From 9363dcd9944d348b1fc50e8c195c8c30ecbc0cc3 Mon Sep 17 00:00:00 2001
From: Erik Ekman <erik@kryo.se>
Date: Thu, 11 May 2023 00:56:51 +0200
Subject: [PATCH] Only change speed after ATR if TA2 bit 5 is unset

Based on ISO 7816-3 8.3:

If bit 5 is set to 0, then the integers Fi and Di defined above by TA1 shall apply.
If bit 5 is set to 1, then implicit values (not defined by the interface bytes) shall apply.
---
 atr.c      | 10 +++++++---
 atr.h      |  2 +-
 atr_test.c |  6 +++++-
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/atr.c b/atr.c
index b1cb128..218c303 100644
--- a/atr.c
+++ b/atr.c
@@ -11,6 +11,7 @@ void atr_init(struct atr *atr) {
     atr->bytes_left = 2;
     atr->first_protocol_suggested = NO_VALUE;
     atr->ta1_value = NO_VALUE;
+    atr->ta2_value = NO_VALUE;
 }
 
 static int handle_t_bits(struct atr *atr, unsigned char data, unsigned *complete) {
@@ -44,7 +45,7 @@ static void handle_ta_byte(struct atr *atr, unsigned char data) {
         atr->ta1_value = data;
         break;
     case 2:
-        atr->ta2_seen = 1;
+        atr->ta2_value = data;
         break;
     }
 }
@@ -100,7 +101,10 @@ void atr_result(struct atr *atr, unsigned *new_proto, unsigned *new_speed) {
     if (new_proto && atr->first_protocol_suggested != NO_VALUE) {
         *new_proto = atr->first_protocol_suggested;
     }
-    if (new_speed && atr->ta1_value != NO_VALUE && atr->ta2_seen) {
-        *new_speed = atr->ta1_value;
+    if (new_speed && atr->ta1_value != NO_VALUE) {
+        if (atr->ta2_value != NO_VALUE && (atr->ta2_value & 0x10) == 0) {
+            // Switch if TA2 bit 5 is cleared.
+            *new_speed = atr->ta1_value;
+        }
     }
 }
diff --git a/atr.h b/atr.h
index 65a08c2..aa1e241 100644
--- a/atr.h
+++ b/atr.h
@@ -23,7 +23,7 @@ struct atr {
     unsigned num_historical_bytes;
     unsigned t_cycle;
     unsigned ta1_value;
-    unsigned ta2_seen;
+    unsigned ta2_value;
 };
 
 void atr_init(struct atr *atr);
diff --git a/atr_test.c b/atr_test.c
index d307e86..cd7599d 100644
--- a/atr_test.c
+++ b/atr_test.c
@@ -34,10 +34,14 @@ static const struct sample {
         .data = {0x3B, 0x93, 0x95, 0x80, 0x1F, 0xC7, 0x80, 0x31, 0x80, 0x6F},
         .len = 10,
     },
-    { // TA(1) and TA(2) supplied, new speed in use.
+    { // TA(1) with speed, TA(2) with bit 5 cleared, new speed in use.
         .data = {0x3B, 0xD2, 0x13, 0xFF, 0x10, 0x80, 0x07, 0x14},
         .len = 8, .speed = 0x13,
     },
+    { // TA(1) with speed, TA(2) with bit 5 set, no speed change.
+        .data = {0x3B, 0xD2, 0x13, 0xFF, 0x10, 0x90, 0x07, 0x14},
+        .len = 8,
+    },
     { // TD(1) present but as last byte.
         .data = {0x3B, 0x80, 0x00}, .len = 3,
     },