies a promise 
+;			 faster but implies a promise
 ;
 ;----------------------------------------------------------------------
 ; MACROS