The m icroprocessor has s e v e r a l f u n c ti o n s t o perform:
com putation. 2 C o n t r o l o f t h e c h a r g e - b a l a n c i n g i n t e g r a t o r and t h e r e c o r d i n g of d o s e . 3 C a l c u l a t i o n of L .“ 4 4 C a l c u l a t i o n of L10/L 90. In vie w o f t h e p a r t i c u l a r e d u c a t i o n a l a p p l i c a t i o n , i t was considered p r e f e r a b l e t o be a b l e t o m e a s u r e Le q and L 10/ L 90 from th e same d ata. This im p lie s t h a t th e m icroprocessor would have t o a c q u i r e sound l e v e l and dose d a t a s i m u l t a n e o u s l y . E s t i m a t e s o f l i k e l y s o f tw a r e p ro c e s s in g c y c l e s showed t h a t t h i s was p o s s i b l e and t h a t t h e Lon c a l c u l a t i o n could be updated c o n s ta n t ly . However, in e q order t o e x e c u t e a l a r g e number o f s t a t e m e n t s i n a g i v e n p e r i o d n e c e s s i t a t e s i n c r e a s i n g t h e m icroprocessor o p e r a t in g f r e q u e n c y and hence i n c r e a s e t h e c u r r e n t c o n s u m p t i o n . I t was t h e r e f o r e n o t co n sid ere d im p o rta n t t o a t t e m p t any L10/L 90 com putation u n t i l th e end o f the measurement p eriod.
During th e measurement p e rio d , th e m icroprocessor i s in a program loop w h e r e i t a c q u i r e s a v a l u e o f s o u n d l e v e l , s p e n d s a predeterm ined p e rio d b a lan cin g th e i n t e g r a t o r and a c c u m u la tin g dose d a t a , makes a c a l c u l a t i o n of th e c u r r e n t L0 q value and the n o u tp u t s t h i s v a l u e . At t h e e n d o f t h e m e a s u r e m e n t p e r i o d , t h e m icroprocessor c a l c u l a t e s t h e v a l u e s o f L 10 and L 90 and s t o r e s them.
7.4.1 Sound Level Time H istory Recording
The dc l o g g e d o u t p u t from t h e s t a n d a r d so u n d l e v e l m e t e r c i r c u i t r y i s d i g i t i z e d and fed i n t o th e m ic ro p ro c e s s o r. An 8 - b i t I / O p o r t i s u s e d t o f e e d a b y t e f r o m t h e m icroprocessor through an R-2R ladder network and one t o one
in p u t o f a c o m p a r a t o r . The o t h e r c o m p a r a t o r i n p u t i s f e d with th e dc logged sound l e v e l s i g n a l and i t s output i s fed back t o th e m icroprocessor. The comparator o u tp u t s i g n a l i s d e t e c te d by t h e m i c r o p r o c e s s o r w h ich i m p l e m e n t s e i t h e r a counter ram p o r s u c c e s s i v e a p p r o x i m a t i o n t e c h n i q u e t o perform t h e A-D c o n v e r s i o n . The a d v a n t a g e s of u s i n g t h e m i c ro p ro c e s s o r’ s I/O p o r t s t o p e r f o r m t h e D-A c o n v e r s i o n d i r e c t l y and f e e d t h e c o m p a r a t o r , a r e t h a t t h e c h i p c o u n t and h e n c e t h e c u r r e n t c o n s u m p t i o n a r e k e p t s m a l l . The conversion t i m e i s m i n i m i z e d b e c a u s e t h e f i n a l v a l u e i s always in a m icroprocessor r e g i s t e r when th e co n v ersio n i s complete.
7 . ^ . 2 Dose Data Recording
To o p t i m i z e t h e dynam ic r a n g e o f t h e Leq c a l c u l a t i o n i t i s im portant t h a t th e m icro p ro cesso r spends a la r g e p r o p o r ti o n of i t s tim e c o n t r o l l i n g th e c h a rg e -b a la n c in g i n t e g r a t o r . I t i s e q u a l l y im p o rta n t t h a t a f u l l s c a l e in p u t would g e n e r a te a l a r g e amount of d a t a over th e f u l l measurement p e rio d so t h a t i f a s i g n a l co rresponding t o th e bottom of t h e ran g e i s fed i n t o th e m eter, i t w i l l be capable of g e n e r a t in g enough d a t a t o a l l o w t h e Lon c a l c u l a t i o n t o be made r e a s o n a b l yc q a c c u r a t e l y a f t e r a s h o r t perio d .
In c o n t r o l l i n g t h e i n t e g r a t o r , t h e m i c r o p r o c e s s o r m u st r e p e a t e d l y t e s t the s t a t e of the comparator u n t i l i t d e t e c t s t h a t an i n p u t t o t h e i n t e g r a t o r h a s c a u s e d t h e o u t p u t t o ramp i n a p o s i t i v e d i r e c t i o n . When su c h a s i g n a l h a s been r e c e i v e d , t h e m icroprocessor e n ab les t h e s w itc h e d c o n s t a n t c u r r e n t t o c a u s e t h e i n t e g r a t o r t o ram p back t o w a r d s z e r o .
The m icro p ro cesso r must r e t e s t t h e s t a t e of the comparator u n t i l t h e s i g n a l i s r e c e i v e d i n d i c a t i n g t o t h e m icroprocessor t h a t t h e i n t e g r a t o r o u t p u t i s z e r o , or v e r y s l i g h t l y n e g a t iv e . The tim e between t e s t i n g and r e t e s t i n g t h e s t a t e o f t h e c o m p a r a t o r , when t h e c u r r e n t i s e n a b l e d , corresponds t o a quantum of dose. For each quantum of dose, the c o n t e n t s o f a r e g i s t e r a r e i n c r e m e n t e d . A f u r t h e r r e g i s t e r , which re c o r d s e lap se d tim e sin c e th e r e s e t b u tto n was l a s t p r e s s e d and h e n c e d e t e r m i n e s t h e m e a s u r e m e n t p e rio d , i s u s e d i n c o n j u n c t i o n w i t h t h i s d o se r e g i s t e r t o compute Le q. The s i z e o f d o se r e g i s t e r r e q u i r e d can be determined by c o n s i d e r i n g th e c o n d i tio n o f minimum o u tp u t t o t h e i n t e g r a t o r , combined w ith th e re q u ir e m e n t fo r a r e l i a b l e answer in a s h o r t ti m e , say 10 seconds. To get w i t h i n ±1 dB of t h e f i n a l a n s w e r , one n e e d s a t l e a s t 10$ r e s o l u t i o n . T herefore a minimum o f 10 q u a n t a m ust be r e c o r d e d i n 10 seconds a t the bottom of th e i n d i c a t o r range. The number of quanta which would correspond t o t h i s a c q u i s i t i o n r a t e f o r a number of in p u t s i g n a l l e v e l s over t h e minimum and maximum measurement d u r a t i o n s i s shown in Table 7.1.
Level 0 dB 25 dB HO dB
Duration Number of Quanta
10 seconds 10 3.16 x 103 1 0 5
1 0 minutes 600 1.89 x 1 0 5 6 x 1 0 7
TABLE 7.1 Dose R e g i s t e r S iz e
I t i s c l e a r t h a t a 3 b y t e r e g i s t e r w ould be n e e d e d f o r a c o n s t a n t l e v e l a t t h e p r o p o s e d t o p o f t h e i n d i c a t o r r a n g e (25 dB), b u t t h a t 3 b y t e s i s i n s u f f i c i e n t t o acc om m oda te a to p o f t h e d y n a m ic r a n g e s i g n a l f o r 10 m i n u t e s . T h is measurement d u r a t i o n was c o n s i d e r e d t o be a p p r o p r i a t e f o r c a p t u r i n g a r e p r e s e n t a t i v e s a m p l e o f t h e s i g n a l , b o t h f o r measurements and f o r th e s a t a t i s t i c a l a n a l y s i s of sound l e v e l data. The use of a t h r e e - b y t e r e g i s t e r would however accommodate a l l s i g n a l l e v e l s which would e v e n t u a l l y r e s u l t in an o n - s c a l e re a d in g .
To d e t e r m i n e t h e t i m e c o r r e s p o n d i n g t o t h e s i z e o f dose quanta, one h a s t o d i v i d e t h e t o t a l number o f c o u n t s corresponding t o th e maximum in p u t by th e t o t a l measurement time:
—Qy = 104 counts per second 6 x 10
To c o m p l e t e t h e t e s t i n g , i n c r e m e n t i n g and an y n e c e s s a r y sw itc h in g in 100 ps i s by no means i m p r a c t i c a l , b u t when the machine c o d e i s d e t e r m i n e d i t r e q u i r e s r u n n i n g t h e